Accurate diagnosis of a plant problem is essential to successful treatment of the problem.  If the problem is not diagnosed correctly, money and time will be wasted on ineffective treatments or worse, the crop will be lost.  It is important to remember that often more than one problem is happening at the same time. For example, a tree may initially be weakened by compacted soil restricting it's root system. It is further weakened by leaf scorch in the summer caused by high temperatures and low rainfall.  Finally is attacked by borers attracted to the stressed plant. In this example, identification of the underlying factors- soil compaction and underwatering - points to "treatments" that could be used to help the tree without the use of chemicals.

To begin diagnosis of a plant disease consider each of the following steps.

1.  Identify the affected plant.  Along with the common and scientific names,  find out the plant's variety name.  Many newer cultivated varieties of plants have increased disease resistance compared to older varieties.  Knowing what diseases the plant already has resistance to will help eliminate disease possibilities.  Specific diseases attack each type of plant, so knowing exactly what species of plant is affected will suggest common disease possibilities.

2.  Carefully examine the diseased area, whether it is one leaf, one cane or vine, one plant or several plants.  Notice how the diseased plants are located in the affected area- or the diseased leaves on the infected plant.

a. Are all the plants in the garden affected or only a portion?  Are they scattered throughout the entire area or all confined to one or more specific areas?    A definite pattern of disease distribution, such as the edge of a planting bed, or a low spot in the garden suggests abiotic causes like wind, drought, standing water, chemical drift, etc. although it does not rule out biotic diseases.  Are all the affected leaves at the bottom of the plant, or the top, or just one vine?  Or are they scattered throughout the entire plant?  Are the affected plant parts physically broken or crushed?

b. What parts of the plants are affected?  Roots, leaves, canes, branches, flowers or fruits?

c. Is only one type of plant affected or are many unrelated plants affected as well?  For example, not only the strawberry bed shows disease symptoms but the lawn looks bad too.

d. When did the disease symptoms first appear?  Most disease problems take time to develop.  Did the symptoms really appear overnight?  If so, suspect a climatic factor such as frost, hail, lightening or high winds.  However, if the condition started at one point of the plant or garden and spread slowly in extent and severity, a biotic disease factor is probably at work.

e.  Have any chemical applications been made, including fertilization or lawn herbicides?

f.  Are any insects present or damaged from other pests like mice, raccoons, skunks, birds, etc?  Are there any holes bored in the stem or trunk of the affected plant?

g.  If the affected plant is a tree, does the trunk go straight into the ground like a telephone pole or can you see the trunk begin to flare out into the root system of the tree?

h.  Is there any standing water in the area, or does the affected area recieve run-off water from the street?

3.  Determine the appearance of a "typical" diseased plant and what symptoms it displays.  Don't look just at the plants or leaves that have totally disintegrated from disease damage.  Look at the entire spectrum of symptoms from the first faint signs of newly infected plants, to moderately infected plants, and finally severely infected plants.  Also compare the infected plant parts to normal plant parts.  Normal plant parts are frequently mistaken for evidence of disease.  Evidence of disease may include any of the following symptoms:

a. Underdevelopment of leaves, shoots or stems including the failure of flowers and normal plant pigments to develop.

b. Overdevelopment of plant tissues.  Galls and witches brooms are good examples of overdevelopment.  The profuse flowering of plants suffering from some root diseases is another type of overdevelopment.

c. Death of plant parts including leaf spots, leaves, stems and fruits.

4.  Review the disease problems common to the plant species.  Compare the symptoms of your plant with the time of year of disease occurrance and the symptoms of these common problems.

Trees, Shrubs and Groundcovers- Ajuga to Forsythia

Trees, Shrubs and Groundcovers- Hawthorn to Mountain Ash (Sorbus)

Trees, Shrubs and Groundcovers- Oak to Zelkova

Turfgrasses/ Lawns

Diseases of Hebaceous Annuals and Perennials- Aster to Zinnia

Tree Fruits and Small Fruits

5.  Use the disease diagnostic key to pinpoint the exact disease problem.

	Tomato Disorder Identification Guide
	Vegetable Pest Diagnostic Key
	Turf Disease Diagnostic Key
	Recognizing Common Apple Diseases
	Fruit Pathology- Disease Diagnostic Key This is a good key for diagnosis of cherry, peach and apple diseases.

 

6.  Finally, professional horticulturists including nursery and greenhouse workers or Cooperative Extension personel can help identify disease problems if provided with all the information above and a fresh sample of the affected plant.