Diagnosis of plant problems is often a very difficult task since there can be many different causes for a given symptom, not all of which are pathogenic organisms. Soil nutrition and texture, weather conditions, lighting, and many other environmental and cultural conditions influence the overall health of a plant. Insect damage can sometimes be confused with plant disease caused by microorganisms or abiotic factors. Knowing a complete history of the plant is essential to making an accurate diagnosis. Also, a plant specimen should be in the early stages of deterioration when it is examined in order for an accurate diagnosis to be made. Once it has decayed, secondary organisms invade the tissue and evidence of the primary pathogen is often obscured.

For these reasons, it is difficult to construct a foolproof key for the diagnosis of plant problems. Even with the necessary laboratory equipment at one’s disposal, it is often difficult to determine the exact cause of a plant’s problem.

The following pages provide an aid to diagnosing some of the common problems of urban plants. This chapter was constructed to help solve consumer’s plant problems — it is not meant for diagnosis of commercial problems or use by laboratory diagnosticians. The information provided is by no means comprehensive, and other resources will be needed for many of your diagnoses.

A Systematic Approach to Diagnosing Plant Damage

Determining what factors caused damage to a plant requires an inquisitive, investigative approach combined with careful observation and the ability to put all the pieces together to reconstruct the event(s) that produced the plant damage. Accurate diagnosis must be made before corrective action can be taken. Even if no corrective measures are available, there is satisfaction in simply knowing what the problem is and what its future development might be.

Living (biotic) factors: Living organisms such as pathogens (fungi, bacteria, viruses, nematodes), and pests (insects, mites, mollusks, rodents, etc.). With living factors, “Something is missing, and something is gained.”

Nonliving (abiotic) factors: Mechanical factors (breakage, abrasions, etc.), physical/environmental factors (extremes of temperature, light, moisture, oxygen, lightning), and chemical factors (chemical phytotoxicities, nutritional disorders, etc.).

If we suspect that it is a living damaging factor, we will look for signs and symptoms to distinguish between pathogens and pests. If the accumulated evidence suggests that it is a pathogen, we will seek evidence to distinguish among fungal, bacterial, viral pathogens, and nematodes. If the evidence indicates the damaging factor is an insect or other animal, we will seek further evidence to distinguish between sucking and chewing types.

The probability of a correct diagnosis based on only one or two clues or symptoms is low. Similarities of symptoms produced on the same plant by completely different factors frequently make the use of symptoms alone inadequate.

In diagnosing plant damage, a series of deductive steps can be followed to gather information and clues from the big, general situation down to the specific, individual plant or plant part. Through this systematic, diagnostic process of deduction and elimination, the most probable cause of the plant damage can be determined. Steps to follow in gathering diagnostic information are presented below. Each step will then be expanded and guidelines presented as we proceed through the diagnostic process. We will first identify the problem, then attempt to distinguish between living and nonliving damaging factors based on the observed damage patterns, development of the patterns with time, and other diagnostic signs.

Factors causing plant damage can be grouped into two major categories: If evidence indicates that the damage is being caused by a nonliving factor, we will seek further evidence as to whether the initial damage is occurring in the root or aerial environment. We will then attempt to determine if the damage results from mechanical factors, from extremes in physical factors (such as extremes of temperature, light, moisture, or oxygen), or from chemical factors (phytotoxic chemicals or nutritional disorders). Once we have identified the plant and limited the range of probable causes of the damage, we can obtain further information to confirm our diagnosis from reference books, specialists such as plant pathologists, entomologists, horticulturists, and/or laboratory analyses.

Define the Problem

Identify Plant and Know Characteristics

Is the growth and appearance of the identified plant normal? Is it abnormal?

Determine that a real problem exists. It is essential that the plant be identified (genus, species, and cultivar or variety) so that the normal appearance of that plant can be established either by personal knowledge or by utilizing plant reference books. Many horticultural plants or structures on those plants such as fruits, seeds, lenticels, etc. may appear to be abnormal to the person who is not familiar with the specific plant. For example, the ‘Sunburst’ honey locust might appear to be suffering from a nutrient deficiency because of its chlorotic yellow-green leaf color, but it was selected because of this genetic characteristic. It is not abnormal for this plant; therefore, it is not a problem.

Always compare the typical diseased plant with a healthy or normal plant, since normal plant parts or seasonal changes are sometimes mistakenly assumed to be evidence of disease. Examples are the brown, spore-producing bodies on the lower surface of leaves of ferns.

These are the normal propagative organs of ferns. Also in this category are the small, brown, club-like tips that develop on arborvitae foliage in early spring. These are the male flowers, not deformed shoots. Small galls on the roots of legumes, such as beans and peas, are most likely nitrogen-fixing nodules essential to normal development and are not symptoms of root-knot nematode infection. The leaves of some plants, such as some rhododendron cultivars, are covered by conspicuous fuzz-like epidermal hairs. This is sometimes thought to be evidence of disease, but it is a normal part of the leaf. Varieties of some plants have variegated foliage that may resemble certain virus diseases. These examples illustrate the importance of knowing what the normal plant looks like before attributing some characteristic to disease.

In describing the plant “abnormality,” distinguish between symptoms and signs. Symptoms are changes in the growth or appearance of the plant in response to living or nonliving damaging factors. Many damaging factors can produce the same symptoms; symptoms are not definitive. Signs are evidence of the damaging factor (pest or pathogen life stages, secretions, mechanical damage, chemical residues, records of weather extremes or chemical applications, damage patterns). Patterns of damage are excellent signs and are definitive diagnostic clues.

