“God has cared for these trees, saved them from drought, disease, avalanches, and a thousand tempests, and floods. But he cannot save them from fools.” – John Muir
- Abiotic and Biotic Agents
- Symptoms and Signs
- Environmental/Abiotic Agents
- Living/Biotic Agents
- Common Tree Diseases in Virginia
- Diagnosing Tree Diseases
- 20 Questions on Plant Diagnosis
- Guidelines for Taking Plant Disease and Insect Samples
- Using the VCE Pest Management Guide (PMG): Home, Grounds and Animals
A tree disease is any disturbance that prevents the normal development of the tree and reduces its economic or aesthetic value. The disease triangle is a simple representation of the three fundamental components involved in the cause of disease in a tree.
Disease result from some disturbance in the normal life process of a tree and are caused by non-living (abiotic) agents and living (biotic) agents.
This chapter describes each of the three components above and how they impact trees. The environment contains non-living agents that impact the health and well-being of trees. These non-living, or abiotic, agents may be physical, chemical, or mechanical. Living, or biotic, agents include microbes and animal pests. For a biotic agent to cause disease in a plant, there must be a susceptible host and, generally, contributing environmental factors. Given a specific type of tree, that tree is susceptible only to certain biotic agents and the degree of susceptibility is determined by existing environmental stresses.
The tree steward can play an important role in mitigating tree problems by:
- Detecting the presence of environmental stresses and causal agents,
- Identifying biotic agents through research and use of the Virginia Tech Entomology and Pathology labs,
- Making recommendations for specific treatments and/or cultural changes to eliminate a problem,
- Educating clients on the needs and proper maintenance of trees in order to help prevent future problems,
- Providing the client with sources for science-based information,
- Giving guidance on finding professional assistance,
- Detecting and reporting trends or the existence of invasive pests or significantly destructive diseases in their city or county.
The information in this chapter is intended to provide an overview of the relevant topics in tree diseases. The VCE Master Gardener Handbook provides additional discussion of these and related topics in the chapters referenced below. The internet provides an abundance of information on the topics. Trainees are reminded to use .edu and respected institution websites, such as the Missouri Botanical Garden and Morton Arboretum, to the greatest extent.
- Understand the impact of environmental stress in causing disease in trees.
- Be familiar with the difference between abiotic and biotic factors.
- Identify the principle biotic factors that cause disease in trees.
- Recognize the difference between symptoms and signs.
- Know the approach for identifying diseases in a tree and determining an appropriate response.
REVIEW: VCE Master Gardener Handbook 2015 (9/18 update)
- Chapter 5: Basic Entomology
- Chapter 6 Plant Pathology
- Chapter 7 Abiotic Stress Effects on Plant Growth and Development
- Chapter 8 Diagnosing Plant Damage
- Chapter 9 Pesticide Use and Safety
Abiotic and Biotic Agents
Agents that cause problems in trees (and other living things) are grouped into two basic categories: abiotic and biotic.
Biotic agents are living, i.e. biological, agents that cause disease. They include microbes that generally cannot be seen with the naked eye, animal pests, and harmful plants. Microbes are living micro-organisms such as fungi, bacteria, and viruses. Animal pests include invertebrate pests, such as insects and mites, and vertebrate pests, such as deer and rodents. Problems caused by microbes and invertebrates are infectious and can be transmitted from one tree to another. Problems caused by vertebrates act more as mechanical agents. Pathogens are microbes and pests that cause tree disease. The term “disease” is frequently used to refer to a disease caused by microbes. However, in some circles the terms “disorder” and “disease” may be used interchangeably.
Symptoms vs. Signs
- Chlorosis – changes in color
- Leaf drop
- Necrosis – dead tissues, dieback, canker
- Epicormic shoots, suckers, water sprouts
- Galls – enlargement of plant partInsect entry/exit holes
- Feeding holes of birds
Visible structures produced by causal agents
- Mold growth
- Fungal bodies – conks and mushrooms
- Bacterial ooze
- Specific odors
- Powdery mildew
- Insect egg, larvae, and body
- Frass – insect poop
Symptoms and Signs
Diseases in plants manifest themselves via symptoms and signs. When diagnosing the problem with a tree, it is important to understand the difference and use appropriate terminology. Symptoms are changes in the plants appearance that indicate problems in the plant. Signs are visible structures produced by causal agents. Examples of each are provided in Table 1 “Symptoms vs. Signs” below.
A given type of tree thrives best under certain environmental conditions. Most types of trees are robust and can survive across a rather wide band of conditions. For example, red maples are indigenous throughout the eastern United States from Texas, throughout Florida, and north into Canada to the Hudson Bay. However, when a tree must endure conditions outside its comfort zone, it becomes stressed and problems occur. These problems may be directly related to the environmental conditions or to damage from biotic agents that are able to invade and damage the tree due to the environment stresses. The native environment for blue spruce is a cold climate at a high altitude. The tree has become popular as an ornamental and is planted in far warmer climates where it tends to suffer from various insect pests.
There are many conditions in the environment that can cause problems in trees when they exceed the tree’s comfort zone. These conditions are considered abiotic agents of diseases. Plant problems arising from these agents are considered physiological diseases or environmental diseases. Although environment problems may impact multiple plants in a given area, abiotic agents are considered non-infectious, that is, they cannot spread from one tree to the next.
The characteristics of the tree, and individual tree, determines its susceptibility to environmental stress. The response of the tree to environmental stress depends of the duration and severity of the stress, the number of exposures to the stress, and whether there is a single stress agent or multiple agents.
In general, symptoms of environmental stress will be widespread and exhibit a somewhat uniform damage pattern across the tree. Environmental stress is thought to account for a large percentage of all plant problems with some sources quoting as high as 70% to 90%.
Surrounding Green and Manmade Areas – The growth, shape, and health of a tree is impacted by whether a tree is growing within a group of trees, by itself in a field where its roots may spread without impediment, or as a street tree where the roots are confined within curbs and paved areas. The specifics of the growing site determine how much the tree must compete for light, nutrients, and water and the vulnerability of the tree to wind and storms. Manmade structures, such as pavement, brick, and glass, can absorb and reflect heat causing increased temperatures that may impact tree health.
Soil – The quality and quantity of the soil (material, texture, pH) impacts the oxygen available to tree roots, water retention, and nutrient absorption. Lack of adequate space limits root growth. Inadequate soil can lead to nutrient deficiency.
Weather – Temperatures and rain levels outside the acceptable range for a given tree or outside the temperature and water levels to which a tree is acclimated can damage the tree. Prolonged periods of highs or lows, unexpected frosts, unexpected high temperatures at times when the temperature is usually cool or cold periods when the temperature usually warms can all impact the health of tree. Either drought or excessive rain for prolonged periods are damaging.
Wind – Gale force winds can uproot a tree. Strong prevailing winds can shape the form of the tree as it grows.
Lightning – Tall trees in open areas are especially susceptible to lightening. When lightning strikes a tree, it causes the sap to boil, generating steam, and causing cells to explode. Strips of wood and bark may peel or be blown off the tree. Depending on how it is struck, the tree may not survive.
Other Severe Weather – Hurricanes, floods and droughts can have devastating effects on trees. Hurricanes can cause branches to break off and whole trees to blow down. Flooding can saturate the roots and lower trunks of trees with brackish water. Prolonged droughts of months and years can cause major declines and even death in trees.
