Newswise — ITHACA, N.Y. – Five years ago, onion growers in New York state started reporting large incidents of premature leaf death in their fields. It affected nearly 75% of growers’ crops and put a dent in the state’s onion industry, which has a four-year average value of $44.7 million, based on data from the U.S. Department of Agriculture. Researchers identified the culprit as Stemphylium leaf blight (SLB), a relatively new and increasingly devastating disease.

A recent study, published in the journal Plant Disease, researchers at Cornell AgriTech have identified better ways to manage SLB and use fungicide more effectively.

“Onion growers in New York are completely reliant on fungicides for managing SLB,” said co-author Sarah Pethybridge, associate professor of plant pathology and plant-microbe biology. “The development of fungicide resistance means growers are faced with a rapidly reducing set of fungicides with which to rotate, leading to increased selection pressure for resistance in the fungicides remaining.”

The increase of fungicide resistance is a major reason why SLB has recently become the dominant foliar pathogen in onions. The disease first appeared in New York in 1985 but only caused significant damage during the 1990, ’91 and ’92 growing seasons, before re-emerging in 2011.

That’s when study co-author Christy Hoepting, an extension vegetable specialist at Cornell Cooperative Extension in Orleans County, started receiving calls from concerned growers about premature leaf death and smaller onion bulbs. She realized the symptoms were similar to SLB, so she sent some field samples to Pethybridge and the study’s lead author Frank Hay, senior extension associate in the School of Integrative Plant Science.

In the laboratory, Pethybridge and Hay grew samples of the fungus on artificial media and exposed them to varying concentrations of each fungicide. By evaluating how different fungicides inhibited the pathogen’s growth, they could determine which fungicides were most effective against SLB.

The researchers plan to develop molecular and genetic tools that will help detect fungicide resistance faster, and they hope to identify a disease prediction model to accurately guide growers’ fungicide use.

This research was supported by funding from the USDA National Institute of Food and Agriculture, a USDA Specialty Crop Block Grant and the Federal Capacity Fund Initiative.

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