"Preventative spray applications may do more harm than good"

Spring is a busy time when our Diagnostic Lab receives many plant samples from growers and landscape professionals requesting plant problem identification and management advice. Some of these inquiries are clearly related to insects, mites, or disease-causing organisms. In a few cases, however, the cause(s) are not so clear, so we draw on experience and insight, as well as detailed information on the plant history.

by Dan Gilrein

Besides the usual factors of pH, fertilizer rate and type, and environment, it helps to know the crop’s treatment history. Some plants have known sensitivities to specific pesticides (insecticides, miticides, fungicides, herbicides, plant growth regulators) and this even includes some organic-compatible products. There have also been issues with certain products used in tank-mix combinations or sequentially over a short period of time. It is almost a rule that problems tend to show up about 10-14 days after the offending treatment was made, though of course sometimes they can be seen much sooner. Symptoms can appear as leaf spots, yellowing, distortion, stunting, marginal burn, and more; the distribution within the crop can also be a telling part of the story.

Spray injury to impatiens flower

After examining application records in many (but not all) such cases I have noticed preventive applications being used without clear logic or reasoning. While I appreciate the added sense of reassurance or ‘insurance,’ this of course comes with a price and little real benefit. Worse, I can often pick a likely culprit or two out of the treatment history which might explain the source of the problem. To help growers in making better judgments throughout production I compiled and made available information on known plant sensitivities to insecticides and miticides, culled from product labels, IR-4 Project data, and experiences.

Given the huge array of crops and growing conditions we certainly don’t have all the answers, but the information is still useful. It is also important to know what these different products can do – and not do, what their various strengths and weaknesses might be. Some are very effective against one or several pests or are mainly suppressive, have very short residual activity, require addition of an adjuvant, show good compatibility with biological controls, etc. Labels are the first source for information, but growers can get more specifics from other growers, dealers, tech reps, Extension and other specialists familiar with these products. The third piece of the puzzle is knowledge of problems particular plants are prone to (twospotted spider mite? Western flower thrips? Potato aphid?) and the plant production history (e.g. originally from cuttings or seed?). Putting all three together, growers can rely less on perception and make sound judgments that maximize benefits and reduce liabilities and risk.

Source: eGroBlog