Climate and climate changes – If the tree is outside its native range of conditions, it will be more susceptible to both abiotic and biotic agents. Many regions are seeing overall changes in temperature and/or rain. Warmer temperatures and increased days of warm temperatures, such as 80-degree days persisting into October in zone 7, can be stressful for some types of trees while permitting the growth of other trees which may not have been suitable for the area in the past.
Soil Compaction – Soil may be compacted by heavy equipment used in construction or by vehicle and human traffic, pressing the soil particles closer together and reducing the pore space between them. The smaller pore spaces reduce both water infiltration and drainage, and slows down the exchange of gases. Roots have a more difficult time penetrating compacted soil. Overall, the amount of water and oxygen available to the roots is reduced. This can cause dying leaves on mature trees and dying branches on younger trees.
Mechanical Injuries – Mowing and maintenance equipment, such as lawn mowers, weed trimmers, and other lawn and garden equipment, can cause significant damage to trees by cutting through or crushing the bark and breaking branches. Cuts into the bark can severely damage or kill sections of the cambium layer that lies just under the bark. This interrupts the flow of sap between the roots and leaves, causing some twigs or branches to die. Items such as ropes, chains, and straps can cause similar damage. When left affixed tightly to a tree, the tree may eventually grow around the agent. Mechanical damage opens the tree or shrub to abiotic organisms, which can damage the woody tissues underneath and lead to decay. Removing grass and weeds underneath the canopy of the tree and adding a 2 to 3-inch layer of mulch will keep mowers and weed trimmers at a respectful distance from the tree trunk. Remember to leave a few inches between the tree trunk and the beginning of the mulch.
Site Disturbance – Any significant change in the conditions around a tree may impact its health. Major disturbances, such as construction, digging, and re-grading the earth, close to the tree may compact the soil around the tree or raise the soil level around the trunk of the tree above the root flare. Digging a trench for utilities or planting other plants may sever the roots of the tree. Constructing a raised bed around the tree exposes the bark to soil moisture and microorganisms that can over time cause tissues in the lower trunk to die. Eventually this can cause root dieback and root collar diseases. Changes such as removal of nearby trees, building of new structures, paving of driveways and walkways change the microenvironment of a tree and may change the light, water, and nutrients available to a tree. Removing a neighboring tree may provide more light for a remaining tree; this may have a positive or a negative impact on the remaining tree depending on the type of tree and other factors impacting the tree.
Planting – Planting the tree at an appropriate place, at the appropriate depth, in an appropriately sized hole, and ensuring that the roots are loose and non-girdling is critical to the tree’s survival.
Pruning – Making incorrect cuts, such as leaving a stub when the branch is removed, or leaving broken branches unpruned provides an open door for biotic agents to infect the tree. Coating pruning cuts with paint or sealer slows the healing and promotes problems.
Maintenance – Inadequate watering, excessive mulch, the use of black plastic under mulch, and inappropriate fertilizing damage the health of the tree.
Air Pollution – Air pollutants are released from cars, generation of electricity from coal and oil, and emissions from other industries. In recent decades, the amount of air pollution has been reduced by governmental regulations and an increasing awareness from the public. A heavily traveled highway or a new industrial plant can add pollutants to the air. Air pollution may cause discoloration in the leaves. Some trees such as magnolia and arborvitae are relatively tolerant to air pollution. However, other trees such as catalpa and Virginia pine are relatively intolerant.
Lawn and Garden Chemicals – Use fertilizers, pesticides, and herbicides for trees only in the precise manner recommended on the label. When using a pesticide or herbicide to address a tree disease, use only products recommended in the VCE Pest Management Guide (PMG). Commercial fertilizer-herbicide mixtures for the lawn can be absorbed by tree roots so it is especially important to avoid applying too much or using them too frequently. Using herbicides, such as glyphosate, to manage weeds around trees can damage the trees, especially if it is a young tree. Remember that even properly applied herbicides applied to nearby areas can be carried by a breeze and injure a young tree.
De-icing Compounds – Compounds used to melt ice on walkways, driveways, and highways contain sodium chloride (table salt) and/or calcium chloride. These chemicals are toxic to trees in quantity and can injure trees when they are absorbed by their roots and leaves. Avoid or minimize the use of de-icing compounds around trees and be especially mindful of the runoff of melted snow and ice carrying these compounds. It is against the law to use fertilizers for de-icing as they contain salts that can damage plants in a de-icing situation.
Power Washing – Power washing of houses, walkways, and driveways is a common practice. Be aware that the bleach and other compounds used in power washing can damage small and medium trees. Bleach on the leaves can leave white spots and margins. The chemical compounds in the power washing liquid that runs off into a planting bed inhibit plant’s absorption of water. To protect plants, close to the house, spray the leaves with water from a hose just before and just after the power washing. This will dilute the mixture reaching the leaves. Also, following the power washing, provide a deep watering to the soil in the planting area. This will dilute the chemicals that have seeped into the soil from the washing.
All trees are not susceptible to all biotic agents. A given type of tree tends to be susceptible to only certain specific agents. Generally, the tree must be under environmental stress for even those specific biotic agents to cause diseases. This section considers microbes (living micro-organisms), animals (vertebrate and invertebrate), and certain plants that may be harmful to trees.
Fungi and Fungal-like Organisms (FLOs) – Threadlike organisms without chlorophyll. They lack the ability to produce their own food and live off living tissue or dead or decaying organic matter. Fungi are the most common biotic cause of plant disease.44p126 There are thousands of different species that cause disease in plants. In addition to entering plants through wounds or natural openings, such as stomata, fungi and FLOs can directly penetrate plant tissue using specialized filamentous structures. Fungus structures, such as mold, mildew, mushrooms, and conks, may be easily visible to the eye. However, other fungus structures, such as those that cause leaf spots, are microscopically small. In addition to leaf spots, fungi may cause blights, rots, cankers, wilts, galls, mildew diseases and rust diseases.
FLOs – Include Pythium, Phytophthora, and organisms that cause downy mildew. FLOs used to be considered fungi. While they share many common characteristics with fungi, FLOs are now understood to belong to a different taxonomic class. Pythium is considered a water mold and tends to develop when growing conditions are too wet. Species of Pythium cause crown and root rot in a variety of plants and blight in turf grass. Phytophthora infestans caused the potato blight in Ireland and northern Europe in the mid-1800s. Downy mildew is not the same as powdery mildew. While powdery mildew is caused by fungi, downy mildew is caused by a group of FLOs. The symptoms of downy mildew vary depending on the host and the specific FLO.
Bacteria – Microscopic one-celled organisms, smaller and simpler that fungi. Bacteria cells have no defined nucleus. Commonly, bacteria are visible only with a microscope. Several hundred bacteria can cause plant disease. Bacteria require a wound or a natural opening, such as stomata, to enter a plant. Bacteria do not grow inside the plant’s cells, but in the spaces between cells. Some produce toxins that cause host plant cells to die. Others produce enzymes that break down parts of the cells and cell walls of the host plant. Still others grow in the area of the xylem causing the host plant to wilt and die. Bacteria can cause symptoms such as galls, wilts, leaf spots, blights, soft rots, and cankers.
