Jul 7, 2015
Downy mildew on cucumbers detected in key areas of Michigan

It looks as though Michigan is on track for a difficult downy mildew year.

This highly destructive and difficult to manage disease has now been documented in three regions of the state including Monroe, Gratiot and Allegan counties. The rapid spread of the pathogen this summer is reminiscent of 2005 when downy mildew spread from one side of the state to the other in a matter of 10 days. This year, the disease was first detected on cucumbers in Monroe County June 22, then in Gratiot County July 3 and most recently in Allegan County July 7. Weather plays a significant role in the development of this disease and the especially wet weather this growing season is especially favorable for downy mildew development. Under this type of weather pattern, cucumbers and melons could become extremely blighted within 10 days if not adequately protected. The blighting from downy mildew can happen so quickly that growers in the past have reported their crop appearing to have been “frosted.”

Time is of the essence with downy mildew and immediate action is urged to protect those crops most susceptible to the disease. In Michigan, cucumbers and melons, including watermelons and cantaloupe, have proven to be much more susceptible to downy mildew than other cucurbits such as pumpkins, squash and zucchini. Cucumbers and melons should be sprayed every four to seven days with downy mildew fungicides. Unfortunately, there are limited products available that can limit this disease, especially when the weather is rainy and wet.

My lab has evaluated new and existing products for downy mildew control each year since 2005 to determine the most effective fungicides. Through these unbiased trials, growers can know if products have remained effective or if alternative products should be used. Last year, many products registered for downy mildew were included in our research. Results showed that Ranman at the high-labeled rate performed significantly better than the other registered products included in the study. Zampro, Previcur Flex and Bravo WeatherStik (at the 3-pint rate) were similar in their ability to limit downy mildew. Zampro was not as effective as Ranman (at the high rate) early in the season, but was significantly better than Gavel, Tanos, Manzate, Presidio and Bravo WeatherStik (at the 2-pint rate). Zing! was not included in the trial last year to compare with other fungicides standards, although the active ingredients should be helpful in managing the disease.

In the fungicide research conducted last year, the downy mildew specific fungicides did not appear to be as effective in limiting downy mildew as they used to be. This could be because the pathogen has changed and is not as responsive to the downy mildew specific fungicides or the disease pressure may have been especially severe in 2014 due to the weather. These trial results have been used to develop the recommendations for 2015 that are listed below that can serve as an example of a spray program to consider for this season. The suggested spray interval is four to seven days and depends on whether the disease has been found in your county and if the weather is wet. If downy mildew has been detected in your county and you’ve experienced unusually wet weather, a four-day application interval is justified. Regardless, the application interval should not exceed seven days.

Spray 1: Ranman (high rate: 2.75 fluid ounces) + Bravo WeatherStik (high rate: 3 pints)
Spray 2: Previcur Flex + Zing!
Spray 3: Zampro + Bravo WeatherStik (high rate: 3 pints)

For pictures and more information, click here. You can also stay up-to-date on cucurbit downy mildew news, including spore counts and research.

Mary Hausbeck, Michigan State University Extension

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