Examine the Entire Plant and Its Community

In defining a plant problem, it is essential to determine the real primary problem. There are foliage symptoms that may occur due to root damage. The primary problem would be root damage, not chlorosis of the foliage — examine the roots. In general, if the entire top of the plant or entire branches are exhibiting abnormal characteristics, examine the plant downward to determine the location of the primary damage. Look for the factor causing the damage at the periphery of the plant damage.

Some pathogens and insects as well as nonliving factors are only damaging if the plant has been predisposed by other primary factors. For example, borers generally only attack trees that are already predisposed by moisture or other physical stress. Premature dropping of leaves by foliage plants (like Ficus benjamina) and of needles by conifers frequently causes alarm. Evergreen plants normally retain their leaves for 3-6 years and lose the oldest gradually during each growing season. This normal leaf drop is not noticed. However, prolonged drought or other stress factors may cause the tree as a whole to take on a yellow color for a short period and may accelerate leaf loss. If the factors involved are not understood, this often causes alarm. The leaves that drop or turn yellow are actually the oldest leaves on the tree, and their dropping is a protective mechanism which results in reduced water loss from the plant as a whole.

Normal versus abnormal needle drop or leaf drop from evergreens

Non-deciduous plants normally retain their leaves for several years but eventually they fall. This drop is usually gradual, and production of new leaves obscures the loss of older leaves.

Normal: If drop is confined to older leaves, alarm is unnecessary because it is a normal response to a condition of stress (e.g., drought). Unfavorable growing conditions such as drought may accelerate leaf fall so that it becomes apparent and of concern.

Abnormal: If newly produced leaves are lost, it is a problem. The drop of current year’s leaves may result from a pathogen or insect attack or from chemical deficiencies or toxicities.

 

Look for Patterns

Here is where we start making the distinction between living and nonliving factors that cause plant damage.

Understand Nonuniform Damage Pattern

Living factors
There is usually no discernible, widespread pattern of damage on a planting. Damage produced by living organisms, such as pathogens or pests, generally results from their using the plant as a food source. Living organisms are generally rather specific in their feeding habits and do not initially produce a wide-spread, discernible damage pattern. Plants become abnormal. Tissues are destroyed or removed, become deformed, or proliferate into galls.

Living organisms are specific (damage may be greatest on or limited to one species of plant).

Living organisms multiply and grow with time; therefore, they rarely afflict 100% of the host plants at one time. The damage is progressive over time. Likewise, the damage generally is initially limited to only one part of the plant and spreads from that initial point of attack with time.

Living organisms usually leave “signs” like excrement, cast skins, mycelium, eggs, etc.

Understand Uniform Damage Pattern

Nonliving factors
Damage patterns produced by nonliving factors such as frost or applications of toxic chemicals are generally recognizable and widespread. Damage will appear on all leaves of a certain age (for example, on all the leaves forming the plant canopy at the time a toxic spray was applied) or exposure (all leaves not shaded by overlapping leaves on the southwest side of a plant may be damaged by high temperatures resulting from intense sunlight). Damage will likely appear on more than one type or species of plant (look for similar damage patterns on weeds, neighboring plants, etc.) and over a relatively large area.

Compare Patterns

The following figures provide a comparison of patterns of living and nonliving factors on plant community, plant, and plant part and discuss these factors.

A. Entire or major portion of top dying: If all or a major portion of a tree or shrub dies, suspect a problem with the roots. Look for damaging factors at junction of normal and abnormal plant tissue.

Gradual decline of entire plant or a major portion of it is caused by living factors such as Armillaria root rot, Verticillium wilt, and root weevil.

Sudden decline is generally caused by a nonliving factor such as a toxic chemical in the soil or drastic climatic changes such as freezing or drought.

B. Single branch dying: If scattered damage occurs in the plant canopy, suspect that the primary problem is related to the foliage or aerial environment, not the roots.

Gradual death of a branch: If scattered branches start to decline and eventually die, suspect a living organism such as a canker pathogen, a shoot blight, or borers.

Sudden death of a branch: If a branch dies suddenly, and especially if affected branches are concentrated on one side of the plant, suspect a nonliving factor such as weather (wind, snow, etc.), animal damage, or chemical drift.

A. Shoot dieback caused by nonliving factors: Sudden dying back of a shoot usually indicates a nonliving cause such as climatic or chemical damage, not a living factor. Damage caused by nonliving factors usually results in a sharp line between affected and healthy bark.

If dieback is more gradual and there is also cracking of the bark and wood, suspect winter injury.

B. Shoot dieback (blight) caused by living factors: Gradual decline of shoots and retention of dead leaves may indicate a living factor.

The margin between affected and healthy tissue is often irregular and sunken.

There may be small, pin-like projections or bumps over the surface of dead bark. These are spore-producing structures of pathogenic fungi.

However, small, woody bumps radiating from all sides of twigs of dwarf Alberta spruce are pulvinus, woody projections where needles were attached. This is a taxonomic identifying characteristic of spruce.

Death of the tips of conifer needles producing a uniform pattern usually indicates a nonliving factor such as a toxic chemical or unfavorable climatic condition. Air pollutants frequently cause tip burn on conifers as do certain soil-applied herbicides or excess fertilizer.

Drought and freezing may have a similar effect. In these cases all needles of a specific growth period are usually affected, and usually the same length on each needle is affected.

The margin between the affected tissue (usually reddish brown) and healthy tissue is sharp and distinct.

