EAST LANSING, Mich. – Downy Mildew (DM), a fungus-like pathogen that can kill cucumber plants, was first detected this growing season in mid-July, leading to worries that it could harm the pickling cucumber crop this year.

The first confirmation of DM in Michigan for 2021 was discovered July 14 in a Saginaw County field, in the state’s Thumb region. A second case was discovered in Saginaw County on July 19, as well as cases in Tuscola and Sanilac counties, in the same region.

On July 22 DM was detected on Michigan’s west side, in Ottawa County. The disease travels via spores through the air and can cause yellow-brown square lesions on each side of the leaf near leaf veins. Seen up close, DM affected leaves can sometimes appear to be infected with common black mold.

Humid and overcast weather in Michigan has also been favorable to the growth and spread of DM in Michigan. The first harvest of pickling cucumbers came off without any big problems, however, experts looking at the situation are concerned that subsequent harvests may not be the same. Growers who raise cucumbers tend to stagger the pickling cucumber crop to create a relatively steady flow of cucumbers to processors, said Mary Hausbeck, a distinguished professor of plant, soil and microbial sciences at Michigan State University (MSU).

Historically, Michigan growers produce over 1.4 million tons of cucurbits valued at about $83 million on 43,000 acres. Michigan ranks number one in the nation for production of pickling cucumbers and is in the top six for fresh market cucumbers and fresh market and processing pumpkin and squash. Downy mildew affects all cucurbits, including cucumber, watermelon, cantaloupe, honeydew, zucchini, gourd, summer and winter squash and pumpkin. The disease reemerged as a problem in Michigan cucumbers in 2005 when the disease spread across the eastern region of the United States and has occurred each year since then.

Downy mildew causes yellow lesions on the top of infected leaves, with a gray to black fuzz on the underside of the leaf giving a somewhat dirty or black velvet appearance. Downy mildew can cause catastrophic losses of cucumbers in a brief period. Unprotected foliage can become completely blighted within 14 days of the initial infection. Late last month Hausbeck said the infection was ongoing. “As of today, we have nearly all the major growing regions confirmed with downy mildew,” she said. “We have quite a cluster in Saginaw, Tuscola and Sanilac counties. All these positives are from large pickling growing operations. It’s an important crop for us.”

Cucumber downy mildew has also been found at MSU’s research plot, located in Ingham County. The DM find in Ottawa County on the west side of Michigan is the only place so far where DM was detected in a non-commercial cucumber patch. Most of the disease has been found in the greater Thumb region, with the only infected commercial crop on the west side being Allegan County. The other Thumb region counties where DM has been detected so far include Bay, Midland and Arenac.

“Downy mildew comes early this year across the state for later pickling cucumber crops,” Hausbeck added. “With the disease ramping up, we still have August and early September to battle downy mildew. We are also having favorable weather conditions for downy mildew and have fewer fungicides than ever to battle the disease.” To keep up to date about the ongoing situation, check out Hausbeck’s plant pathology research lab web page at, https://veggies.msu.edu/downy-mildew-news.