June gloom is here; always a welcome respite from the erratic heat waves of May but a month that can be a problem for plants susceptible to the mildew fungus. I’ve composed the following do’s and don’ts to help your garden remain as free as mildew as possible.
Do select and grow mildew resistant plants. Roses, Crepe Myrtles and other plants that are notorious for harboring mildew usually have varieties that have been selected to resist the powdery white scourge.
Don’t fertilize. Nitrogen based fertilizers increase susceptibility to mildew
Do use copper dormant spray on deciduous plants. These sprays will help kill mildew when it germinates in spring.
Do wash the plants foliage; mildew spores cannot germinate on the ground, so wash them off during mildew season, however do not use this method if you have other fungi infesting your plants (like rust) that are spread by water.
Don’t allow weeds and un-composted leaf litter to collect around susceptible plants (this is good advice for all plants).
Do use simple, organic methods of control (see below).
Don’t let mildew get you down; it’s an obligate parasite which means it needs its host alive to survive. At worst it will stunt your plant but it almost never kills plants.
Paper on stomatal density and powdery mildew infestation: http://www.ncbi.nlm.nih.gov/pubmed/22476473
Paper on nitrogen fertilization and mildew: http://hri.tdwdev.com/Docs/Publications/JEH/JEH_2008/JEH_2008_26_4/JEH%2026-4-197-203.pdf
Organic Mildew Treatments
The Cornell Baking Soda Formula and other controls: http://www.organicrosecare.org/articles/recipe_mildew_control.php
Milk Spray: 9 parts water to one part milk; probably works by clogging stomata so that the fungus can't enter the plant. It's reported that skim milk works best.
Neem oil can have a preventative effect on mildew, but I've had too many reports of plants burning from its use to recommend it.