Damage by living organisms, such as fungi and insects, to needles usually occurs in a random, scattered pattern and rarely kills all needles of a particular growth period. Needles are usually affected over varying lengths and often appear straw yellow or light tan in color. Black fruiting bodies of causal fungus may be present on diseased needles.

Delineate Development

As already mentioned, another clue for distinguishing between living and nonliving factors causing plant damage is to observe the development of the pattern.

Living organisms generally multiply with time and produce an increasing spread of the damage over a plant or planting with time, and therefore are progressive.

Spots are usually uniformly and evenly distributed over the leaf surface, and generally will be of uniform size. Color is usually uniform across the spot.

Injury from chemicals taken up by plants from soil through roots or from air through leaves usually results in scorching (necrosis) of leaf margins and interveinal areas. If severe, necrotic tissue may drop out giving a ragged appearance. Similar patterns are produced by moisture stress. If uptake of toxic chemical is to a fully expanded leaf, toxicity is marginal and interveinal. If uptake is to an unexpanded leaf, toxicity occurs in veins.

Nonliving factors generally damage the plant at a given point in time; for example, death of leaf tissue caused by a phytotoxic chemical is immediate and does not spread with time. There are exceptions. If a nonliving damaging factor is maintained over time, the damage will also continue to intensify with time. For example, if a toxic soil or air chemical is not removed, damage to plants within the contaminated area will continue to develop, but damage will not spread to plants in uncontaminated areas. Nonliving factors are not progressive. This again reemphasizes the necessity of piecing together multiple clues to identify the most probable factor causing plant damage.

Determine Causes

Patterns of damage distribution and time patterns in development of damage have been valuable in making the gross distinction between damage caused by living factors and damage caused by nonliving factors. Additional clues must be obtained to distinguish among factors within the living and nonliving categories.

Distinguish Among Living Factors

To further identify which subcategory of living factor caused the damage, a close examination of the symptoms and signs is required.

Symptoms are the modified appearance of the affected plant, for example necrotic tissues, chlorosis, cankers, galls, leaf distortion.

Signs are the presence of the actual organism or evidence directly related to it: visual observation of the insect on the leaf, presence of fungal mycelium, spores, insect egg masses, insect frass, mite webbing, etc. Signs can be used as clues in identifying the specific living organism that produced the plant damage.

A combination of clues from both symptoms and signs are required for preliminary distinction between pathogen and insect-mite damage.

Symptoms and Signs of Pathogens: Differentiating between bacterial and fungal pathogens is not always clear cut.

Table 6-1: Symptoms and signs of fungal and bacterial leaf spots

Fungal diseases

Fungal leaf spots and stem rots are characterized by various symptoms: dry texture, concentric rings, discoloration, and fruiting structures. Fungal leaf spots and stem rots are usually dry or papery. This is especially true in dry climates. The most distinguishing clue of a fungal disease is the presence of signs: mycelium and fruiting bodies of the fungus itself. The fruiting bodies range in size from microscopic to those easily detected with the naked eye. They are found within the leaf spot or stem rot area. Each type of fungus has its own characteristic structures which enable plant pathologists to identify them.

Zones of different color or texture may develop, giving the spot a bull’s eye effect. The dead tissue (tan) is in the center of the spot where the fungal spore germinated. Then as the fungal mycelium front moves outward from the point of dead tissue to healthy, not yet infected tissue on the perimeter, the foliage color changes from dead tan in the center to healthy green on the perimeter.

Spots are not limited by leaf veins since mycelium grows on leaf surface.

Foliar pathogens: The leaf spots caused by fungi generally have distinct margins. Many times they are circular with concentric rings resulting from growth of the mycelium from the center point of initial infection outward (much like crocheting a doily). The condition of the leaf tissue and associated color ranges from dead (necrotic tan) in the center, to recently dead (darker brown ring), to dying (darker ring with possible light yellow, chlorotic edge indicating the advancing edge of the fungal infection). The margins of fungal leaf spots (Figure 6-6) and stem rots can be brightly discolored, such as purple (Fusarium stem rot) or yellow (Helminthosporium leaf spot), making these symptoms quite striking.

Root and stem Pathogens: Root rot and vascular wilt result from fungal infection and destruction of root and stem tissues. The most common visual symptom is gradual wilting of the above-ground shoots.

Bacterial diseases

Bacteria do not actively penetrate healthy plant tissue like fungi. They enter through wounds or natural openings such as leaf stomata or twig lenticels. Once bacteria enter the plant, they reproduce rapidly, killing the plant cells.

Bacterial leaf spots are often angular because they are initially limited by the leaf veins.

Color of the bacterial spots is usually uniform. Bacteria are one-celled organisms that kill as they go. Tissue may first appear oily or water-soaked when fresh, but upon drying becomes translucent and papery tan.

Bacterial galls: In some cases, toxic materials are produced that cause plant tissues of roots, stems, or leaves to grow abnormally as in crown gall.

Bacterial leaf spot disease: The bacteria usually enter through leaf stomata. Symptoms include water-soaking, slimy texture, fishy or rotten-odor, confined initially between leaf veins resulting in discrete spots that have straight sides and appear angular. Many bacterial leaf spots, such as Xanthomonas leaf spot on Philodendron (also called red edge disease), expand until they reach a large leaf vein. This vein frequently acts as a barrier and inhibits the bacteria from spreading further. A chlorotic halo frequently surrounds a lesion. Lesions may enlarge through coalescence to develop blight lesions. Some lesions exude fluid containing bacteria. Water-soaking frequently occurs in bacterial leaf spot diseases such as Erwinia blight of Dieffenbachia. Holding the leaf to light usually reveals the water-soaking. The ability of bacteria (usually Erwinia species) to dissolve the material holding plant cells together results in a complete destruction of leaf or stem integrity. Some fungi also produce this symptom but not usually as extensively as Erwinia. In general, bacterial infections show this characteristic more than fungal infections. In final stages, cracks form in the tissue and disintegration follows.

