Mary Hausbeck
Plant Pathology

There has been a bit of a stir following the report of downy mildew on cucumbers growing in greenhouses in Ontario, Canada. 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. Cucurbits that are growing in low tunnels and cucurbit crops that are now being planted are at risk since the sporangia are in our geographical region. It is important to be extra vigilant in scouting those cucurbits growing in low tunnels and newly planted fields. However, fungicide sprays are not needed at this time. 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 5 regions in Michigan. The counts from the spore traps will be posted onto my website (http://www.plantpathology.msu.edu/labs/hausbeck/ForGrowers.html) along with any field outbreaks and other information. The easiest way to search for this webpage is to Google “Hausbeck growers,” and it should be at the top of the list of results. I suspect that there has been downy mildew in Canadian cucumber greenhouses each spring which is why we’ve had the disease each of the last several summers. Spray recommendations for downy mildew are summarized in the two tables accompanying this article.


Please note: Gavel 75WG and Dithane are not registered on some vine crops.

Please note: Gavel 75WG and Dithane are not registered on some vine crops.