Downy mildew on field cucumbers detected in Ontario

Cucumber growers in east Michigan are urged to take preventive measures against downy mildew.

A pickling cucumber field in Ontario, east of Windsor, was found to have downy mildew. This downy mildew occurrence was reported yesterday (July 6) and it is thought that the infection occurred within the last week. In past years, Michigan has found its first outbreak 5 to 14 days after an Ontario report. Therefore, Michigan growers are advised to begin a preventive spray program immediately (see Table 1).

Table 1. Fungicide recommendations for downy mildew on cucumber.

Table 1

A relatively new problem

Michigan cucumber and pickle growers have battled downy mildew, incited by the water mold, Pseudoperonospora cubensis, for five consecutive years. Prior to 2005, Michigan growers did not have to worry about this problem. Downy mildew is well-known for causing catastrophic losses in a brief period of time. Unprotected foliage can become completely infected and appear to be frosted within 10 days of initial infection (see photos). This downy mildew pathogen is resistant to commonly used fungicides including Ridomil Gold-based products and the strobilurin fungicides (i.e. Cabrio, Quadris, and Flint). Results from our 2005-10 research identified a limited number of fungicides that are effective, but must be applied every five days when the weather favors disease.

Downy mildew on a cucumber leaf
Photo 1. Downy mildew on a cucumber leaf. Photo credit: Mary Hausbeck, MSUE.

Defoliation of cucumber
Photo 2. Defoliation of cucumber caused by downy mildew. Photo credit: Mary Hausbeck, MSUE.

Spore trapping

The downy mildew pathogen does not overwinter in Michigan or nearby states. The downy mildew spores travel on air currents and must move from areas where it can overwinter (such as southern areas of the United States or protected cucumber production greenhouses) into Michigan each growing season. The downy mildew reproduces via tiny, microscopic spores that act as seeds of the pathogen. To help growers determine the level of downy mildew threat, a monitoring system has been in place since 2006.

In 2011, spore traps were placed at four sites in Michigan. These spore traps continuously sample the air and collect spores into the trap. These spores are imbedded on a film that is removed and taken to the laboratory for identification and quantification. A compound microscope is needed to have enough magnification to identify any downy mildew spores that are on the tapes. Currently, the spore counts have been low, but due to the nature of the trapping and counting, our updates lag by five or more days. Click here to see the current spore counts. The spore traps help to alert us to any influx of spores into those production regions, but should not be the only tool used to time fungicide sprays. Since we do not have a trap in each field, it is possible that we can miss an isolated spore mass coming into a particular region.

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Related article: Downy mildew outbreak in southeast Michigan on cucumbers

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

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