Influx of cucurbit downy mildew sporangia detected in eastern Michigan

Cucumbers and melons are at risk for downy mildew. Protective fungicides should be considered.

Michigan growers of cucumbers for slicing and pickling have battled downy mildew caused by the Pseudoperonospora cubensis pathogen each year since 2005. Prior to 2005, cucumber varieties were genetically resistant to this destructive blight and fungicide protection wasn’t necessary. In addition to cucumbers, other cucurbit crops are also at risk for this downy mildew and include cantaloupes, pumpkins, watermelons, gourds, winter and summer squashes and zucchinis.

In Michigan’s trials, cucumbers and melons, including cantaloupes and watermelons, appear to be far more susceptible to downy mildew than the other cucurbits such as pumpkins, squashes, gourds and zucchinis. The downy mildew pathogen is not able to overwinter in any region that experiences a hard frost and it cannot withstand Michigan winters, even with the relatively mild winter temperatures of 2012.

The cucurbit downy mildew is different than the downy mildews that infect other crops such as cabbage, basil, impatiens, roses, broccoli, etc. In fact, the downy mildew pathogens are each quite specific to a particular plant or crop type. The recent Michigan outbreaks of downy mildew on cabbage seedlings, greenhouse basil and impatiens have nothing to do with each other or cucurbit vegetables.

cucmber leaf with downy mildew
A cucumber leaf with downy mildew infection.

underside of leaf
Underside of a cucumber leaf with downy mildew infection
and dark-colored sporulation.

Downy mildew reproduces via a spore-type called a sporangium that can move relatively long distances on air currents. Initially, it was thought that the downy mildew would overwinter in Florida, Texas or Mexico and gradually move northward as the spring growing season progressed and cucurbit crops were planted. However, Michigan and other states in the Great Lakes region sometimes experience a downy mildew outbreak on cucumbers weeks before many of the more southern states. Of course, northern greenhouses can provide a warm environment for both cucurbits and downy mildew through the winter. In the spring, the cucurbit downy mildew sporangia can readily escape from infected greenhouse cucurbits via the vents in their quest to find cucurbits, preferably cucumbers or melons.

To assist growers in knowing if and when cucurbit downy mildew sporangia have arrived in Michigan via air currents, two spore traps were recently established in fields located in the eastern part of the state. These volumetric traps sample the air continuously and the particles in the air are impacted onto sticky tapes that are then examined under the microscope and downy mildew sporangia identified.

Spore trap
Spore trap in a field of cucumber seedlings.

This week, downy mildew sporangia have been detected in both of our sampling sites. See the current daily spore trap totals. While the counts in the regions are very low, past research indicates that these low counts are a precursor to a downy mildew disease outbreak. Sometimes the sporangia counts remain low for a few weeks and then abruptly spike with high concentrations. Since downy mildew must be controlled preventively, growers of cucumbers and melons on the east side of the state are encouraged to protect their crops with fungicides proven to be effective against downy mildew. See the cucurbit downy mildew spray recommendations.

Growers who have their cucurbits in low tunnels can apply the fungicide Presidio through the drip irrigation for downy mildew protection. Once the tunnels are removed, downy mildew fungicides with a different mode of action can be used as sprays.

Downy mildew has not been found on a cucurbit crop in Michigan. In the past, we’ve relied on reports of the disease being detected in large production fields. Early detection of downy mildew in cucumbers and melons among home gardeners would be very helpful to both small and large commercial cucurbit growers in the state. Homeowners typically do not use preventive fungicides and therefore their gardens could serve as early indicators of a likely downy mildew outbreak. Earlier this month, a workshop was held on campus to help to train Master Gardeners in the identification of this devastating pathogen in the home garden. These activities are part of a Project GREEEN extension program to broaden our educational and control efforts regarding downy mildew on cucurbits in Michigan. See the confirmed downy mildew reports in Michigan.

Another feature of our cucurbit downy mildew efforts this year include weather-based pest management tools that can be found at the Enviro-weather vegetable page. Growers recognize that long periods of overcast, wet weather favor downy mildew development, whereas sunny, hot and dry weather tend to limit downy mildew spread and development. Through the Project GREEEN extension program, the weather data most likely to impact management decisions regarding downy mildew will be available for the Michigan growing regions.

Dr. Hausbeck’s work is funded in part by MSU’s AgBioResearch.

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