Aug 5, 2022
Conditions could lead to escalation of cucurbit downy mildew in Michigan

The first symptoms of cucurbit downy mildew disease on pickling cucumber plants in commercial Michigan fields has been found by Michigan State University’s Hausbeck Plant Pathology Research Lab in Muskegon and Allegan counties.

The statewide spore trapping network to detect downy mildew spores in the air has positives for six counties from the sampling period that ended July 17, in Bay, Allegan, Monroe, Muskegon, Ingham and Berrien counties.

“In all of the years that our lab has run this spore trapping program, I’ve not seen so many counties have positives from the air samples all at once,” MSU plant pathologist Mary Hausbeck said in a report distributed by Michigan Farm Bureau. “This indicates an influx of downy mildew sporangia in the state at a statewide level.”

The lab’s Downy Mildew Spore Trap webpage is updated with the latest results of confirmed positives for field disease and spore trap air samples. The results from the spore traps are a few days behind due to processing of the spore tapes.

The lab can distinguish between cucumber and hop downy mildew spores using molecular tools and both are reported. To see spore trap results and current downy mildew news, visit the lab’s webpage.

“I believe that the delayed downy mildew development in the state is the result of the high temperatures and dry conditions that we have been experiencing,” Hausbeck said in the report.

Because the cucurbit downy mildew pathogen (Pseudoperonospora cubensis) prefers cooler temperatures, the current moderate temperatures, along with recent rainfall, has set the stage for a rapid escalation of the disease in Michigan if fungicide sprays are delayed.

Suspect samples should be submitted for a diagnosis. See instructions on how to submit samples.

Photo: 1, 2: Early symptoms of downy mildew on cucumber with the yellow-brown tissue bordered by the leaf veins. Photos 3, 4: The dark spores of the cucumber downy mildew pathogen can be seen on the underside of the leaf. Photo: David Perla, MSU

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