Viruses – Particles that live inside cells and infect other living organisms. Viruses are visible only with an electron microscope. They are parasites that require a living host to grow and multiply. By disrupting the normal processes of the cell, viruses take over plant metabolism and use the plant cell to produce more viruses. There are several hundred different viruses that can cause plant disease. They are generally named based on the disease they cause, such as Tobacco Mosaic Virus. Once infected by virus, little can be done to cure the diseased plant but cultural practices are important to avoid spreading the viral infection to other plants.
Of all the microbes viral infections can be the most difficult to diagnose. They produce no readily observable signs and symptoms can be subtle, often looking very similar to nutrient deficiencies or herbicide injury. Common symptoms of viral infection are poor growth, mottling (alternating darker and lighter patterns in the leaves, to include light and dark green or yellow patches or streaks), ring spots and wavy line patterns, leaf crinkling, and distortion.
Nematodes – Simple, multi-cell animals, worm-like in appearance and mostly microscopic. Nematodes infect people, animals, and plants, but those that damage plants do not attack people and animals. Plant pathogen nematodes feed on plants by puncturing the cell wall and sucking out the cell contents. This causes the plant to decline and even die. Roots of infested plants may have knots or galls, root lesions, excessive root branching, injured root tips, and stunted root systems. The damaged root system causes wilting, yellow leaves, and reduced size and number of leaves. The puncture opening enables fungi and bacteria to invade the plant. As with other biotic agents, the types of plants attacked by the nematode depends on the type of nematode.
This section considers insects, mites, deer, birds, rodents and harmful plants.
Insects – Adult insects have 6 legs and 3 body parts. Most adults have wings. While most insects are beneficial, some can damage trees. Insects can be categorized as chewing, sucking, and boring. Each group produces a characteristic pattern of damage.
Chewing – Chewing insects eat plant tissue and defoliate plants. Some eat the entire leaf. Others eat the tissue between the veins of the leaf, known as skeletonizing. Still others, known as leaf miners, eat the tissue between the tops and underside of the leaf, hollowing out the leaf. The following are chewing insects.
- Beetles – either the larvae or adult stage, depending on the species. Examples include Japanese beetle, viburnum leaf beetle, and willow leaf beetle.
- Caterpillars – worm-like larval stages of moths and butterflies. They may eat the entire leaf or irregular areas. Examples include Lymantria dispar, eastern tent caterpillar, webworms, leaf rollers, bagworms, and oak blotch leaf miner.
- Sawflies – wasp-like insets whose larvae can resemble naked caterpillars or may be slug-like. The name comes from the saw-like appendage that the female adult uses to lay eggs. Sawfly caterpillars generally eat the entire leaf but the slug-like forms eat the tissue between the veins (skeletonizing).
Sucking insects – Sucking insects have piercing mouthparts that penetrate leaves, twigs, branches, flowers, or fruit and feed on a plant’s juices by sucking out the sap. Symptoms include fading or mottled leaf color, curling and twisted leaves, wilting foliage, and malformed flowers. Honeydew is a sticky secretion produced by some of the sucking insects. Honeydew creates a growing environment for sooty mold, a common name for several species of fungi, and the presence of sooty mold indicates sucking insect activity. Examples of sucking insects are discussed below.
- Aphids – small, soft-bodied insects that usually cluster on stems or undersides of terminal leaves. Aphids are also known as plant lice. Aphids cluster where they can feed on succulent growth, such as the underside of young leaves and on developing stems. While aphids may be yellow, green, black, or red, the color may be hidden by a waxy white coating. Aphids secrete honeydew as a waste material. The ants in the picture below are ‘herding’ the aphids for their sweet output.
- Scales – During most of their lives, scale insects are legless and motionless and do not resemble insects at all. They may be circular, oval, or pear shaped, flat or convex. Scale can be either armored (hard) or soft based on whether the scale produces a waxy shell that gives the soft-bodied insect under it some protection. After the eggs hatch, the immature form of the insect is known as a crawler because they are able to move about the plant to find a feeding site. Once they select their site, they insert their beak to feed and then molt, losing their legs, eyes, and antennae. The insect then never moves again. The unarmored scale produce honeydew. The scale shown below is crape myrtle bark scale, a recent alien arrival which is making its way around Virginia. Interestingly, the larger somewhat fluffy creature is the larva of a small lady beetle which sucks out the scales. For additional information on scale see “VCE Pub ENTO-106NP, Scale Insects.”
- Mealybugs – Mealybugs have soft and mushy bodies covered with a white, waxy coating. Unlike scale, mealybugs can move throughout their lives, although that movement is slow. They produce honeydew. The ladybug above looks very similar, but they have different diets and are found in very different places: carnivorous/tree bark with scale, above vice vegetarian/soft green plant tissue especially houseplants and plant nurseries.
- Whiteflies – Whiteflies resemble tiny white moths. The immature stage resembles scale. Both the adults and immature stages suck sap from the leaves of host plants and produce honeydew. They are a common nuisance in greenhouses.
- Thrips – Thrips are minute, slender insects with narrow fringed wings held flat on the back and asymmetric sucking mouthparts. Adults may be yellow, brown, or black. Immature thrips are lighter in color and do not have wings. Thrips most frequently damage flower buds, causing them to turn brown and die. They are known for frequently spreading viruses. (Thrips is both singular and plural.)
- Leafhoppers – Adults are 1/8 to 1/4 inch long with wedged shaped bodies, pointed heads, and wings that are held over their backs like a roof. The nymphs are similar to the adults but smaller with short wings. They are active and either move sideways or jump when disturbed. Most feed on the upper surface of terminal leaves, leaving the leaves with coarse, white stippling. Most species feed on only one or several closely related species of plants. Some leafhopper species produce curled and stunted terminal leaves. Other species may transmit bacterial capable of producing scorch-like symptoms. A few species of leafhoppers secret honeydew.
Boring insects – Insect borers tunnel and feed beneath the bark. They are considered one of the most destructive pests of ornamental trees. Most are larvae of certain moths and beetles. Stressed trees may give off certain chemicals that evaporate into the air and attract adult insects looking for suitable places to lay their eggs. Females lay eggs in crevices or around wounds in the tree bark. When the eggs hatch, the larvae tunnel beneath the bark to feed and grow. The larvae eat inner bark, phloem (transporting nutrients), and xylem (transporting water). This destruction disrupts the flow of nutrients and water between the roots and the canopy resulting in branch dieback, structural weakness, decline and even death of the tree. The borer group of insects include the emerald ash borer, dogwood borer, and lilac borer.
Mites – Mites are not insects but are more closely related to spiders. They are very small, about 2mm, about the size of a period, and have 8 legs and a one-part body with no distinct heads. Mites suck out the chlorophyll in leaves leaving tiny white spots. Because of their small size, they may be present and not detected on a tree with no obvious damage until the infestation is heavy. At that point, the leaves turn yellow and brown and eventually die. Mites usually attack lower areas of a tree first and then move up the tree. A traditional way of checking for mites is to put a white sheet of paper under the branch, tap the branch, and look for moving black spots on the paper.
Various animals, such as deer, birds and rodents can damage trees. While they are living agents, they do not infect trees as do biotic agents but act more like mechanical agents.
