Long-dormant disease becomes most dominant foliar disease in New York onion crops

New York is the fifth largest onion bulb producer in the United States, with a production of more than 110,000 metric tons on more than 2,800 hectares. Most of these onions are grown on soils with high organic matter content, where leaf disease management is crucial for productivity and profitability. These leaf diseases include Botrytis leaf blight, leaf spot, blight, late blight and Stemphylium leaf blight.

Stemphylium leaf blight (SLB) begins as small tan to brown lesions that coalesce and lengthen leaf length. These lesions turn dark olive brown to black, contributing to the loss of the green leaf surface, which can affect bulb size and quality and prevents the tops of the plants from lodging naturally when the bulbs are fully mature.

Until recently, GLS was considered a minor foliar disease as it has not caused much damage in New York City since the early 1990s. However, New York onion growers have recently seen an increase in crop dieback, and scientists at Cornell University were surprised to discover that GLS was the cause.

"We set out to determine the cause of the decline problem," said Frank Hay, one of the Cornell scientists. "One surprise was that Stemphylium leaf blight had long been considered a minor onion disease but had become the dominant leaf disease in New York onions. "

Hay and his colleagues discovered that the re-emergence of LBS occurred as the disease developed resistance to fungicides. Their research led to the withdrawal of some fungicides from commercial production as widespread resistance developed in the field.