Downy Mildew Outbreak in Cucumbers

In Michigan, a tremendous, statewide outbreak of downy mildew on cucumbers has growers of all vine and cucurbit plants scrambling to protect their crops. In late July, nine counties confirmed downy mildew reports in a week, with five reporting in just one day. At press time, three more counties in the state reported downy mildew outbreaks, according to Mary Hausbeck, a plant pathology professor and Extension specialist with Michigan State University. Hausbeck’s lab monitors downy mildew sporangia in the air as it comes in and she says the main concern is over how quickly the disease is spreading.

Previously, downy mildew was not a concern, especially for growers of cucumbers, because of advancements in breeding technology; however, the pathogen seems to have changed, making all cucumbers susceptible, Hausbeck says.

Surrounding states in the Midwest — Ohio, Indiana, Illinois, and Wisconsin — are reporting downy mildew outbreaks on cucurbits (melons, squashes) and other vine crops, and downy mildew is showing up in the Northeast, Southeast, and most recently in California, reports Hausbeck.

“This is a significant issue, as we have a downy mildew outbreak of major proportions this year; it is more widespread more quickly than in previous years,” says Hausbeck. “Conditions are so favorable for the spread of downy mildew that we can see it’s going to be difficult to control.”

With Michigan being the number one state for pickling cucumbers, the concern is widespread and growers in the state are on high alert to protect their crops. A July 29 Vegetable Crop Advisory Team Alert implored growers to immediately treat all cucumbers and melons across the state with fungicides, while closely watching all other vine crops for peculiar symptoms. While cucumbers and melons are the most susceptible to downy mildew, the Alert reminded growers that although pumpkins, winter and summer squash, zucchini, and gourds have not readily developed downy mildew in past years, the atypical weather has been extremely favorable for downy mildew development, so those crops are not immune from the disease.

Fungicide recommendations can be found at http://plantpathology.msu.edu/labs/hausbeck/ForGrowers.html.

On The Phytophthora Front

While Phytophthora has been the more recent newsmaker due to the wet weather parts of Michigan have been experiencing this year, and continues to be a significant problem, the widespread outbreak of downy mildew is “stealing the thunder from Phytophthora” this year, Hausbeck says. The two pathogens are related, as both are water molds that spread in very wet conditions. The summer has already seen a number of nights with heavy dew, which is very conducive to both downy mildew and Phytophthora. Typically heavy dew nights begin toward the end of August, says Hausbeck.

“We continue to make strides in understanding how Phytophthora spreads, with new tools and fungicides available to help growers manage the pathogen,” Hausbeck says. Two of the newer fungicides available for Phytophthora and downy mildew control are Presidio (fluopicolide) from Valent U.S.A. and Revus (mandipropamid) from Syngenta Crop Protection, says Hausbeck.

“These are products that we have tested and continue to test and find that they are helpful,” she says. “We are also working with genetic resistance to see if we can use more of the plants’ genetic make-up to defend against Phytophthora in peppers and tomatoes.”

In a research program through Hausbeck’s lab, three graduate students are currently working in conjunction with growers to help move the industry toward new solutions, she adds.

“In our recent research, many of our key studies are being conducted with grower cooperators where we come onto their farms, and we are so appreciative that we have such good collaboration with the industry,” Hausbeck says. “We are very grateful to the good growers who always step up to the plate and help us work to find solutions.”