Deer – There are certain types of trees that deer particularly enjoy eating. This damages the appearance of the tree and may decrease the ability of the tree to feed itself. To protect the tree, spray the leaves with deer repellent or build a fence around the tree. Bucks rub their antlers against young tree trunks to scrap velvet. Usually the deer select trees around 4 inches in diameter. This can cause significant damage to the tree’s vascular system. When the bark has been severed all around the circumference, the tree cannot survive. If less than 50% of the circumference is affected, the tree can usually recover. Deer also may rub trees during mating season to attract females or mark territory. Deer tend to return to the same trees. To protect the tree, apply a wire or plastic sleeve around the tree trunk or build a tall (4 feet) fence around the tree and close to the tree trunk. Deer are also vectors for the introduction of pathogens to the tree.
Birds – Many woodpeckers feed on insects living in the decaying cavities of trees and other areas of trees where there is insect activity. The birds are looking for wood-boring beetles, carpenter ants, and other insects. These woodpeckers tend to prefer dead wood and the holes they leave tend to randomly spaced. Some woodpeckers are sapsuckers, meaning that they prefer tree sap as their primary food but will eat insects, seeds, and nuts as secondary food sources. The most common and most destructive sapsucker in North America is the American yellow-bellied sapsucker, a migratory bird that is seen throughout eastern North America. Sapsuckers tend to choose a favorite tree and return to that tree frequently. Unlike the insect-eating woodpeckers who tend to find their insects in dead wood, the sapsuckers are pecking holes and eating sap from live wood, making closely arranged rows of holes in the bark. Although some trees appear to seal the sapsucker wounds without harm (Atlas cedar, for example), the United States Forest Service suggests that the activities of the yellow-bellied sapsucker can degrade the wood of and even kill many varieties of trees. While a small tree can be killed in a single season, two or more years of feeding would be required to kill trees with diameters greater than 8 inches.
Rodents – Mice, voles, squirrels, and rabbits can damage young trees by eating the soft bark around the trunk base. Voles eat roots. In addition to the bark, rabbits may also eat tender buds. This occurs most frequently in the winter. Their eating creates small wounds in the tree that allow fungi and other micro-organisms to attack the tree. If an animal eats the bark entirely around the circumference of the young tree, the tree will die. The tree can be protected from this damage by wrapping a ¼ inch wire mess or other tree guard material around the bottom 24 inches of the tree, sinking the mesh two inches below the ground.
Some plants cannot make their own food and parasitize other plants to obtain nutrients and water. Examples include mistletoe and dwarf mistletoe, which have chlorophyll and can photosynthesize so they are not completely dependent on the host tree. However, they take nutrients and water from the host. Mistletoe usually select hardwoods as host trees in the Eastern US. Mistletoe is not a fast grower, and most healthy trees can tolerate some mistletoe plants. Heavy mistletoe infestations can make the tree less vigorous: if the tree has additional abiotic or biotic stressors, the tree could die.
Dwarf mistletoe is practically leafless and occurs only on conifers. Dwarf mistletoe is mostly seen in the western United States and is considered more damaging that true mistletoe. It can cause witches brooms and swelling of infected portions of the tree branch. Forms of damage include loss of growth, defects in the host, and death of the host.
Vines can damage trees in multiple ways. They compete with the tree for light, water, nutrients, and space. They may girdle the tree and constrict the trunk and branches.
Probably the most problematic vines with regard to trees are evergreen vines and fast-growing vines. Some of the most damaging vines are ivy, Japanese honeysuckle, wisteria, and kudzu. Deciduous vines and vines that grow more slowly, such as clematis and passion flower, are less likely to damage a tree. Although highly undesirable, poison ivy is not considered an aggressive grower and is not considered a significant tree killer. However, poison ivy can damage a tree by eventually choking the tree or branches and adding extra weight to the crown.The appropriate way to remove vines is not to pull them off the trees but instead to cut the vine as it surfaces from the ground at the base of the tree. The roots of the vine should then be pulled out of the ground around the tree. The vine on the tree should be left alone to die and fall off.
Common Tree Diseases in Virginia
This section provides an overview of some of the more problematic tree diseases in Virginia. The purpose of the section is to provide the tree steward an awareness of these common diseases. When VCE publications are available with more in-depth information, they are referenced at the end of the discussion of the given disease. Specifics for chemical treatments are found in the VCE Pest Management Guide (PMG), discussed in Section 10.
Diseases Caused by Pathogenic Microbes
Anthracnose – Anthracnose is a general term for diseases caused by a several closely related fungi. Each species of the fungus affects only certain specific tree species and symptoms vary depending on the tree species. The following paragraphs addresses anthracnose in flowering dogwoods and sycamore trees as these are especially problematic in Virginia. Anthracnose also commonly occurs on white oaks, elms, and maples. Other species of the fungus infect linden, tulip tree, hickory, birch, and walnut, although this last group is usually only slightly affected.
Anthracnose in Flowering Dogwood – Two types of anthracnose affect flowering dogwoods.
- Spot anthracnose is caused by the fungus Elsinoe corni. Symptoms appear in early spring on the dogwood bracts as small circular lesions with purple borders and light centers. Lesions appear later on the leaves. Later in the season, the centers may fall out giving a shot hole appearance. Cool, wet springs tend to produce more severe symptoms. Usually the disease causes little damage. Since the fungus spores overwinter in fallen leaves, the most practical control measure is to remove debris from the base of the tree. If the disease was severe the previous year or a cool, wet spring is expected, spraying with fungicides may be useful.
- Discula Anthracnose is caused by the fungus Discula destructiva. In addition to lesions on the bracts and leaves, discula anthracnose causes dieback in the branches with lower branches dying first. Lesions on the bracts and leaves are irregularly shaped with purple margins and vary in size unlike the uniform, small lesions cause by spot anthracnose. Infected leaves may remain on the tree during winter. The infection can spread to the trunk, causing cankers, and then the death of the tree. The fungus overwinters in leaves and stem cankers. Spores are spread by cool, wet weather in the spring. The disease is most common and severe at high altitudes and is less common in the eastern part of the state. Discula anthracnose is difficult to control and requires both cultural and chemical methods.
For additional information, see VCE Pub 450-611, “Foliar Diseases of Dogwood,” by Mary Ann Hansen, Department of Plant Pathology, Virginia Tech.
Anthracnose in Sycamores is caused by the Apiognomonia veneta fungus. Like anthracnose in dogwoods, sycamore anthracnose usually occurs in cool, wet weather in spring. However, the symptoms are different. In sycamores dark, angular areas occur along leaf veins and expand to cover the entire leaf. The disease can kill more than 90% of new shoot growth, cause the dieback of twigs, the development of cankers, and even death in larger branches. Repeated infections can result in abnormal branching to include the growth of witches’ brooms. Although the symptoms are severe, the sycamore is usually able to develop a second set of leaves by midsummer and the disease is rarely fatal.
See VCE Pub 450-604, “Anthracnose – Fungal Disease of Shade Trees.”
Phytophthora Root Rot
Phytophthora Root Rot is caused by various species of Phytophthora fungi. It affects conifers to include Leyland cypress, arborvitae, and pine. It can also affect dogwoods and various ornamental shrubs such as camellia, boxwood and azaleas. The fungi first infect the fine absorbing roots and may move to larger roots and the root collar. Roots deteriorate, become brown, and severely degrade the ability of the root system to support the tree. Trees wilt, needles turn yellow and fall from the tree. Eventually this leads to the death of the tree.
Phytophthora thrives best in heavy, warm, moist soils and areas where water can collect around plant roots. All species of the fungus are able to live indefinitely in the soil. Once symptoms appear, chemicals are often ineffective in controlling the disease. Do not install susceptible plants in areas that tend to stay moist.
