By Beth K. Gugino
Still no confirmed reports in Pennsylvania but pathogen inoculum is building in nearby states!
Purplish-gray sporulation of downy mildew on the underside of a pumpkin leaf. Lesions resulting from bacterial diseases will remain yellow or tan in color on the underside of the leaf even under humid conditions.
This past week, downy mildew has been confirmed on cucumber and cantaloupe in Delaware and also on giant pumpkin (Cucurbita maxima) in Maryland. Previous reports from Maryland and New Jersey have only been on cucumber. To my knowledge, there have been no confirmed reports in Pennsylvania. In PA the past few years, the first reports have been on cucumber and occurred on 10 July 2013, 22 June 2012 and 5 July 2011.
Although the recent local weather has been favorable for downy mildew development, the upper level weather patterns have not been conducive for moving the pathogen from those sources across Pennsylvania. However, all this could change as soon as the weather patterns start moving up the east coast. Favorable conditions for downy mildew development include overcast skies with leaf wetness or dew periods of up to 6 hours and cooler temperatures between 50 and 77°F.
Downy mildew can easily be confused with the bacterial diseases angular leaf spot or bacterial leaf spot, both of which have problematic in cucurbits this season. To distinguish downy mildew, look for the purplish-gray sporulation on the underside of the leaf. If you do not see downy mildew sporulation in the field (most obvious in the morning as the dew dries), place several symptomatic leaves in a sealed bag overnight and then check for purplish gray sporulation the next day.
Source:psu.edu