Powdery mildew is often associated with roses and other plants requiring a good amount of moisture to sustain growth.
My vegetable gardens in the last few years have been stunted and in the case of 2010, a complete loss due to mildew. Powdery mildew is most active at temperatures between 60 and 80 degrees F. Leaf temperatures above 95 degrees can kill the powdery mildew.
This year I plan to combat this awful intruder.
Thanks to a fact sheet from the CSU Extension office I’m prepared to work on this year’s crop.
I missed a portion of the cultural controls I could have used over the winter to help reduce spore creation or survival like removing all affected plant parts and debris in fall to decrease survivability in winter.
So its off to step two – control agents.
Preventative Control Agents
Sulfur is cheap and effective. PM has not formed a resistance to it so it can be easily used. Although I have read that sulfurs effectiveness is based on the temperature in which you apply it. Sulfur acts largely through the vapor phase. Its relatively inactive at cool temps below 65F, and can be phytotoxic at temperatures above 85F. It also should be applied before symptoms appear. Apply wettable sulfur to plants (e.g. Safer Garden Fungicide) only on days when air temperatures will not reach above 90, and within two weeks of oil spray applications.
There is also a product called Serenade which is a biological control that uses bacteria to kill the fungal pathogen.
I have also read that a 50 / 50 mixture of milk and water sprayed on plants BEFORE mildew appears is a good deterrent. I will be testing that theory on my honeysuckle that gets mildew later in the season each year. I will spray it a few times during the growing season. Stay tuned.
Eradicant Control Agents
If symptoms of PM are already present, use horticultural oils like Neem or jojoba oil to reduce or erradicate the infection. Never apply oils within two weeks of a sufur application and do not spray when temperatures will be above 90 degrees.
As plants grow additional applications of fungicides may be necessary.