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If disease is a possibility, check the following general guidelines for identifying the different causes of disease:
A fungal pathogen often causes round leaf spots, stem rots with a dry/papery texture, concentric rings, discoloration, or wilt. Fruiting structures (sometimes microscopic) may form on affected tissue.

A bacterial disease can take the form of galls (swollen areas), irregularly shaped leaf spots, wilting (then yellowing and dying), or rot (often a wet rot).

A viral pathogen can inhibit chlorophyll formation, causing degrees of yellowing or mottling, stunting, distortion, or dieback of part of the plant. Viruses usually debilitate rather than kill, as they are parasitic.

Nematodes are microscopic roundworms that cause disease-like symptoms. Stem nematodes feed on stems and cause shortening of internodes. Root nematodes feed underground, damaging the root system; this leads to moisture and nutrient stress which shows up as wilting and stunting. Foliar nematodes cause angular leaf spots.

If an insect problem seems likely, check the following:

A chewing/rasping insect feeds on plant tissue and can cause ragged/chewed or missing leaves (caterpillars, slugs, beetles, grasshoppers), rolled leaves (leafroller), tunnels in between upper and lower leaf surfaces (leaf miners), holes in stems, branches or trunk; sections of tree dying; or premature yellowing (wood borers), girdled or dead stems (cutworms, twig girdlers, or stem borers), or general decline of plants due to root damage (soil dwelling insects)

A sucking insect feeds on plant fluids and injects toxins into the plant. The toxins can cause leaf spotting or stippling (aphids, leafhoppers, spider mites, other bugs), leaf distortion such as curling or puckering (or more commonly this can be damage from leafhoppers and thrips), or poisoning of entire plants, resulting in stunted growth and/or yellowing (scale, mealybugs, mites, aphids, whitefly).

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Non-living Factors
Damage caused by mechanical factors is usually revealed by close visual examination; check for broken or girdled stems or roots, also bruised, punctured, or broken leaves.
Damage due to physical factors often results from environmental extremes. Cold damage is characterized by death of exposed foliage. Container plants are susceptible to cold damage to the root system, characterized by blackened or spongy roots with lack of new growth or root hairs, usually near the container edge.

A rapid change from low light to high light intensity, or vice versa, can cause yellowing of leaves, reduced growth, and leaf drop or death. Too little light can reduce, delay, or prevent flowering.

Excess heat usually causes scorch symptoms on leaf tips and interveinal areas. Portions of leaves shaded by other leaves, or leaves on the shady side of the plant, may be undamaged. Frequently, heat damage will occur uniformly over all plants in an affected area.

Drought and waterlogging produce many of the same symptoms on the above-ground parts of the plant, mainly chlorosis (yellowing leaves), abscission (shedding older leaves), and wilt. Waterlogging of the root zone also results in oxygen deficiency, leading to a halt in root growth and metabolism, death of the roots, and wilt.

Damage due to chemical factors - inappropriately used pesticides or excessive rates of pesticides can cause symptoms, such as leaf burn, distortion, chlorosis, or bleaching, depending on the chemical. On a field basis, pesticide or fertilizer damage symptoms frequently are associated with application patterns.

Nutrient deficiencies show up as yellowing, stunting, or death of older plant leaves or new growth, depending on the missing nutrients.

Refer to expert information. Talk to your Extension agent or local garden center, explaining the symptoms thoroughly, or check all the symptoms against a good chart or reference book, keeping in mind the factor you have determined as the likely cause. If it appears insects or pathogens are causing the problem, be sure to positively identify the pest so you can choose the most effective control measure.

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