Fire blight is caused by Erwinia amylovora bacterium. The bacteria can attack about 75 species of the rose family such as pear, ornamental pear, apple, and crabapple trees. It also occurs on pyracantha, spirea, hawthorn, and mountain ash. The bacteria enter the plant though blossoms, new shoots, and wounds. Infected areas produce bacterial ooze that can lead to further infections. First flowers and then leaves wilt and turn brown but stay attached to the tree. Infected areas have a shriveled appearance as if scorched by fire. Twigs and then branches die back. A young tree may die in a single season but more mature trees can endure several years of infection before they die. The bacteria overwinter in cankers (dead wood representing a wound on the tree) and dead, dried fruit.
To control the disease, prune out diseased wood and cankers 10 to 15 inches below the infection. Pruning is best done in late summer or winter. The pruned wood should be destroyed and tools disinfected. Diseased plants should be replaced with resistant varieties. See the PMG for chemical treatments.
Bacterial Leaf Scorch
Bacterial leaf scorch is caused by Xylella fastidiosa bacterium. The bacterial is introduced into the tree’s xylem by sucking insects such as sharpshooter leafhoppers and spittlebugs. Once the bacterial has enter the system, it impacts water movement in the tree. Symptoms are similar to those caused by drought or root disease. The leaves show yellow and brown areas, generally with a characteristic yellow band between the green and brown areas. Trees impacted by drought or root disease do not have this yellow band.
Other symptoms vary depending on the host species and include yellowing and browning of the leaves and premature leaf drop. Symptoms usually appear in the late summer and fall. Infected trees may leaf out later than normal in the spring and leaves may be stunted. While the disease continues from year to year, the severity can vary. Over time the tree gradually declines and eventually dies.
In Virginia, the disease most often infects oaks, elms, and sycamores, but several other species are susceptible to include maple, hackberry, mulberry, sweet gum, and gingko.
There is no cure for the disease, but maintaining the health of the tree can help the trees survive longer. If the tree is removed, it should be replaced with a species that is not susceptible to the bacteria.
For additional information see VCE Pub 3001-1433 “Bacterial Leaf Scorch of Landscape Trees.”
In addition to the diseases above, other frequently encountered conditions caused by microbes include botryospharia canker, conifer needle cast, powdery mildew, rusts, and verticillium wilt. These diseases are caused by various fungi.
Significant Insect Pests of Virginia
Emerald Ash Borer
Emerald ash borer (EAB) attacks all species of ash trees that grow in Virginia. EAB was first reported in Fairfax County in 2008 and, as of 2016, has been identified in 46 counties, about half the counties in the state. There are approximately 187 million ash trees in Virginia.
The adult borer is about ½ inch long and bright, metallic green. EAB generally mate in early June and lay their eggs in the crevices of the bark on the ash tree. When the eggs hatch, the larvae create S-shaped tunnels just behind the bark. Woodpeckers often feed on the larvae, leaving jagged holes in the bark. The mature borers emerge through D-shaped holes.
Observable symptoms of tree decline do not occur until 2-5 years after the tree has been infested. The tree starts to thin out and the crown declines as branches in the top of the tree die. Epicormic branches sprout from the trunk, often occurring in areas where living and dead sections of the tree meet. When dead bark is removed, S-shaped tunnels are observable. The tree rarely recovers after infestation is identified. There is no treatment. Infested trees should be removed and destroyed to prevent further spread of the EAB. Because the borer can be transmitted in firewood from the ash tree, it is important to never move firewood from one geographic location to another. For additional information see VCE Pub 2904-1290, “Emerald Ash Borer” by Eric R. Day and Scott Salom, Department of Entomology, Virginia Tech, 2016.
Lymantria dispar is the most important tree defoliating insect in the eastern United States (Ref MBG). Adult moths do not feed and live for only a few days. Adult females are white and in July lay eggs in tan colored masses on tree trunks and the underside of branches. Egg masses may be found on rocks at the bottom of a tree or even on recreational vehicles. Caterpillars merge from the egg masses the following April. The caterpillars may be up to two inches long and are hairy with blue and red dots along their backs and a beige head.
Caterpillars rest in the leaf litter at the base of the tree during the day. At dusk, they crawl up the trunk and feed on the leaves, returning to the base of the base of the tree at dawn. Trees that lose a lot of leaves will produce a second set. This second set will tend to be smaller and lighter in color that the initial leaves. Healthy trees can withstand this defoliation once or twice, but unhealthy trees may die after only one defoliation. Death of a weakened tree is not directly caused by the defoliation, but by invasion of secondary organisms.
Lymantria dispar caterpillars favor most species of oak but also feed on aspen, willow, birch, apple, and basswood. When the caterpillar populations are high, older caterpillars may feed on maple, hickory, elm, and many conifers. There are some tree species, such as the tulip tree (yellow poplar), that are never eaten by the caterpillars.
Outbreaks are cyclical with multiple year periods of low activity followed by several years of high activity. Much of the cyclical nature of the outbreaks is dependent on the Entomaphaga maimaiga fungus. Wet weather in spring tends to increase the presence of the fungus which attacks the Lymantria dispar larvae, leading to a reduction in Lymantria dispar activity. Chemical treatment against the moths is usually aerial application of insecticides when the caterpillars are young. Lymantria dispar caterpillars are frequently mistaken for eastern tent caterpillar or the fall webworm. Lymantria dispar never build tents or webs. See VCE Pub 444-750. 
Eastern Tent Caterpillar
The eastern tent caterpillar, Malacosoma americanum, is recognized by the silky tents that the larvae spin in branch crotches. Adult moths are dark tan and lay their eggs in a 1 inch long, black collar around a twig. The eggs overwinter and hatch in March or April. Caterpillars are brown and hairy with a white stipe down the back, yellow brown borders, and row of blue spots on each side.
The larvae gather in the crotches of branches to spin their web nests. They leave the web to feed during the day and return to the web at night. In 4 to 6 weeks, caterpillars can reach 2 to 2 ½ inches in length. When it is time for the caterpillars to pupate, they leave the web nest and can be seen crawling on other plants, walkways and whatever is around. While these wandering caterpillars are a nuisance, they are not feeding at this time and do not damage the plants on which they may be found.
Eastern tent caterpillars favor black cherry, chokecherry, and apple trees but they also feed on hawthorn, pear, plum, and other flowering fruit trees. If the outbreak is especially severe, the entire tree may be defoliated. Since defoliation generally occurs early in the season, wild cherry trees can generally replace their leaves with new ones. If trees do not have enough time to grow new leaves, they may die.
To control infestations, cut off twigs with egg masses in fall and winter. In smaller trees, web masses can be removed and destroyed by hand, working at night after the caterpillars have returned to the nest. The caterpillars may be killed with insecticide spray in the spring. Since the webs protect the caterpillars from the insecticide, spraying must occur during the day when the caterpillars are away from the web. See VCE Pub 444-274, “Eastern Tent Caterpillar.”
Fall webworms, Hyphantria cunea, form webs over the end of branches. Webs are created by newly emerged caterpillars. As the caterpillars grow, they expand the webs to cover more leaves. Caterpillars feed inside the webs. While the webs are a nuisance, the caterpillars do not significantly damage the tree because defoliation occurs later in the summer and fall rather than during a period of active growth.
