Being a good fruit or vegetable grower means becoming a good observer of plants and knowing how the culture, insects, diseases and weather all interact in producing the final outcome as to why the plant doesn’t look or function in a certain way.
Diagnosis is the art of recognizing plant problems from their symptoms, a determination of a disorder based on critical observation. Sometimes, diagnosis is fairly clear-cut and simple. When pests are obvious and easily identified, the problem may easily be solved, but disorders are not always easy to identify, especially when they appear for the first time in the experience of the observer.
This is why it is absolutely imperative to answer as many questions as you can about the plant, such as, where it was growing. For example: exposure, soil type, light conditions, fertilizer given, chemicals used, etc., so that you can rule out the things that did not cause the problem and narrow it down to the things that might have. You need to recognize the condition of the plant you are looking at, and by doing so, will strengthen your own powers of observation.
To make a diagnosis, it is necessary to know something about the environmental stresses, pathogens, parasites, and other problems, which may be common to an area. You need to understand basic procedures of diagnosis, know what background information to seek and how to interpret it. In other words, you need to set up some steps that you must go through to ensure success.
What is the plant?
Do you have a representative sample? What does a healthy specimen look like?
Careful close examination of the sick plant, along with other plants in the general area, will determine how they are different. Evaluation of symptom expression will aid in determining what is wrong with the plant.
Do you know how the crop was produced? (Soils, irrigation program, fertilization, fungicides, insecticides, herbicides, and other cultural practices.)
What has been the weather history? (Drought, hail, flooding, winds, frost, lightning, etc.)
How is the problem distributed over the planting? Does it affect all the plants or only a few? Is it happening in low-lying areas or is the topography of the land not important? Differences in soil type, drainage, or water-holding capacity, as well as aeration and nutrient-holding ability may cause plant disorders. If only one plant or a few plants in a small area show symptoms, then a local disturbance can be suspected. Consider soil, climate, chemicals, parasites, etc. If the presence of symptoms is seen on more than one kind of plant, this is a clue that the disorder is the result of some kind of environmental factor.
What is the plant microclimate? (Wet areas, shady, dry areas, different soil, etc.)
Observe and be able to recognize symptoms. Never miss them. A symptom is any change in the known structure, appearance or function of a plant population, which causes it to be different from normal, including any external abnormalities such as leaf discoloration, malformations or other obvious variations, which are unnatural.
What kinds of symptoms do you see? Leaf spots, galls, wilting, cankers, plant elongation, twisting, distortion, etc. are examples of symptoms.
What part of the plant is not functioning properly? Roots, leaves, flowers, etc. If the entire plant appears to be dying uniformly, chances are that there is something wrong with the roots or crown. When only a part of the plant is affected, the problem may be localized.
What is the history of symptom development? How long has it been going on?
Abnormal growth or disorders may be caused or strongly influenced by the growing conditions and specific management practices to which plants have been exposed. Plants do not always respond to stress immediately, and symptoms may not show up for weeks or months after the damage occurred. This is especially true for trees. Therefore, it is absolutely necessary to know the plant background before making a diagnosis.
Observe and recognize signs. A sign is finding a causal agent, such as an insect or insect remains, fungal leaf spot or lesion, etc. This is a sure means of diagnosis. Sometimes, the causal agents may have disappeared by the time the plant symptom is observed. You should be able to recognize most common diseases on sight.
Knowledge of crop rotation or the lack of it is necessary in diagnosing parasitic diseases and is also useful in diagnosing environmental disturbances. Parasites may build up in one crop and cause serious damage to subsequent, more sensitive crops.
Herbicides used on one plant may accumulate in the soil and cause damage or completely retard growth of other plants.
Pest control success may be determined from the absence or presence of various insects, diseases or weeds.
When all of the above factors are considered, then a person should be able to arrive at a conclusion as to the causal agent. If the causal agent is determined to be non-parasitic, appropriate management steps should be undertaken to correct the problem. If, on the other hand, the cause is determined to be due to an organism (plant pathogen), the next step would be the development of a pest management strategy.
Can you recognize symptoms vs. signs?
Insects and diseases leave signs or evidence that the causal agent is now there or has been there before. A symptom is how a plant responds physically to an attack by an insect or disease. Here are some examples:
Honeydew is a clear, sticky liquid that is made up of undigested plant fluids that aphids excrete (sign). This attracts ants, which like to feed on the honeydew (sign). The sooty mold fungus, which is a black deposit that covers leaves, stems and twigs, attaches itself to the honeydew, which provides a good food source for the fungus (sign).
Tip burn or hopper burn caused by leafhoppers rasping the tips of leaves or needles, causing them to turn yellow or brown (symptom).
Galls are swellings of stems or leaves (symptom) that come in various sizes, shapes, colors or textures (smooth, hairy, or warty), which usually contain larvae of beneficial wasps that parasitize insects (sign).
Curled leaves, (symptom) especially on new growth will reveal aphids feeding inside (sign).
Twisted and/or distorted leaves and stems (symptom) can be caused by herbicide spray drift of phenoxy herbicides such as 2,4-D. Tomatoes and grapes are excellent indicator plants for this damage.
Holes in foliage and fruit is usually due to feeding of chewing insects, such as cabbage looper on cabbage, Japanese beetle feeding on grape, etc. (sign). Leaf miners feed between leaf surfaces such as the boxwood leaf miner (sign). Leaf skeletonizers devour the area between the leaf veins, leaving the veins behind (sign).
Scale insects can vary from pinhead size to an eighth of an inch or larger sucking insects living under a protective shell. They can be white, rust colored or dark, and come in many sizes and shapes on bark, leaves and fruit (sign).
Spittlebugs produce a white froth from the undigested plant juices (sign). The insect can be found inside the froth (sign).
Wilting of plants is a symptom that can be caused by low soil moisture, or waterlogged soil, which can deplete oil oxygen, causing root rots and then the plant wilts. Wilting can also be caused by insect feeding and the transmission of bacterial diseases, causing the plant to wilt. Larval stages of borers can enter plants, causing them to wilt (symptom).