Whether in an effort to save existing plants or to prevent plant disease problems from recurring, it is important to know “What went wrong?” Diagnosis is the process of gathering information about a plant problem and determining the cause.
Once the cause has been determined, it is then possible to recommend a solution or remedy. Diagnosing plant disease problems can involve considerable detective work.
Sometimes there is insufficient information and other times, the primary cause of a problem is hidden by more obvious, but less important, problems. Success in diagnosing plant problems depends on how much we know about the host plant, the plant problems in general, and the quality of information obtained from the client.
For example, 10 tomato plants all similarly damaged are brought to you. All have yellow leaves, stunted growth, and very few feeder roots. You learn from questioning the grower that he applied one-half of a 20- pound bag of 10-10-10 fertilizer to a tomato plot that measures 60 square feet.
The grower put 10 pounds of fertilizer on 60 square feet, which translates to a rate of 166 pounds per 1,000 square feet. This is almost 10 times the normal rate of 20 pounds per 1,000 square feet of a 10-10-10 fertilizer.
The grower’s fertilization rate is enough to kill fine feeder roots. The diagnosis is damage to the roots caused by over-fertilization.
Read Also: Plant Diseases Caused by Abiotic (Non-living, Nonparasitic, or Non-infectious) Agents
Basic Steps in Reaching a Plant Disease Diagnosis
1. Identify the plant – The better your plant identification skills the faster you will be able to diagnose a problem. Most references on plant pests and diseases are organized by plant, so knowing the plant is the essential first step in using many reference books.
2. What is normal? Your familiarity with the normal appearance and cultural requirements of the plant will enable you to differentiate normal changes from symptoms of a problem.
3. What is the problem? To make a disease diagnosis, you need to know: the pattern of distribution of the diseased plants or plant parts, the plant species or cultivar involved, the site where the plant is growing (field, orchard, garden, greenhouse, inside the house, etc.), and previous crop history of the site.
For example, uniform damage to many species in an area, to all plants on one side of the field or garden, or to all shoots on one side of the tree indicates that the cause may be an abiotic factor. Also, if the damage is well-demarcated in a garden or in a plant, it may suggest that some abiotic factor is involved.
On the other hand, if there is evidence of the progressive spread of the disease from an initial focus to other plants of the same cultivar or species or to different parts of the plant, it may indicate that an infectious agent is involved.
4. Examine the plant and note symptoms and signs – For a presumptive diagnosis of diseases in plants, look for the symptoms and signs of the disease.
The characteristic internal or external alterations of a plant in response to a disease-causing agent are called symptoms (leaf spot, necrosis, blight, canker, wilt, lesion, gall, witches’ broom, rot, chlorosis, mosaic, etc.).
Sometimes, the pathogen that causes the disease produces its own character growth or structures on the diseased plant that are of diagnostic value. These are referred to as signs of the disease (mold, mildew, sclerotia, mushrooms, conks, etc.)
Read Also: Plant Diseases Caused by Living (Biotic, Parasitic, or Infectious) Agents
5. Tentative diagnosis – Based on your knowledge of the plant and information from reference books, formulate a tentative diagnosis. This will help you focus your examination of the plant and assist in collecting relevant information.
6. Double-check the diagnosis – Once you have arrived at a diagnosis unless it is an obvious diagnosis, double-check it. Ask other master gardeners or extension educators for their opinions. Read through the reference books about your diagnosis to make certain everything matches. Additional laboratory work may be needed to confirm your diagnosis.
7. Types of plant disease diagnosis – Verbal descriptions by a telephone call or evaluation of a sample provide the most common diagnostic opportunities. However, a site visit provides more complete information.
To make a telephone diagnosis, you must completely rely on information provided by the caller in order to make your diagnosis. There will be common, familiar problems, such as powdery mildew of apples, when a little information easily leads to a correct diagnosis.
In other cases, it will be very difficult to make a diagnosis over the telephone and it may be necessary to evaluate a sample.
Much of your diagnostic work will be done with plant samples. Usually, the sample will provide the clues necessary to solve the problem. But when the sample only confirms the identification of the plant, you must concentrate on acquiring information to reach a diagnosis.
Your job will be to learn about the plant’s cultural and environmental conditions, the care the plant has received, and whether the sample is representative of the problem affecting the plant.
A site visit provides the greatest opportunity to gather information, but a successful plant diagnosis also depends on a combination of factors: your knowledge of the plant involved, your understanding of the plant’s basic cultural requirements, and your recognition of the potential problems that might affect it.
It also depends on your ability to gather information, both through observation of the plant and discussion with the client.
Read Also: Guide to Plant Diseases and Disease Management
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