The adult moth is white, usually with dark spots on the wings. It lays its eggs on the underside of leaves from May to July. Caterpillars emerge within two weeks and immediately begin creating webs and feeding on leaves. This feeding stage lasts from 4 to 8 weeks. Caterpillars are covered with silky hairs. Their bodies are pale yellow to green with black strips on their back and yellow stripes on their sides. When it is time to spin their cocoons, caterpillars crawl to a protected space such as ground litter or just below the surface of the soil. Adult moths emerge in late spring or early summer. There are two generations per year.
Fall webworms feed on over 100 species of deciduous trees. Nests can be cut out on smaller trees. Insecticides may be used and are most effective when the webs first appear. See VCE Pub 2808-1013, “Fall Webworms.”
Hemlock Wooly Adelgid
Hemlock Wooly Adelgid, Adelges tsugae, is a non-native aphid-like insect that arrived in the United States in the 1920s. The insect has infected hemlocks throughout the Appalachian region. In Virginia it feeds on Eastern Hemlock and Carolina Hemlock. As many as 80 percent of the hemlocks in the Blue Ridge Parkway and Shenandoah National Park have died due to this infestation.
The insect has multiple forms and life stages. During the crawler stage, nymphs attach themselves to the hemlock bark, needles, and twigs. They produce a white waxy substance that provides a protective coating. These cottony masses remain after the insect has gone and give the insect the “wooly” in its name. The insect feeds on the sap at the base of the hemlock needles, depriving the tree of nutrition. Trees usually die within 3 to 5 years but may die in a single year if the infestation is severe.
Systemic insecticides and dormant oil may be used to control the insect. Several beetle species have been imported and released as biological control agents. See VCE Pub 3006-1451, “Hemlock Wooly Adelgid.”
Diagnosing Tree Diseases
A given type of tree needs certain environmental conditions to prosper (amount of moisture, type of soil, amount of light, temperature, etc.). Outside those conditions, the tree will be stressed and that stress will eventually be reflected in changes in the tree’s condition. The tree is susceptible only to certain pathogens microbes and pests). Generally, a necessary condition for pathogens to cause significant problems in a tree, the tree is already experiencing environmental stress.
This section emphasizes certain essential aspects of diagnosing tree problems. Section 8 of this chapter presents “20 Questions on Plant Diagnosis” a factsheet from the Ohio State University Extension describing a detailed step-by-step process for diagnosing a plant problem. Section 9 defines the process for taking plant samples for VT lab. Section 10 discusses the structure, use, and importance of the VCE Pest Management Guide (PMG) for Home Grounds and Animals.
First, identify the type of tree. If the species is not known, then determine the genus or family. Two examples follow:
- The tree is a conifer with scale-like leaves. Then it must belong to the Cupressaceae (Cypress) Family.
- The tree is a deciduous broadleaf and produces acorns. Then it must belong to the Oak Genus in the Fagaceae (Beech or Oak) Family.
Begin the diagnostic process with a basic familiarity of the desired environment for that tree type. Books such as Common Native Trees of Virginia and Trees of Eastern North America are excellent resources. The internet contains a wealth of information on trees. As much as possible, stay with educational sites such as the those of state extensions; recognized institutions, such as the Missouri Botanical Garden or Morton Arboretum; or government sites such as those by the federal or state departments of agriculture or forestry service.
Determine the micro-organisms and pests to which the tree is most susceptible. The first reference for this should be the PMG. Books such as the Ortho and Southern Living problem solving books discuss plant diseases by plant type. Prior to the internet, these were critical resources and they can still be useful. However, the internet contains an abundance of information that is easily accessed. Just search for “Problems of Tree Type X”. Again, stay with the recognized authoritative sites as described above. Even if the tree’s potential problems are contained in PMG, also look on the internet for pictures and additional information on recognized causes of disease.
Knowing the tree type, the environment it needs to flourish, and the pathogens (micro-organisms and pests) to which that tree type is most susceptible, provide the basis for examining the diseased tree. The tree steward should both observe and discuss with the homeowner the environmental conditions that the plant has been experiencing. In these observations and discussions, the tree steward is looking for stresses on the tree. The Tree Steward examines the tree to look for symptoms and signs and talk with the homeowner about the development of the disease pattern (time, severity, parts of plant/s affected).
The identification of environmental stresses may alone account for the observed symptoms. However, the tree steward should also compare the observed symptoms and signs to the symptoms and signs of the diseases to which the tree is known to be susceptible to determine if there is correlation. In many cases, relating symptoms and signs to an assessment of environmental conditions and potential pathogens will lead to a diagnosis. If the tree steward is not confident with a diagnosis at this point, then he/she should take a sample of affected parts of the tree (leaves, twigs, bark and roots) and provide that example to the VT Diagnostic Lab. The procedure for taking that sample and submitting it to the VT lab is provided in a later section.
Once a diagnosis has been determined, the tree steward should provide a recommendation for addressing the problem. For environmental stress issues, example recommendations could be:
- Water the tree deeply at regular intervals when there is little rain.
- If the present location is too moist (or gets too much light or too little light), move the tree to a more suitable location, if small enough, or replace it.
If the cause of the problem is a micro-organism or pest, discuss remedial actions such as cultural practices and application of a fungicide or insecticide. The PMG must be the source for any recommendation involving the application of chemicals.
20 Questions on Plant Diagnosis
This section reprints, with permission, Ohio State University Extension Factsheet “20 Questions on Plant Diagnosis,” PLPSTH-GEN-3, which is an excellent step-by-step guide and very much worth reading for its logic, insights and illustrations. However, please note that Sections 9 and 10 of this chapter provide Virginia-specific information that should take precedence over the OSU Factsheet Questions 16: How do I take samples? and 20: What are my recommendations? In all of these instructions, moreover, it should be remembered that tree stewards are not plant pathologists. Their role is to know how to look at a tree and its surroundings for critical information; what kind of questions to ask; and how to research or refer the problem for more expert help. It can be overwhelming to think that you are trying to identify one among a seeming million pests. But you are can be most effective in eliminating possible diseases and concentrating on finding the really useful information. If there is a locally important problem, especially if it concerns a specific type of tree, then it may be worthwhile to bone up on the details: for example, the Emerald Ash Borer, if it has not arrived in your locality. (If it has, you probably already know!)
Guidelines for Taking Plant Disease and Insect Samples
Megan Tierney, ANR Horticulture Agent, VCE York/Poquoson authored the following guidelines for submitting samples of diseased plants and insect problems through the York Poquoson Extension Office. This procedure should be modified for the local TS Master Gardener extension office.
“A diagnosis is only as good as the sample and information you provide”
Lesson from the York Poquoson VCE office:
Samples are only accepted Monday-Wednesday at the York Poquoson VCE office
so samples don’t sit at the post office and expire over the weekend,
which leads to no results.
Include the following with the sample:
1. Plant parts must be living diseased parts (all dead = no results), sections of branches, no single leaves, etc., contained in a sealed bag.
2. A zip top sandwich bag full of soil from the root system of the plant including living root matter (lab cultures for fungus/bacteria that can be the cause for disease and does basic pH test). Plants and soil should be separated or the soil saran wrapped.
3. Completed homeowner questionnaire forms *One per plant, not customer.
4. Include printouts or email pictures that show the overall health of the plant or close-ups of issues in color. Email the pictures to the address specified by the local office.
