by J Benton Jones, Jr.

A Diagnostician’s Does and Don'ts

From many diagnostic visits, I have learned and established the “does” and “don’ts” required to be a successful diagnostician.

I was with a group of County Agents, consultants and chemical dealers, walking fields that had been variously treated with herbicides and fertilizers and now the plants were showing the effects of what can happen when such chemicals are misused. Walking into a soybean field with plants at about the 3-leaf stage, the leaf margins of most plants looked “burned,” the result of a late aerial application of a pre-emergence herbicide – that is what we were told.  The farmer asked me what the possibility would be for a manganese deficiency occurring later in the season. I collected leaf and soil samples for laboratory analysis to make this assessment. Receiving the assay results, the plants were found to be potassium deficient, with the potassium soil test level “low.” The problem visually seen that day was not herbicide-damaged soybean plants, but the result of a potassium deficiency whose typical leaf symptom is marginal “firing” of the leaves. I then asked the right questions, “what was the previous crop?”, and “what fertilizer had been applied for the soybean crop?” The previous crop was silage corn, and no fertilizer was applied for the soybean crop. In addition, the soil type was one classed as only “moderately fertile;” therefore, easily susceptible to potassium deficiency if not adequately fertilized.

A greenhouse tomato grower asked me to make a visit to determine why some tomato plants had leaves with dark “scars” on them. The greenhouse was glass-covered, the crop being grown in a home-made NFT hydroponic system. There were open water surfaces and the air in the greenhouse felt “humid.”  What was happening? With night cooling, condensation accumulated on the interior structure and water dripping from the joined structural pieces was falling on the leaves, causing the leaf “scaring.” The grower thought that there was a disease condition developing and was prepared to apply a fungicide to his tomato plants. 

First, one needs to know how to deal with bias. Initially, don’t listen to what others might say or tell you as to their evaluation of a crop’s developing characteristics and appearance. Go prepared with an open mind, be familiar with the cultural procedures and crop to be evaluated, and gather information that will refresh our mind on the cultural practices and crop to be evaluated. Make notes and refer to already existing information. Make a judgment and then begin to ask questions to either verify or eliminate causes. Collect plants, plant tissue, and rooting media for later examination and analysis, placing in suitable containers, and then properly store and transport to maintain their integrity. Look for the obvious, but also be aware that external factors need to be considered, such as past and current weather conditions. Use a spade to lift roots from the rooting media, carry a magnifying glass and small vials for collected insects. For field-crop evaluations, determine what exists upwind. “Scouting” is one form of diagnosis, mainly used for identification of insect type and population levels. But those who “scout” fields for insects are also keen observers who can be a valuable information source when you are making a diagnosis. Record your conversations with the grower and other observers, take photographs and/or video. Seek professional assistance when dealing with issues beyond your professional knowledge level and skills.

Called as an “expert” witness in a fertilizer dealer versus tomato grower court proceedings, it seemed that the outcome of the case was going to be determined solely based on testimony by those who were party to the individuals involved. At the time it was determined that the tomato crop was not going to produce high yield-quality fruit, no one took a soil or leaf sample for analysis, examined the plant roots, had an assay made of the applied fertilizer, or taken close-up photographs of individual plants.  Although two years had lapsed, I obtained past year’s soil test records, a soil sample and residue of the applied fertilizer for analysis, past weather records and data on applied fertilizer, amount and kind, and that being applied through the irrigation system that year. Weather conditions at planting and the time the crop was determined not to be economically harvestable, combined with past and obtained data, a scenario of the interacting factors that lead to the crop failure was formulated and presented to the court. An appropriate finding was then based only on the fact that the fertilizer type ordered by the grower was not supplied, but that its use did not contribute to the crop loss. If the court’s decision was only based on the testimony from those who viewed the crop, the fertilizer company would have had to pay for the crop loss, based on the erroneous conclusion that the use of the fertilizer supplied was the cause for the crop failure.

Most diagnostic ventures are like putting a jigsaw puzzle together, requiring all the pieces rightly fitted together to complete the picture.  Experience is probably the best teacher. A misdiagnosis can be costly, so having all the facts, piecing them together that makes sense, is the diagnostician’s challenge, but its reward is comforting when an effect is properly attached to its cause.