Friday, September 30, 2011

Powdery Mildew

What causes the white, cottony growth on the top side of many plants including beans, cucumbers, squash, papaya and roses? This is a fungus, called powdery mildew (PM).  There are many different kinds of powdery mildew fungi, but they are rather host specific. This means that one specific mildew fungus will only infect those plants in a particular genus or family; so you don’t have to worry about the PM on the beans attacking the papaya or the PM on your mango attacking the tomatoes.  Powdery mildew fungi like temperatures between 60 and 80 degrees F.  Infected leaves often turn yellow to brown, and may shrivel up and die, making the plant unproductive.

Control –there are a number of relatively safe materials to use against this fungus: wettable sulfur, horticultural oils and Kaligreen (potassium bicarbonate).  Cucurbits (melon, squash cucumbers) can be sensitive to sulfur.  Do not apply when the temperature is near or over 90 degrees and do not apply within 2 weeks of an oil spray.  Apply these materials in the earliest stages of disease development for best results.  Before spraying, it is helpful to remove the leaves that are heavily infested.  Do not dispose of them on the ground since they are loaded with fungal spores. 
A recipe for the control of PM from the University of Hawaii CTAHR calls for one tablespoon of baking soda mixed with 2 ½ tablespoons of vegetable oil per gallon of water; spray every 7-10 days. 
In addition, there are two biological fungicides which work against the powdery mildew fungus.  One is a parasitic fungus.  The other, Serenade, which is registered in Hawaii, is a bacterium that helps prevent the powdery mildew from infecting the plant. 
Powdery mildews can and do attack healthy plants, but older plants that are less vigorous and plants that are stressed are more susceptible to infection.