Vascular wilt: In some cases the bacteria poison or plug the water conducting tissue and cause yellowing, wilting, browning, and dieback of leaves, stems, and roots.

Viral diseases

Viruses are “submicroscopic” entities that infect individual host plant cells. Once inside a plant cell they are able to infect other cells. Viruses are obligate parasites. They can only replicate themselves within a host’s cell. Because the virus commandeers the host cell to manufacture viruses identical to itself, the plant cell is unable to function and grow normally. In the virus infected plant, production of chlorophyll may cease (chlorosis, necrosis); cells may either grow and divide rapidly or may grow very slowly and be unable to divide (distortion, stunting).

Vein clearing (chlorosis) with interveinal tissue remaining green usually indicates a virus disease or uptake and xylem translocation of a herbicide such as diuron. This is in contrast to the leaf veins remaining green with surrounding chlorotic tissue associated with nutrient deficiencies such as iron deficiency.

Mosaic is a patchwork of green and yellow areas over the surface of a leaf. The leaf may also be puckered and distorted. These symptoms usually indicate a virus disease, especially if yellow areas blend gradually into green areas. If margins are distinct, mottling may indicate a nutritional problem or genetic variegation.

The symptoms of most virus diseases can be put into four categories:

1. Lack of chlorophyll formation in normally green organs:

Foliage may be mottled green and yellow, mosaic, or ringed (yellow or other pigmented ring patterns), or be a rather uniform yellow (virus yellows).

Veins: Vein clearing is a common first symptom of some viral diseases. The veins have a somewhat translucent or transparent appearance. In vein banding there is a darker green, lighter green or yellow band of tissue along the veins.

2. Stunting or other growth inhibition: The reduction in photosynthesis due to less chlorophyll leads to shorter internodes, smaller leaves and blossoms, and reduced yield.

3. Distortions of leaves and flowers: Witches’ brooms or rosettes result from nonuniform growth within a tissue or uncontrolled growth.

4. Necrotic areas or lesions: Being obligate parasites, viruses require the survival of their host plant for their own procreation. Hence, viruses rarely cause death. Necrosis that does occur is usually confined to discrete areas of the plant; necrosis rarely occurs to such an extent that the entire plant is killed.

Viruses typically discolor, deform, or stunt plants rather than induce necrosis or cause death. Expressed symptoms (chlorosis, stunting, distortions) can be valuable clues for virus identification, but can be easily confused with symptoms induced by other problems such as nutritional disorders, spray injuries, or certain feeding damage induced by mites or insects. In addition, because of their extremely small size, the virus or signs of the virus are not visible to the unaided eye. The virus particles are detectable within the plant cell through the electron microscope.

Viruses are transmitted from plant to plant by insects, mites, fungi and nematodes, rubbing, abrasion, or other mechanical means (including grafting or other forms of vegetative propagation). Viruses are occasionally transmitted in seed. Because of the nature of virus transmission, virus symptoms generally spread with time from one infected plant tissue to other plant tissues or from one infected plant to other plants in the community.

Nematodes

Plant nematodes are microscopic roundworms that damage plant tissues as they feed on them. Many feed on or in root tissues. A few feed on foliage or other above-ground organs.

Shoot nematodes: (Aphelenchoides spp.): Foliar nematodes feed inside leaves between major veins causing chlorosis and necrosis. Injury is most often seen at the base of older foliage. When plants with a net-like pattern of veins become infested with foliar nematodes, the tissues collapse in wedge-shaped areas and then change color.

Root nematodes: The most common above-ground symptoms caused by root-infesting nematodes result from damaged root systems. Moisture and nutrient stress symptoms and general stunting are common. The root lesion nematodes (Pratylenchus spp.) and burrowing nematodes (Radopholus similis) destroy the root cortex tissues as they feed. The root-knot nematodes (Meloidogyne spp.) inject growth-regulating substances into root tissues as they feed, stimulating growth of large tender cells to provide themselves a permanent feeding site, and causing overgrowth of root tissues around them to form visible, swollen galls or knots. Other root nematodes stunt growth, apparently by killing root meristems.

Symptoms and Signs of Insects, Mites, and Other Animals

Insects

The location of the feeding damage on the plant caused by the insect’s feeding, and the type of damage (damage from chewing or from sucking mouth parts) are the most important clues in determining that the plant damage is insect-caused and in identifying the responsible insect.

An insect’s life cycle (complete or incomplete) is important when attempting to detect the insect or design a control program.

