Fruit & Vegetable Magazine

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Downy mildew reported in greenhouses


May 28, 2009
By Michigan State University Extension

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downymildewMay 28, 2009, East Lansing, MI – There has been a bit of a stir in
Michigan following the report of downy mildew on cucumbers growing in
greenhouses in Ontario, Canada.

May 28, 2009, East Lansing, MI – There has been a bit of a stir in Michigan following the report of downy mildew on cucumbers growing in greenhouses in Ontario, Canada.

downymildew 
Downy mildew on a cucumber leaf. 

The concern comes from the fact that the seeds (or sporangia) of the downy mildew are readily carried on air currents so it is certain that the downy mildew is being released into the outdoor growing environment each time a greenhouse vent is opened. The ability of these sporangia to survive once they are carried out of the greenhouse depends on whether they land on a suitable host (preferably cucumber or melon) and the weather during their flight. A day that is moist and overcast will be more favorable for survival of the sporangia than a day that is bright with low relative humidity.

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There have not been any field reports of downy mildew in Michigan or surrounding states. Currently, there are no field reports of downy mildew in Canada.

The spore traps for downy mildew are being set up in five regions in Michigan. The counts from the spore traps will be posted at this website along with any field outbreaks and other information.

According to Michigan researchers, it’s suspected there has been downy mildew in Canadian cucumber greenhouses each spring, which is why state growers have had the disease each of the last several summers.