Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: FRIDAY, April 30, 1993 TAG: 9304290087
SECTION: CURRENT PAGE: NRV-15 EDITION: NEW RIVER VALLEY
SOURCE: Joe Hunnings
DATELINE: LENGTH: Long
The most effective approach is to know what questions to ask to narrow down the possibilities.
For example, you should consider recent rainfall and fertilization if you notice brown, dry edges on the leaves of your plants, because both dry weather and excess fertilizer can cause such damage.
And both excessively dry soil and waterlogged soil can cause the same plant reaction - wilt. Even insect damage can sometimes be confused with plant diseases caused by microorganisms. Causes of plant damage
Factors causing plant damage can be grouped into two major categories. Living factors include pests (insects, mites, rodents, rabbits, deer) and pathogens (disease-causing microorganisms, including fungi, bacteria, viruses, nematodes). Non-living factors include mechanical problems, (breakage, abrasions), environmental factors (temperature, light, moisture, oxygen, lightning, wind), and chemicals (fertilizer or pesticide excess, nutritional disorders).
A systematic approach
\ Define the problem: Closely examine the entire plant and others around it. Take note of all the symptoms. For example, if the plant has insects, examine the leaves with insects on them. Do they have brown spots, holes, chewed edges? Are they turning pale green or yellow? Is there more than one kind of insect present?
\ Look for patterns: Do other plants have the same problem? Are they all in the same place or in different locations? Are they all the same type of plant?
Damage to different types of plants in a particular spot can indicate non-living factors. Damage to a few species of plants or only to plants of the same species can indicate living factors.
\ Examine spread of problem: Is it spreading gradually? This can indicate living factors. If it occurred suddenly and remains in a particular spot or on a particular plant, this points toward non-living factors.
\ Determine likely cause of damage: Based on the pattern and spread, decide if the problem is more likely caused by living factors or non-living factors. If living, is the problem a pathogen or a pest?
Living factors
If disease is a possibility, check the following general guidelines for identifying the causes:
A fungal pathogen often causes round leaf spots, stem rot 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 or missing leaves (caterpillars, slugs, beetles, grasshoppers), rolled leaves (leaf roller), tunnels 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 (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).
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 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 scorching 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.
Chemical damage - inappropriately used pesticides or excessive rates of pesticides - can cause symptoms such as leaf burn, distortion, chlorosis or bleaching, depending on the chemical. In fields, 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. Explaining the symptoms thoroughly to your extension agent or garden center, 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 identify the pest positively so you can choose the most effective control measure.
Check the obvious
It never hurts to look again for obvious problems. Is the stem/trunk badly damaged? Has the plant been sitting in a saucer of water for a week? Has your neighbor sprayed weed killer lately? Does your cat enjoy fern salad? Is your dog deprived of fire hydrants? Do your children "help" you by cleaning the plants with furniture polish or window cleaner?
For more information on selection, planting, cultural practices and environmental quality, contact your local Virginia Cooperative Extension Service office.
Joe Hunnings is the Virginia Cooperative Extension Service agent for agriculture in Christiansburg. If you have questions, call the Montgomery County extension office at 382-5790.
by CNB