Feeding habits

Chewing damage or rasping damage

• Entire leaf blade consumed: Entire leaf blade may be consumed by various caterpillars, canker worms, and webworms. Only tougher midvein remains.
• Distinct portions of leaf missing: Distinct notches cut from leaf margin (black vine weevil adult), circular holes cut from margin of leaf (leaf cutter bees), small randomly scattered holes in leaf (beetles, chafers, weevils, grasshoppers).
• Leaf surfaces damaged: “Skeletonization” of leaf surface. Slugs, beetle larvae, pear slug (pear sawfly larvae), elm leaf beetle, and thrips.
• Leaves “rolled”: Leaves that are tied together with silken threads or rolled into a tube often harbor leafrollers or leaftiers (e.g., omnivorous leaftier, Cnephasia longana).
• Leaf miner damage: Leaf miners feed between the upper and lower leaf surfaces. If the leaf is held up to the light, one can see either the insect or frass in the damaged area (discolored or swollen leaf tissue area). Examples: boxwood, holly, birch, elm leaf miners.
• Petiole and leaf stalk borer damage: Petiole and leaf stalk borers burrow into the petiole near the blade or near the base of the leaf. Tissues are weakened and leaf falls in early summer. Sectioning petiole reveals insect-larva of small moth or sawfly larva. Examples: maple petiole borer.
• Damage from twig girdlers and pruners: Twig girdlers (like the twig girdling beetle) may chew all the way around (or girdle) branches of the host plant. Pruners like the vine weevil may eat irregular holes in plants.
• Borer damage: Borers feed under the bark in the cambium tissue or in the solid wood or xylem tissue. Examples: Mountain pine beetle and smaller European elm bark beetle galleries. Damage is often recognized by a general decline of the plant or a specific branch. Close examination will often reveal the presence of holes in the bark, accumulation of frass or sawdust-like material or pitch. Examples: raspberry crown borer, Sequoia pitch moth.
• Root feeder damage: Larval stages of weevils, beetles, and moths cause general decline of plant, chewed areas of roots. Examples: sod webworm, Japanese beetle, root weevil.

Sucking damage

In addition to direct mechanical damage from feeding, some phloem-feeding insects cause damage by injecting toxic substances when feeding. This can cause symptoms which range from simple stippling of the leaves to extensive disruption of the entire plant. Insect species which secrete phytotoxic substances are called toxicogenic (toxin-producing) insects. The resulting plant damage is called “phytotoxemia” or “toxemia.”

• Spotting or stippling: Results from little diffusion of the toxin and localized destruction of the chlorophyll by the injected enzymes at the feeding site. Aphids, leafhoppers, and lygus bugs are commonly associated with this type of injury.
• Leaf curling or puckering: More severe toxemias such as tissue malformations develop when toxic saliva causes the leaf to curl and pucker around the insect. Severe aphid infestations may cause this type of damage.
• Systemic Toxemia: In some cases the toxic effects from toxicogenic insect feeding spread throughout the plant, resulting in reduced growth and chlorosis. Psyllid yellows of potatoes and tomatoes and scale and mealy bug infestations may cause systemic toxemia.
• General (uniform) “stipple,” flecking, or chlorotic pattern on leaf: Examples: adelgid damage on spruce needles and bronzing by lace bugs.
• Random stipple pattern on leaf: Examples: leafhoppers, mites.
• Leaf and stem “distortion”: Associated with off-color foliage = aphids (distortion often confused with growth regulator injury). Examples: rose aphid, black cherry aphid, leaf curl plum aphid.
• Galls, swellings: May occur on leaf and stem tissue and may be caused by an assortment of insects. Examples: aphids, wasps, midge, mossyrose gall wasp, poplar petiole gall midge, azalea leaf gall.
• Damaged twigs are split: Damage resembling split by some sharp instrument is due to egg laying (oviposition) by sucking insects such as tree hoppers and cicadas. Splitting of the branch is often enough to kill the end of the branch. Examples: cicada.
• Root, stem, branch feeders: General decline of entire plant or section of a plant as indicated by poor color, reduced growth, dieback. Examples: Scales, mealybugs, pine needle scale.

Insect life cycles

Knowledge of life cycles assists in identifying the damaging insect.

Incomplete life cycle: Insects resemble the adult upon hatching, except they are smaller and without wings. As the insect grows, it sheds its skin or molts, leaving cast skins as a diagnostic sign. The adult stage is most damaging. Lygus bugs, leafhoppers, and grasshoppers are examples of insects with incomplete life cycles.

Complete life cycle: Eggs, larva (wormlike or grub-like creature that may feed on various plant parts), pupa (relatively inactive, often enclosed in some form of cocoon), and adult insect are completely different in appearance. The larval stage with chewing and rasping feeding is most damaging. Examples of insects with complete life cycles are butterflies, moths, weevils, beetles, and flies.

Other animal damage

Arachnids have sucking mouth parts and 8 legs instead of 6 like the insects. Spider mites have an incomplete life cycle (mite resembles adult throughout life cycle). Damage is often a characteristic stipple pattern on leaf which then becomes pale color on underside (severe infestation causes leaf bronzing and death). Presence of “dirty” foliage indicates small fine webbing on the underside of the foliage mixed with eggs and frass. Eriophyid mites cause distorted new growth, leaf margins roll, leaf veins swell, and distort the leaf (symptoms often confused with growth regulator damage).

Crustacea: Sow bugs and pill bugs feed on decaying vegetation. Not considered to be damaging to live plants.

Mollusca: Slugs and snails. Feeding injury to low-growing foliage resembles skeletonizing or actual destruction of soft tissue. Signs: Presence of ‘silvering’ and slime trails on foliage.

Miscellaneous animals: Millipedes and centipedes (arthropods) feed on decaying plant vegetation (many small legs, brownish or white in color, vary in size from 1/2 – 2”). Not considered to be injurious to live plants.

Small mammals: Chewing of bark and cambium tissue on small trees and shrubs is most frequently by rodents (mice, rabbits, squirrels, and possibly beavers). Signs: Note teeth marks.

Large mammals: Branches torn or clean cut by cattle, goats, deer, and horses.

