The powdery mildew fungus is a common disease found in the garden. The leaves and flower buds of plants show a powdery milky white film and flower buds may not open normally. Humid weather makes the mildew worse. If you discover this fungus on your plants, they should be removed or treated quickly. This will help prevent the disease spreading throughout the garden or row of crops, such as squash.
Flowers, shrubbery and vegetables are all susceptible to powdery mildew. Several fungi may cause the powdery mildew, each preferring different host plants. So if one type of plant shows symptoms, a nearby tree or shrub may not be symptomatic. Flowers such as zinnias, snapdragon and dahlia are all potential hosts for the disease. Shrubs such as crape myrtle, azalea, spirea and rhododendron are also susceptible. The vegetables in your garden, including squash, cucumber and peas may also show mildew symptoms.
Effect of Seasons
The fungus may be present in the soil, or come in on a new plant. Powdery mildew can overwinter in the soil. It is an airborne spore that attaches to host plants in the spring. Wind and water, even raindrops splashing, can transfer a spore to a healthy plant. High humidity increases the conditions for spore formation and low humidity increases the dispersal of spores. This fungus thrives in warm and humid nights and cooler days.
The early signs of powdery mildew are curling or discolored leaves and a dwarfed growth. Small white spots begin to appear on leaves as the mildew progresses. These spots will grow together and may eventually develop a mycelial mat that is embedded in the plant. This is how the fungus draws nutrients and eventually life from the host. In trees the effect may be seen as leafing out later than normal.
Gardeners use 3 tbsp. of a product called horticultural oil in combination with 2 tbsp. baking soda in a gallon of water. The oil by itself is often effective. Gardeners also use neem oil and potassium bicarbonate. Spraying the leaves as frequently as once a week will be necessary. Remove the most infected leaves or buds. Changing the soil may help potted plants not become reinfected. Increased air circulation is also helpful in deterring the fungus. Do not water from above the plant and remove any debris that is near the base.
A prepared commercial product may be indicated in cases of severe infestation. Check with a local cooperative extension service for recommendations and application instruction. Be aware of precautionary measures such as gloves and masks when using chemicals. Myclobutanil is one chemical that is effective on many types of fungi. Others are triadimefon and propiconazole, which are systemic fungicides. Dusting sulfur is a good first intervention; this product will remain even if the plant gets wet in a rainfall.