Aids in Diagnosing Plant Diseases

Effective disease control depends primarily on early, accurate identification of the disease and the causal agents. In most cases, it is too late to control a disease on a plant once the disease appears. However, timely control measures can prevent the disease from spreading to other plants.

Field diagnosis of a disease problem involves some detective work looking for symptoms, signs and patterns. Symptoms are the plant's response to certain stresses which may be caused by the environment or by a pathogen. Signs are actual parts of the pathogen such as fruiting structures, galls, ooze or mycelium (mold). The pattern of the disease in an area provides many clues about the cause. Compiling all of this information will help in correctly identifying the problem.

STEPS IN FIELD DIAGNOSES:

1. Identify the affected plant. Determine the scientific name as well as the common name because common names are frequently used for distinctly different plant species. For example, there are many different species of pine trees each with different plant diseases unique to that species.

2. Determine what diseases have been reported to occur on the plant being examined. Consult Utah plant disease control bulletin No. 13 and books such as:

Diseases and Pests of Ornamental Plants -- P.P. Pirone

Vegetable Crop Diseases -- C.R. Dixon

Westcott's Plant Disease Handbook -- K. Horst

Diseases of the Forest and Shade Trees of the United States -- USDA Handbook No. 386

How to Control Plant Diseases in Home and Garden -- M.C. Shurtleff

Fungi on Plants and Plant Products on the United States -- D. Farr, G. Bills, G. Chamuris, A. Rossman.

Ortho Problem Solver -- Ortho Chemical

APS Compendium of plant disease series: Alfalfa, Apples and Pears, Barley, Bean, Beet, Blueberry and Cranberry, Chrysanthemum, Citrus, Corn, Cotton, Cucurbit, Elm, Flowering Potted Plants, Grapes, Onion and Garlic, Ornamental Foliage, Pea, Peanut, Raspberry and Blackberry, Rhododendron, Rice, Rose, Sorghum, Soybean, Stone Fruit, Sweet Potato, Tobacco, Tomato, Tropical Fruit, Turfgrass, and Wheat diseases.

Diseases of Temperate Zone Tree Fruit and Nut Crops -- J. Ogawa, H. English

Diseases of Trees and Shrubs -- Sinclair, Lyon and Johnson.

Diseases of Turfgrasses -- H. Couch

3. Compare the diseased plant with a healthy plant growing nearby to better assess symptoms and signs. Some normal plant parts are frequently mistaken to be evidence of the disease. For example, the leaves of a healthy sycamore are covered by a conspicuous fuzz which is confused with fungus mycelium. Small galls on the roots of legumes such as beans and alfalfa may be normal nitrogen-fixing nodules.

4. Determine the distribution of the disease within the field or garden. Is there more than one species of plant affected? If so, the disease is probably caused by climate, chemicals or some other cultural factor. If the condition is uniformly distributed in a low spot in the field or only at the edge of a planting, a soil or water factor or toxic chemical should be suspected. Parasitic diseases progress with time and rarely infect 100% of the plants in an area. When the problem affects all of the plants in a particular area, the cause of the problem is probably a deficiency or excess of a soil nutrient; The result of drought, frost or hail; or a toxic chemical such as herbicide or air pollution.

Plant pathogens rarely cause a condition to appear suddenly. They usually begin at one point and spread slowly to other plants. If the symptoms show up "overnight" or in one or two days, suspect a climatic factor or toxic chemical.

5. Has the same crop or plant been grown in the area in previous years? Were there problems growing other species of plants in the same location? Have any herbicides or chemicals been used in the area? Have there been any unusually cold, hot or wet climatic conditions in the past
few days?

6. Many above-ground symptoms are the result of root rots. If leaves are small, yellow or show wilting; or if the plant has poor terminal growth and very little fruit or flower production, then root rot should be suspected. Most of the plant diseases in Utah are caused by soil-borne pathogens. Dig around the base of the tree looking for dead roots or dead areas in the bark. Smaller plants (e.g. flowers, vegetable plants) should be completely removed and the roots examined. Healthy roots should be white or cream color. If they are brown or black, the plant probably has root rot. See Plant Disease Control Nos. 6 &21.

7. Premature dropping of needles by conifers is a frequent concern of many urban dwellers. Conifers normally retain their needles for 3 to 6 years and gradually lose the oldest needles each season. However, drought or some other climatic factor may accelerate needle loss so that the entire tree will appear yellow. There is generally no need for concern if only the oldest needles are dropping. See Plant Disease Control No. 11.

8. Look at all parts of the plant. Are the symptoms only present on the leaves, stems, flowers or fruit; or is the entire plant involved?

9. Consult the following table to determine some possible causes of poor plant growth. If the cause of the condition cannot be readily determined, it may be necessary to request help from your county agent. Sometimes the symptoms and signs are not characteristic enough to allow an accurate diagnosis of the problem. In these cases, a sample may have to be taken to isolate and identify the causal agent.

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

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use. Always read and follow the label.