Birds: Yellow-bellied sap-sucker (even rows of holes in the tree trunk). Missing flower petals, puncture splitting of bark.

Distinguishing Among Nonliving Factors

If patterns of damage in the field planting and on the individual plant are uniform and repeated, this indicates that a nonliving factor is the probable cause of the damage. We will now examine additional information and clues to determine whether the nonliving damaging factor was a mechanical, physical, or chemical factor.

Look for changes in the three categories of nonliving factors of the affected plant’s environment:

• Mechanical Factors – (damage/breakage) – plant damage caused by site changes – “construction damage,” transplanting damage, “lawn mower blight,” abrasion, bruising.
• Physical Factors – environment or weather changes causing extremes of temperature, light, moisture, aeration.
• Chemical Factors – chemical pesticide applications, aerial and soil pollutants, nutritional disorders.

Mechanical factors

Close visual examination and questioning will often determine if the stems or roots have been broken or girdled or if the leaves have been bruised, punctured, or broken. For example, if a large Ficus elastica is dropped while being transplanted and the stem is broken, rapid wilting of the portion of the plant above the break will occur. Examine the plant site for signs of recent excavation, construction, paving, etc.

Physical factors (environmental factors)

Primary sources of diagnostic information are damage patterns and weather records to pinpoint time and location of weather extremes. Records are “signs” of the factor that caused the plant damage.

Temperature extremes

Heat: The highest leaf temperatures will occur in the early afternoon when the sun is located in the southwest quadrant of the sky. Therefore, lethal leaf temperatures produced by solar radiation absorption will occur primarily on unshaded leaves on the outer surface of the plant canopy on the southwest side. Portions of leaves shaded by other leaves or leaves on the shaded northeast side may be undamaged. Most severe damage occurs on the leaves most exposed and furthest from the vascular (roots, stem, leaf vein) source of water, on leaves on the outer perimeter of the plant, leaf tips, and interveinal areas. A recognizable pattern related to leaf tissue that would have the highest potential temperature and be most readily desiccated will occur uniformly over all plants in the area.

Cold: Damage will occur on the least hardy plants and will be most severe on the least hardy tissues of those specific plants. In fall acclimation, cold hardiness is first achieved by the terminal buds, and then with time the lower regions achieve hardiness; the branch crotches are often the last tissues to achieve cold hardiness. Generally the root systems will not survive as low a temperature as will the tops – root systems are damaged at higher temperatures than are the tops. On the other hand, after hardiness has been achieved, if warm temperatures induce deacclimation (for example, in the early spring), the terminals (buds) are first to become less cold hardy.

The portion of plant damaged will indicate if low temperature damage occurred before the plant achieved cold hardiness in the fall, or if it occurred after cold hardiness was lost in the spring. Reverse patterns are produced.

On a given structure (like a leaf or bud), the damage will be death of exposed, nonhardy tissues in a recognizable (repeated) pattern. Frost damage to foliage (e.g., conifer needles) in the spring will uniformly kill all needles of a given age from the tip of the needle back toward the stem a given distance on each needle. Frost cracks are longitudinal separations of the bark and wood generally on the southwest sides of the trunk -most likely to occur because of daily, wide temperature fluctuations. Freezing death of dividing cells on outer portions of leaf folds inside the bud will cause distorted or lace-like leaf blade because of non-uniform cell division and growth during leaf expansion. Cold damage to the root system is primarily a concern with container-grown plants where the root temperature fluctuates more and can be expected to reach lower temperatures than would occur with the same plant if field-grown. Cold damage to the root system can be detected by examining the roots. Damage generally occurs from the periphery of the root ball (near the container edge) and evidence includes blackened or spongy roots with lack of new growth or new root hairs. Above ground symptoms generally will not be evident until new shoot growth in the spring; at that time leaf expansion may be incomplete (small leaf size) because of the restricted uptake of water and nutrients by the damaged root system. With increased air temperatures, the water loss from the shoots and leaves may exceed the root uptake capacity, and the plants may defoliate due to this water deficiency.

Plants Vary in their Cold Tolerance: The cold tolerance (hardiness) of various plants in the landscape has been rated by the USDA. The most recent (2012) version is based on weather data from 1976–2005. To check your USDA hardiness zone, check the USDA Plant Hardiness Zone Map.

Light extremes

Plants can acclimate to various conditions, but the primary requirement for acclimation is time. Plants respond adversely to rapid changes in the environment. Rapid change from low to high light intensity will result in destruction of the chlorophyll pigments in the leaf (yellowing and necrosis = sunburn). Rapid change from high to low light intensity will result in reduced growth and leaf drop; new leaves will be larger. “Sun leaves” are smaller, thicker and lighter green in color than are “shade leaves.” Flowering will be reduced, delayed, or absent under low light.

Oxygen and moisture extremes

Here we are primarily considering the root environment where oxygen and moisture are inversely related. Water logging (moisture saturation) of the root environment results in oxygen deficiency. Without oxygen, root metabolism and growth come to a standstill. Consequently, uptake of water and nutrients is restricted with subsequent wilting and nutritional deficiency symptoms occurring on the above-ground portions of the plant. Drought and water logging produce many of the same symptoms on the above-ground portion of the plant. The first symptoms will be chlorosis and abscission of older leaves. Under severe, continuing moisture stress, wilting and necrosis will occur on tips and interveinal regions of recently expanded leaves and new growth.

Chemical factors

Chemical injury patterns on an individual plant

A general uniform pattern of damage occurring over several plant species and over a relatively large area indicates a nonliving factor such as a chemical phytotoxicity. Questions-answers, records, the plant symptoms, and knowledge about the mobility within the plant of the common chemicals (nutrients and pesticides) should help determine which chemical caused the damage.

