Master Gardener^sm Column for the Week of July 4, 2005
Shirley Waggoner-Eisenman
Brunswick County Extension Master Gardener^sm Volunteer

One of the most difficult tasks is diagnosing a plant’s problem since there are many things that can cause damage to a plant. Soil nutrition and texture, weather conditions, lighting, and many other environmental and cultural conditions influence the overall health of a plant.

Knowing the complete history of the plant is essential to making an accurate diagnosis. 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 plant and evidence of the primary pathogen is often obscured.

To correctly diagnose on only one or two clues or symptoms and get the proper diagnosis is highly unlikely. Similarities of symptoms on the same plant by completely different factors frequently make the use of symptoms alone inadequate. Factors causing plant damage can be grouped into two major categories: living factors--organisms such as pathogens (fungi, bacteria, viruses, nematodes), and pests (insects, mites, mollusks, rodents) and non-living factors—mechanical factors (i.e. breakage, abrasions, etc.), physical, environmental factors (extreme temperature, light, moisture, oxygen), and chemical factors (chemical phytotoxicities, nutritional disorders, etc.).

The living damage factor will show signs and symptoms that will distinguish between pathogens and insects. If accumulated evidence suggests it is a pathogen, try to distinguish if it is fungal, bacterial, or viral pathogens, or nematodes. If evidence indicates that the damaging factor is insect or animal, look for damage of sucking or chewing types.

If the damage is caused by a non-living factor, week further information to determine if the initial damage is occurring in the root or aerial environment. This information should allow you to determine if the damage is mechanical or chemical.

It is essential that the plant be identified (genes, species, and cultivar or variety) so that the normal appearance of the plant can be established either by personal knowledge or by utilizing plant reference books. Many plants may appear to be abnormal to a person who is not familiar with the specific plant. Compare the diseased plant with a healthy or normal plant; normal plant parts or changes may sometimes be mistakenly assumed to be evidence of disease.

In establishing the plant “abnormality” distinguish between “symptoms” and “signs.” Symptoms are changes in growth or appearance of the plant in response to living and non-living damaging factors.

Many damaging factors can produce the same symptoms; symptoms are not definitive. Signs are evidence of the damage 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.

It may be necessary to examine the entire plant and its community. A foliage symptom may occur due to root damage. If so, the entire problem would be root damage, not chlorosis of the foliage. If the entire top of the plant exhibits 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.

Pathogens and insects as well as non-living factors may cause damage if the plant has been predisposed by other primary factors. An example—borers generally attack only trees that are already predisposed by moisture or other physical factors. Premature dropping of leaves by foliage plants and of needles by conifers frequently causes alarm. Evergreen plants normally retain their leaves for three to six years and lose the oldest gradually during each growing season. This normal leaf drop is not noticed. 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.

More information on diagnosing plants will be continued in future columns.

Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call (910) 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.

For further information or assistance, please e-mail:
Charlie Spencer, Brunswick County Extension Master Gardener^smVolunteer

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Date Created 7/1/2005