5. For insect identification, no live insects will be accepted. They must be drowned in rubbing alcohol or sealed in a plastic bag until they die (at least a week) before they can be accepted.
6. For insects, please indicate as much information as possible about the habitat (where they were found: example, kitchen).
7. For weed identification the lab needs as complete a plant as possible, i.e., roots, flowers, fruits, and seeds.
8. Make sure plant owner name is on all bags/containers so that they stay together.
9. Place together for further action by the extension office.
Good Sample: Includes soil sample with roots present, several leaves and pictures– branches would make it better
For more detailed information on submitting good samples to the Plant Disease Clinic, view this webinar from Mary Ann Hansen and Elizabeth Bush of Virginia Tech’s Plant Disease Clinic: https://youtu.be/rCEE8NNntMg
Using the VCE Pest Management Guide (PMG): Home, Grounds and Animals
The PMG provides over 300 pages of information on managing problems of vegetables, fruits, ornamentals, lawn, and even discusses parasites on pets and other vertebrates as pests. Because of its length and the many topics covered, it can be challenging to use the document in digital form. The intent of this section is to make the TS aware of the relevant portions of the PMG for diagnosing tree problems and provide a general approach for using the document. Page numbers presented here are for the 2017 edition of the PMG.
Chapter 4, p.4-11 through p. 4-73 of the PMG addresses problems with trees. This includes the following sections:
- Diseases of Landscape Trees, p. 4-11
- Table 4.2 – Fungicide Use
- Insects of Trees, Shrubs, Annuals, and Perennials, p. 4-27
- Index to Insects and Mites by Host
- Table 4.3 – Timing for Borer Treatment
- Table 4.4 – Timing for Scale Insect Treatment
- Table 4.5 – Control Measures for Major Pests and Pest Groups
- Table 4.6 – List of Common Insecticide Mixtures
- Table 4.7 – Directions of Pesticide Usage
- Organic Controls for Insects of Home Ornamentals, p.4-73
- Table 4.8 – Organic Control Use
As discussed in Section 7, the identification of a tree problem begins with knowing the type of tree. The PMG provides information on the fungi and insects that most commonly cause problems on a given tree type in Virginia. The TS should consult Table 4-2 – Fungicide Use, on p.4-13 through p. 4-26, to determine possible fungal infections for the given tree. For example, if the tree is an ash, expected fungal infections are anthracnose and rust, p. 4-13. The TS should then consult the “Index to Insects and Mites by Host”, p. 4-30 through p.4-32 to determine possible insect or mite infections. In our example, ash trees are susceptible to aphids, Flower gall mites, borers, defoliators, Emerald Ash borer, lacebug, leafminer, leaf roller, rhinoceros beetle, sawfly, scale insects, and spider mites, p. 4-30.
Knowing the fungi and insects to which the tree is susceptible provides a baseline of possibilities to consider when the TS examines the diseased tree. That baseline, of course, has to also include the possibility of abiotic agents as well as biotic agents other than fungi. The TS should consult other references for the other relevant biotic agents for the given tree. The TS then assesses the symptoms, signs, and other relevant aspects of the tree, as described in Section 8, to determine which agent(s) are the most likely cause of the tree’s problems. If the tree’s symptoms align with a specific fungus, the TS then returns to Table 4-2 to determine the appropriate treatment. Table 4-2 list fungicides applicable to the specific fungi, and recommends when to apply the fungicide, cultural controls, and precautions /remarks. If the ash tree has anthracnose, then chlorothalonil and mancoseb are two of the recommended fungicides. They should be applied at bud break or at the first sign of disease. (This probably indicates that treatment will occur the following spring.) Fallen leaves should be collected and either burned or buried to reduce overwintering of the fungal inoculum.
If the tree’s symptoms align with a certain insect or mite, the TS consults Table 4.5 for control measures. If the ash tree is determined to have leafminers, then imidacioprid is the approved control measures, p.4-50. The treatment should occur in mid- to late June after eggs have hatched. When using imidacloprid, see “Bee Advisory Box,” The PMG also provides the URL for a related factsheet on leafminers.
Remember, any recommendation of chemicals (fungicides or pesticides) must be consistent with the controls listed in the PMG.
- What agents cause disease in trees?
- Which damages the most trees, environmental stress or pathogens?
- Which micro-organism is responsible for damaging the most trees?
- Insects can be categorized into three groups by the way they inflict damage on trees. What are those three categories?
- What bird is most likely to damage a tree and why?
- What is the first thing to know when diagnosing a tree problem?
- What is the authoritative source for chemical treatments of tree problems?
The following are additional sources that provided general information used in the development of this chapter:
- Appleton, B. (2015). 24 Ways to Kill a Tree. Virginia Cooperative Extension.
- Appleton, B, French, S. Tree and Shrub Planting Guidelines. Virginia Cooperative Extension.
- GardenNotes #331. (2011). Colorado State University. http://www.ext.colostate.edu/mg/gardennotes/331.pdf
- Heizer, J. (2017). Pests and Diseases of Hampton Roads. Bartlett Tree Co.
- Isleib, J. (2012). Signs and symptoms of plant disease: Is it fungal, viral, or bacterial? Michigan State University Extension. http://msue.anr.msu.edu/news signs_and_symptoms_of_plant_disease_is_it_fungal_viral_or_bacterial
- Insect and Disease Problems. (2011). International Society of Arborists. www.treesaregood.org
- Pests and Problems by Pest. Missouri Botanical Garden. http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-thehome-gardener/advice-tips-resources/pests-and-problems/pests-and-problems-bypest.Aspx
- Tree and Plant Advice: Help with diseases & Help with pests. Morton Arboretum. http://www.mortonarb.org/trees-plants/tree-and-plant-advice
- Nortman, D. (2015). Tree Pathology. Virginia Cooperative Extension, York County/City of Poquoson. Presentation to Peninsula Tree Class of 2015.
- Ornamental Plant Insect Pests, Insect Pests of the Home Landscape. PlantsGalore.com.
