UC weed scientist demonstrates symptoms of herbicide spray drift Unwanted vegetation has been an agricultural nuisance since farming supplanted gathering 10,000 years ago. When herbicides became widely available to farmers in the 1940s, it lightened the drudgery of the hoe. But this inexpensive and effective weed management tool isn’t without repercussions, one of which is accidental drift from a field being treated with an herbicide to a nearby crop where the herbicide can cause harm. Farmers can take measures to prevent pesticide drift, such as avoiding herbicide applications when it is windy, using nozzles that create relatively large herbicide drops that are less likely to float, and carefully positioning the spray nozzles to direct the herbicide only where it is needed. Nevertheless, farm advisors and pest control advisers are regularly asked to diagnose plant symptoms that might have resulted from pesticide drift. Improving their knowledge of pesticide drift symptoms is the goal of an annual project of UC Davis Cooperative Extension weed ecologist Tom Lanini. Each year, he deliberately sprays dilute mixtures of a variety of herbicides onto trees or vines in order to observe the consequences. Last month, a soon-to-be removed collection of peach trees at UC Davis was treated with very low rates of 15 different herbicides. Farmers and pest control advisors were invited to browse through the orchard about a week after the treatments to see what happened to the trees. Three herbicides that cause significant drift symptoms Three herbicide treatments resulted in particularly dramatic symptoms – Rely 200, Gramoxone and Shark. Rely 200, with the active ingredient glufosinate, is marketed by Bayer Crop Science as an alternative to glyphosate products (like Roundup) when glyphosate-resistant weeds are present. The peach tree treated with a low rate of Rely had symptoms that could easily be confused with shot hole disease, Lanini said. Shot hole is caused by a fungus that infects trees typically after a warm, wet winter or after a prolonged wet spring. “With shot hole, the symptoms go all the way through the leaves, but the Rely damage looks like a bullseye,” he said. Syngenta’s Gramoxone, which contains paraquat, is a contact herbicide that kills plant tissue only where it comes into contact with the trees. Areas of the trees not touched by the spray drift are unaffected. Shark, an herbicide marketed by FMC Agricultural Products that contains carfentrazone, produced the most dramatic symptoms in the trial. Not only were the leaves damaged, the fruit itself was covered with unsightly brown spots.
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Department of Agriculture and Natural Resources, University of California. All contents November 20, 2009 Regents of the University of California. All rights reserved. |
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