The Online Gardener's Handbook
Chapter 1: Causes of Plant Injury
Getting to the Root of the Problem
Table of Contents
- Diagnosing Plant Problems
- Tolerance Levels
- Related Links
Diagnosing Plant Problems
The accurate and timely diagnosis of a plant problem is important
if the most effective control measures are to be selected and implemented.
This, in turn, will minimize the damage to the plant's yield and
But before you can diagnose a plant problem, you must know the
plant's normal appearance and growth pattern. This helps identify
problems early on. It is equally important to be able to describe
any abnormalities. For example, does the plant show wilt, dieback
or discoloration; how are symptoms distributed on the plant, and
on other plants in the area; which species are affected, and so
Inspect the plant(s) closely for webbing, cast skins, excrement,
and other signs of insects, as well as for the pest themselves.
Many insects, such as aphids, caterpillars and beetles can easily
be seen on plants. Others such as maggots are not likely to be seen
until damage is done. If these pests have been a problem in the
past, preventive measures will be required. Look for moulds, mildews,
mushrooms and other spore producing structures; these are the signs
of fungi. Look for the presence of a cloudy fluid (bacterial ooze)
which might indicate an infectious disease. Generally, damage to
leaves cannot be repaired once the plant has been infected.
Don't forget that many pest problems occur out of sight, under
the soil surface. You may need to dig up a plant to look for root
damage or soil insects.
Bear in mind that weather conditions, site, soil conditions and
cultural practices such as chemical use, fertilizer application,
and watering practices will also affect plant growth. A greater
discussion of such factors is found in the section on abiotic diseases.
They must all be considered when making your diagnosis. Often, it
is a combination of factors that create the problem. Plants under
stress from poor environmental conditions, for example, are more
susceptible to infectious diseases and pests.
To identify the problem, turn first to the appropriate commodity
section in this manual. Read through the descriptions of pests and
diseases for a particular host until one is found which fits the
symptoms you have observed. Both the fruits and ornamentals sections
have diagnostic keys to help you in this. For each problem listed
in this manual, symptoms are described and management options suggested.
When diagnosing the problem, it is important that you not guess.
Only the most common pest problems are listed in this publication.
If pests or diseases are encountered which do not fit any listed
in this publication, consult other references and specialists. If
you are unsure of the diagnosis, consult nurseries or garden centres,
professional landscapers or a Master Gardener in your area. It is
also possible to submit samples of diseases or insects to a commercial
laboratory, such as the University of Guelph's Pest Diagnostic Clinic
for diagnosis, however there is a fee associated with this service.
Once you know what is causing the problem, you can then decide
whether controls are necessary, and if so, how to implement them.
In making your decision, remember that some pest damage is normal
on garden plants and that minor damage usually will not have a lasting
impact on them.
Consider the health of your plant, as a plant that is stressed
by other factors will be less able to withstand pest damage than
a healthy plant. The timing of the damage will also be important,
as plants are often more susceptible to pest damage at certain stages
of their development. For example, vegetables are extremely vulnerable
to insects, diseases and weed competition as seedlings, but can
tolerate some damage later in development, particularly if it is
a non-harvestable part. Consider also the location and degree of
damage. For example, damage to the outer leaves of a cabbage can
be removed, and it may be possible to cut off minor blemishes on
Finally, your available control methods should fit into your decision
making. If the method you want to use takes a long time to take
effect (for instance some natural enemies) or will only provide
partial control, you may want to implement it when pest populations