- Raid, R. (2011) Plant Pathology Guidelines for Master Gardeners. University of Florida, Everglades Research & Education Center. http://erec.ifas.ufl.edu/plant_pathology_guidelines/index.shtml
The Virginia Master Gardener Handbook, 2015 Edition
- Home and Garden Information Center, Trees & Shrubs: Common Problems. University of Maryland Extension. https://extension.umd.edu/hgic/trees-shrubs/commonproblems-trees
- Home and Garden Information Center, Mechanical injury – Trees and Shrubs. University of Maryland Extension. https://extension.umd.edu/hgic/mechanical-injury-trees-andshrubs
- Insects. University of Minnesota Extension. https://www.extension.umn.edu/garden/insects/
- Common Native Trees of Virginia Tree Identification Guide. (2012). Virginia Department of Forestry. http://dof.virginia.gov/infopubs/_fhr/FHR-2017-01_pub.pdf
- Williams, S. D., Boehm, M. J., & Qu, F. Plant Disease Factsheets: PLPATH-GEN-1, PLPATH-GEN-5, PLPATH-GEN-6, PLPATH-GEN-7, PLPATH-GEN-8. Ohio State University Extension. https://ohioline.osu.edu/factsheet/plpath-gen-1
- The Virginia Master Gardener Handbook. (2015). Virginia Cooperative Extension. ↵
- Francl, L. J. (2001). The Disease Triangle: A plant pathological paradigm revisited, The Plant Health Instructor. https://www.apsnet.org/edcenter/instcomm/TeachingArticles/Pages/DiseaseTriangle.aspx ↵
- Pendergast, D. & Pendergast, E. (2003). The Tree Doctor, A Guide to Tree Care and Maintenance. Firefly Books. ↵
- Clatterbuck, W. K., Vandergriff, D. S., & Coder, K. D. Understanding Lightning and Associated Tree Damage. Texas A&M AgriLife Extension. https://agrilife.org/treecarekit/after-the-storm/understanding-lightning-associated-tree-damage/ ↵
- DeJong-Hughes, J., Moncrief, J. F., Voorhees, W. B., & Swan, J. B. (2017). Soil compaction: causes, effects, and control. University of Minnesota Extension. ↵
- Purcell, L. (2014). Mechanical Damage to Trees: Mowing and Maintenance Equipment. Purdue University Extension. https://www.extension.purdue.edu/extmedia/fnr/fnr-492-w.pdf ↵
- Pratt, P. W. & Schnelle, M. A. (2017). Site Disturbance & Tree Decline, Oklahoma State University. Oklahoma State University Extension. http://factsheets.okstate.edu/documents/hla-6429-site-disturbance-and-tree-decline/ ↵
- Agriculture Information Bulletin No. 372, Air Pollution Injures Trees. (1974). Forest Service, U.S Department of Agriculture. https://www.na.fs.fed.us/spfo/pubs/misc/tree_care/aib372-e.htm ↵
- Askew, S. D., Wycoff, S. B., Bergh, J. C., Bush, E. A., Day, E. R., Dellinger, T., Derr, J. F., Hansen, M. A., Hong, C. X., Laub, C. A., Likins, M., McCall, D. S., McCoy, T., Miller, D. M., Nita, M., Parkhurst, J. A., Parson, R., Paulson, S. L., Pfeiffer, D. G., Rideout, S. L., Schultz, P. B., Wilson, J., Yoder, K. S.. (2021). 2021 Pest Management Guide - Home grounds and animals. Virginia Cooperative Extension. https://resources.ext.vt.edu/contentdetail?contentid=2377 ↵
- Beckerman J. (2009). Downy Mildew. Purdue University Extension. https://www.extension.purdue.edu/extmedia/bp/bp-68-w.pdf ↵
- Esser, R. P. What are Nematodes? Organization of Nematologists of Tropical America Florida, Inc (ONTA). http://www.ontaweb.org/photos-and-links/what-are-nematodes/ ↵
- Day, E. R., (2020). Aphids. Virginia Tech Department of Entomology, Virginia Cooperative Extension. https://resources.ext.vt.edu ↵
- Day, E. R., (2015). Scale Insects. Virginia Tech Department of Entomology, Virginia Cooperative Extension. https://pubs.ext.vt.edu/2808/2808-1012/2808-1012.html ↵
- Knuth, S. (2011). Yellow-Bellied Sapsucker. Virginia Department of Game and Inland Fisheries. http://12l8.blogspot.com/2011/03/yellow-bellied-sapsucker.html ↵
- Glen, C. (2016). Does Mistletoe Harm Trees? North Carolina Cooperative Extension, Chatham County Center. https://chatham.ces.ncsu.edu/2014/12/does-mistletoe-harm-trees-2/ ↵
- Aulakh, J. S. (2016). Woody Vines - Identification and Control. Connecticut Agricultural Experiment Station. https://portal.ct.gov/-/media/CAES/DOCUMENTS/Publications/Fact_Sheets/Valley_Laboratory/WoodyVinesIdentificationandControlpdf.pdf ↵
- Climbing Trees are Tree Killers. (2020) The Tree Care Guide. http://www.thetreecareguide.com/climbing-vines-tree-killers/ ↵
- Spengler, T. (2020, July 20). Vines and Trees: Do Vines Harm Trees by Growing on Them. Gardening Know How. https://www.gardeningknowhow.com/ornamental/trees/tgen/do-vines-harm-trees.htm ↵
- Spengler, T. (2020, July 20). Vines and Trees: Do Vines Harm Trees by Growing on Them. Gardening Know How. https://www.gardeningknowhow.com/ornamental/trees/tgen/do-vines-harm-trees.htm ↵
- Hansen, M.A. Foliar Diseases of Dogwood. Department of Plant Pathology, Virginia Tech. ↵
- Stripes, R. J. and Hansen, M. A. Anthracnose - Fungal Disease of Shade Trees. Department of Plant Pathology, Virginia Tech. ↵
- Pests and Problems by Pest, Sycamore Anthracnose. Missouri Botanical Garden. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/sycamore-anthracnose.aspx ↵
- Stripes, R. J. and Hansen, M. A. Anthracnose – Fungal Disease of Shade Trees. Department of Plant Pathology, Virginia Tech. ↵
- Benson, D.M. & Jones, R.K. (2000). Phytophthora Root Rot and its Control on Established Woody Ornamentals. North Carolina State University, College of Agriculture and Life Sciences. ↵
- Pear Disease - Fire Blight. (2017). Penn State Extension. https://extension.psu.edu/fire-blight-of-pear ↵
- Gypsy Moth. Missouri Botanical Garden. https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/caterpillars/gypsy-moth.aspx ↵
- Bush, E. (2012). Integrated Pest Management for Plant Diseases in the Home Garden and Landscape. Virginia Cooperative Extension. http://pubs.ext.vt.edu/PPWS/PPWS-15/PPWS-15.html ↵
- State of the Forest: Annual Report on Virginia’s Forests. (2017). Virginia Department of Forestry. http://www.dof.virginia.gov/infopubs/_sof/SOF-2017_pub_print.pdf ↵
- Day, E. and Salom, S. (2016). Emerald Ash Borer. Virginia Cooperative Extension. ↵
- In July 2021, the Entomological Society of America announced a change to the name of the insect known by the common name "Gypsy Moth." Please see this statement from the ESA: "A The existing common names for the moth Lymantria dispar and the ant Aphaenogaster araneoides were identified as containing a derogatory term for the Romani people. In June, the ESA Governing Board elected to remove the common names for both species from the ESA Common Names of Insects and Related Organisms List. ESA will seek to convene a volunteer group to propose a new common name for L. dispar, which would then be made available for ESA member comment and subject to approval by the ESA Committee on Insect Common Names and the ESA Governing Board. In the meantime, ESA encourages people to refer to the insects by their Latin names." Entomological Society of America. (2021). "Better Common Names Project." https://www.entsoc.org/better-common-names-project ↵
- Roberts, E. A. (2001). Gypsy Moth in Virginia: An Update. Virginia Cooperative Extension. ↵
- Eastern Tent Caterpillar and Forest Tent Caterpillar. (1992). University of Maryland Extension. ↵
- Day, E. R. (2014). Eastern Tent Caterpillar. Virginia Cooperative Extension. ↵
- Day, E. R. (2008). Fall Webworms. Virginia Cooperative Extension. ↵
- Hemlock Wooly Adelgid. (2015). National Park Service. https://www.nps.gov/grsm/learn/nature/hemlock-woolly-adelgid.htm ↵
- Salom, S. and Day, E. R. (2016). Hemlock Wooly Adelgid. Virginia Cooperative Extension. ↵