How annoying is powdery mildew? Very! It always seems that the minute we have every other pest and scourge finally under control, back comes the powdery mildew. Unless you have a really extensive outbreak, it’s relatively harmless, but make sure you take action at the first sign of it. If you don’t you’ll have some ugly deformed plants and a severe case will kill them.
As for the conditions that promote powdery mildew, here’s a passage from the Perdue University Department of Botany web page: “High relative humidity at night and low relative humidity during the day with temperatures of 70 – 80F (conditions that prevail in spring and fall) favor powdery mildew. Maintaining conditions that favor rapid drying of foliage will help reduce disease incidence. Locate susceptible plants in open areas where they will not be crowded. Plants in shade are more prone to mildew than are those growing in sunlight. Prune to thin out any dense foliage; this will increase air movement and favor rapid drying of foliage. Avoid nightly sprinkling during August and September; instead, soak the soil as needed.”
Organically grown plants tend to be much more resistant to fungus diseases than those grow with synthetic chemicals and fertilizers because the soil they are planted in is full of beneficial fungi that counteract the harmful stuff. However, excessive nitrogen will promote lush, tender green growth that is very susceptible to powdery mildew, so this late in the season use a more balanced fertilizer on your plants.
There are lots of organic treatments for powdery mildew. You can buy commercially prepared products of which there are many — look for the OMRI (Organic Materials Review Institute) label, or you can mix your own.
A very simple treatment can be mixed using 1 oz. of milk to 9 oz. of water. Spray at the first signs of mildew and every week after for about 3 – 4 weeks. You can keep spraying as a preventative weekly if you like.
Another remedy suggested in the Sunset Western Garden book uses garlic: mix 3 oz. of minced garlic with 1 oz. of vegetable oil and let soak for 24 hours. Strain, stir in 1 tablespoon of liquid castile soap and store in a sealed container. It will keep at room temperature for several months. To use, mix 2 tablespoons of the garlic mixture in 1 pint of water. Spray every week for 3 – 4 weeks.
These treatments can be used on ornamental plants and food crops, though I would wash any edibles before eating.
If you notice that particular plants get a bad case of powdery mildew it might be time to consider replacing them with something that is more resistant.
For instance this rose — Brass Band I think — always had a bad case of mildew, which is a sign that it’s not suited to the conditions in my yard or area. I’m close enough to the ocean to be affected by the marine layer that creeps in every night carrying a lot of moisture with it. It’s out of here next year, to be replaced by a rose that is more resistant.