Patterns of injury symptoms on an individual plant that develop because of deficiency, excess, or toxicity of a chemical differ, depending primarily upon whether the chemical causes damage directly on contact or is absorbed and distributed within the plant through phloem-translocation or through xylem-translocation.

Symptoms from direct contact of chemicals with the plant

Shoot foliage contact: Symptoms from shoot contact chemicals occur over the general plant canopy. If the toxic chemical is applied directly to the above-ground parts of the plant (shoot-foliage contact chemical), the physical pattern of application may be detected (e.g., spray droplet size, etc.). If the toxic chemical is spray-applied, the pattern of spray droplets or areas where spray accumulated to runoff along the leaf edges will show the most severe damage. If it is a toxic gas (a volatile chemical acting as an aerial pollutant), the areas between the leaf veins and along the leaf margins where the concentration of water within the leaf is lower will be the first to show damage. Injury from foliar applications of insecticides, fungicides, and fertilizers is primarily of the direct-contact type and is typified by chlorotic-necrotic spotting, especially interveinally and along leaf edges and other areas where chemical concentrates and is least diluted by inter-cellular moisture. Examples of shoot-foliage contact chemicals are foliar-applied fertilizer salts and the herbicides paraquat, acifluorfen, dinoseb, and herbicidal oils.

Root contact: Toxic contact chemicals in the root zone, including excess fertilizer, result in poor root development. Symptoms from root-contact chemicals are localized where the chemical contacts the root, but produce general symptoms in the shoot. The shoots may show water and nutrient stress symptoms (e.g., reduced growth, wilting, nutrient deficiency symptoms). The injury symptoms on the shoot and foliage from root damage by direct contact with toxic chemicals or excessive salts resembles a drying injury; the roots are unable to obtain water. Roots are injured and root tips may be killed. This will result in a general stunting of the plant. In severe cases, wilting can occur even though the soil is wet. Lower leaves generally wilt first and this is followed by a marginal drying of the leaves. Many factors injuring or inhibiting root growth may produce similar shoot symptoms. Nematodes, soil compaction, cold weather, salinity, nutritional disorders, and certain herbicides (dinitroanilines, DCPA, and diphenamid) cause root inhibition.

Symptoms of deficient or toxic translocated chemicals

The effects of mobile chemicals absorbed by the plant are dependent upon whether the chemical is transported in the phloem or in the xylem. If transported solely in the xylem system, the chemical will move upward in the plant in the xylem-transpiration stream. Toxic symptoms from xylem-translocated chemicals occur primarily in the older foliage. Deficiency symptoms of xylem-transported (phloem-immobile) nutrient ions will occur first in the new growth.

If the chemical is translocated in the phloem, it may move multidirectional from the point of absorption (i.e., it may move from the shoot to the root or the reverse). Toxic symptoms from phloem-translocated chemicals occur primarily in the new growth and meristematic regions of the plant. Deficiency symptoms of phloem-retranslocated nutrient ions occur first in the older foliage.

Xylem translocated chemicals move primarily upward in the plant to the foliage. A chemical is translocated upward in the xylem (apoplastic movement) of the plant from the point of absorption. Symptoms occur in tissues formed after the toxicity or deficiency occurs.

Toxic chemicals, xylem translocated: When toxic chemicals are translocated to fully expanded, older leaves, the toxicity symptoms generally appear on the leaf margins and interveinal areas. When toxic chemicals are translocated to immature, young leaves, the toxicity symptoms generally appear associated with the veins, especially the midrib.

• Photosynthetic-inhibiting chemicals: Injury from translocated toxic chemicals is primarily to the foliage. Plant injury generally progresses from the lower, older foliage to the top. Individual leaves show greatest injury (chlorosis) along their tips and margins or along the veins. Examples of xylem-translocated herbicides include the photosynthetic inhibitors such as the triazine, urea, and uracil herbicides.
• Shoot-inhibiting chemicals: Examples of toxic chemicals absorbed by the roots and translocated in the xylem to the shoots are the “shoot inhibiting herbicides.” The shoot inhibitors cause malformed and twisted tops with major injury at the tips and edges of the leaves; looping of the leaves may occur since the base of the leaf may continue to grow while the leaf tips remain twisted together. Thiocarbamate herbicides cause these symptoms on both grasses and broadleaves. Alachlor and metolachlor herbicides cause similar injury symptoms on grasses.

Deficient nutrient Ions, xylem-translocated (phloem immobile): Several nutrient ions are immobile after upward translocation in the xylem and incorporation in plant tissue. They cannot be withdrawn when deficiencies develop in the root zone and retranslocated in the phloem to the new growth. Deficiency symptoms of phloem-immobile nutrient ions develop on the new growth. Boron and calcium are quite phloem-immobile, which means that if the external supply becomes deficient, the symptoms of boron and calcium deficiency will appear first on the new growth. And, with severe deficiencies, the terminal bud dies. Iron, manganese, zinc, copper, and molybdenum are also relatively phloem-immobile and are not readily withdrawn from the older leaves for translocation through the phloem to younger leaves and organs. Deficiency symptoms are most pronounced on the new growth. Phloem-translocated chemicals move multidirectionally from point of application or source of the chemical to the meristematic regions.

