University of Kentucky plant pathologists say they continue to get questions on whether to treat cornfields with a foliar fungicide as a potential yield booster even in a dry year where disease problems are not prevalent.While in soybeans, there is evidence that certain strobilurin fungicides sometimes enhance yield even when diseases are not present at a level sufficient to affect yield, in corn, the research base to date is much more mixed, said Paul Vincelli, plant pathologist with the UK Cooperative Extension Service.“Most of the studies I’ve seen, including a number conducted in Kentucky, show no statistically significant improvement in yield from foliar fungicides applied to corn in trials when disease activity is minimal through grain fill,” he said. “This past week I obtained data from a neighboring state showing some field trials where a yield increase was observed from strobilurin fungicides when disease activity was low. However, these studies represent less than half of the total number of studies I’ve reviewed. “Furthermore, only a fraction of the total number of trials I’ve reviewed showed yield increases that would more than pay for the cost of the application at $4 per bushel corn prices. In those cases where these yield increases occurred there was no common denominator that would allow one to predict whether a field would benefit by a fungicide application in the absence of disease. Finally, it should be noted that the use of fungicides for general yield enhancement, even though disease activity is below yield-threatening levels, probably increases the risk that fungicide resistance might eventually develop.”These products are excellent for controlling gray leaf spot and northern leaf blight, but incidences of these diseases are very low this year. Dry conditions are not conducive to the proliferation of these diseases.According to rainfall maps at the UK Ag Weather Center, counties west of Interstate 65 received substantial rainfall during the past week, as did other parts of the state, Vincelli noted. Cornfields approaching and at tasseling benefited greatly from this rainfall, as silking is the most sensitive stage of corn to drought. Although it is not apparent how much yield loss may have already taken place in some fields, last week’s rains came at a critical time for many cornfields in western Kentucky and probably did little to “kick-start” diseases. “If there is no disease threat at tasseling, it seems to me, based on research I’ve done and seen from other states, that the threat to grain fill from these diseases is low, especially if the hybrid has some resistance,” he said. “I cannot predict what the conditions will be for the next eight weeks, but if you are not seeing disease at tasseling, it is likely to not be a major factor affecting yield.” Many questions remain about foliar fungicide use in corn, Vincelli said. But his best assessment is that routine spraying of cornfields doesn’t look advantageous for this year. Some fields might benefit, however, depending on how many of the risk factors are present. Risk factors include high-value specialty corn production, disease-favorable weather, disease activity at tasseling, irrigation, high yield potential, late planting, no-till production, continuous corn production and susceptible hybrids.“If I were a producer planning to spray, I would try to arrange to leave one or more untreated strips,” he said. “An unreplicated, untreated strip is not a valid experiment, but at least it would give the producer a crude indication as to whether they might have gotten some benefit from the application. Even crude information is better than no information, and no information is what the producer will have if he treats the entire field and leaves nothing untreated.