Powdery mildew is a fungal disease that results in a powdery gray or white coating on the leaves and stems of infected plants. It starts out as a few spores on the leaves, and quickly spreads, eventually yellowing the leaves and causing premature leaf drop.
Damage to Plants:
Besides being unattractive, powdery mildew results in leaf yellowing and droppage, stunted plant growth, distortion of buds, blooms, and fruit, and eventual overall weakening of the plant.
Disease Life Cycle:
Spores overwinter on diseased plant parts, and begin asexual production of new spores once the weather warms. New spores are carried on the wind to other parts of the plant, or to other nearby plants. Spores never stop producing more spores, so if infected leaves are not destroyed, the problem will only get worse.
Treatment and Prevention:
Powdery mildew thrives in temperatures between 60 and 80 degrees. Dry, shady conditions are ideal, as are areas with poor air circulation. Planting disease resistant cultivars and making sure you allow for good air flow are two ways to guard against powdery mildew. Inspect plants regularly during warm, dry conditions, and remove any leaves that show signs of infection. Destroy (do not compost!) infected plant parts. A spray made with baking soda, if applied weekly at the first signs of infection, can protect plants against further damage.
Plants that are badly infected should be ripped out and destroyed to prevent the disease from spreading further.
A commercially available organic option is Neem oil, which both treats existing powdery mildew and protects the plant against further infection.
Interestingly enough, the most effective measure in preventing and treating powdery mildew is to spray the foliage of your plants daily with plain water from the hose.
Powdery mildew hates water! The only caveat with this method is to be sure you do it early in the day so that the foliage completely dries before cooler evening temperatures arrive, otherwise you may invite other fungal diseases, such as black spot, into your garden.