Toxic chemicals, phloem translocated: Injury from phloem-translocated toxic chemicals is primarily to new leaves and roots because of translocation of chemical to the meristems. Whether taken up by the roots or shoots, these compounds are moved through the living plant cells and phloem (symplastic movement) to both the root and shoot tips. The young tissue (shoots or roots) will be discolored or deformed and injury may persist for several sets of new leaves. Examples of phloem-translocated toxic chemicals, whether absorbed by the roots or shoots, include the herbicides 2,4-D, dicamba, picloram, glyphosate, amitrole, dalapon, sethoxydim, and fluazifopbutyl. These compounds move to the meristems and typically injure the youngest tissues of the plant.

Deficient nutrient ions, phloem mobile: If phloem mobile nutrient ions become deficient in the root zone, these ions may be withdrawn from the older plant tissue and retranslocated in the phloem to the new growth. In such situations, deficiency symptoms will first occur on the older leaves. Elements that may be withdrawn from older leaves and retranslocated in the phloem to younger leaves and storage organs include nitrogen, phosphorus, potassium, magnesium, chlorine, and in some plant species, sulfur. In plant species where sulfur can be withdrawn from the older leaves and translocated to the newer growth, deficiency symptoms may initially occur on the older leaves or over the plant in general. In plants where sulfur is not readily retranslocated, the older leaves may remain green and the sulfur deficiency symptoms occur only on the new growth.

Diagnostic Keys

These are useful keys to keep at help desks. Use them to match common problems with their causes and control methods. For specific control recommendations, always refer to the “Pest Management Guide: Home Grounds and Animals” 456-018.

Table 6-3: Tree Fruit & Nuts Diagnostic Key

Table 6-4: Small Fruits Diagnostic Key

Table 6-5- Ornamental Shrubs and Trees Diagnostic Key

Table 6-6: Annual and Perennial Flowers Diagnostic Key

Attributions

Prepared by James L. Green, Extension Horticulture Specialist, Oregon State University; adapted for Virginia (2009)

• Diagnostic Keys revised by Elizabeth Bush, Plant Disease Diagnostician (2022), Mary Ann Hansen, Plant Disease Diagnostician (2009, 2022), and Eric Day, Insect Identification Lab Manager, Virginia Tech (2009)
• Sabrina Morelli, Arlington Extension Master Gardener (2021 reviser)
• Elizabeth Brown, Bedford Extension Master Gardener (2021 reviser)
• Adria C. Bordas, Extension Agent, Agriculture and Natural Resources (2015 reviser)

Image Attributions

• Figure 6-1: Normal leaf drop among older leaves nearer to trunk versus abnormal leaf drop from newer leaves towards tip of branch. Johnson, Devon. 2022. CC BY-NC-SA 4.0.
• Figure 6-2: Patterns of dieback on plant canopy. A. Shows canopy death due to urban tree decline B. shows branch death due to dogwood twig borer (Oberea tripunctata). Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 5454691 by Jason Sharman from Bugwood.org CC BY-NC 3.0 US and image 3056077 by James Solomon from Bugwood.org CC BY 3.0 US.
• Figure 6-3: A. Dieback on branch due to cold injury B. Dieback on branch due to Phomopsis blight (Phomopsis juniperivora). Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 5049098 by Joseph OBrien from Bugwood.org CC BY 3.0 US and image 0485003 by David J. Moorhead from Bugwood.org CC BY-NC 3.0 US.
• Figure 6-4: Needle damage. Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 1241649 by Susan K. Hagle from Bugwood.org CC BY-NC 3.0 US and image 1241572 by USDA Forest Service from Bugwood.org CC BY-NC 3.0 US.
• Figure 6-5: Chemical damage from: A. Sulfur dioxide in a consistent pattern across pecan leaf; B. Flouride injury on birch which is concentrated around leaf margins. Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 1505014 by USDA Forest Service – Region 8 – Southern from Bugwood.org CC BY 3.0 US and image 1494137 by University of Georgia Plant Pathology from Bugwood.org CC BY 3.0 US.
• Figure 6-6: Example of fungal leaf sports, photo shows entomosporium leaf spot. Spots usually vary in size, are generally round, and occasionally elongate on stems. (Diplocarpon mespili) on chokeberry leaf. Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 5368844 by Paul Bachi from Bugwood.org CC BY-NC 3.0 US.
• Figure 6-7: Example of bacterial leaf spots on leaf, photo shows bacterial spot of stone fruits (Xanthomonas arboricola pv. pruni) on plum leaf. Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 0162018 by U. Mazzucchi from Bugwood.org CC BY-NC 3.0 US.
• Figure 6-8: Vein clearing, photo shows Tobacco etch virus (Potyvirus TEV) on tobacco leaf. Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 1440033 by R.J. Reynolds Tobacco Company from Bugwood.org CC BY 3.0 US 
• Figure 6-9: Mosaic leaf pattern, photo shows alfalfa mosaic (Alfamovirus Alfalfa mosaic virus) on soybean leaf. Johnson, Devon. 2022. CC BY-NC-SA 4.0. Includes image 5605168 by Craig Grau from Bugwood.org CC BY-NC 3.0 US.
• Figure 6-10: The highly invasive spotted lanternfly is a phloem-feeding insect. Image 5524251 from Joseph OBrien, USDA Forest Service, Bugwood.org, CC BY 3.0 US
• Figure 6-11: Examples of cold damage. Johnson, Devon 2022. CC BY-NC-SA 4.0. Includes image 5049093 by Joseph OBrien, USDA Forest Service, Bugwood.org CC BY 3.0 US and image 5363644 by Howard F. Schwartz, Colorado State University, Bugwood.org, CC BY 3.0 US