Powdery mildew can reduce the yield and aesthetic qualities of greenhouse-grown vegetables and herbs. Edible crops grown for harvest in hydroponic or ground beds, as well as transplants for re-sale, are all at risk. There are many powdery mildews that, taken together, have the potential to infect a wide range of crops. Fortunately, many are host-specific, meaning they infect only one or a limited number of crops.
Vegetable and herb crops that are susceptible to powdery mildew are: cucumber, squash, tomato, lettuce, Swiss chard, basil, rosemary, sage, parsley, lemon balm, mints and hypericum. Fruiting plants, which are sometimes produced under cover, are prone to powdery mildew, as well; these include strawberry, raspberry and blueberry.
Initial symptoms may appear as a chlorosis or discoloration of foliage, as well as a leaf curl. As the disease progresses, necrotic patches may accompany stunted and curling leaves. Inspect plants for white-colored fuzzy mycelial growth on the upper and/or lower leaf surfaces. Spores, known to as conidia, can build up quickly under ideal environmental conditions.
Temperatures between 62 and 72°F and relative humidity higher than 70 to 85 percent favor powdery mildew. Temperatures above 86°F will dry out spores, making conditions unfavorable for growth. The time from first infection until obvious symptoms are present ranges from three to seven days.
Infections often occur in the spring or fall, when cool nights followed by warm sunny days create rapid temperature changes. Unlike many foliar diseases that require free moisture on plant surfaces for disease development, powdery mildew is inhibited by these conditions. Wind is the primary means of dispersing spores (conidia), which live for seven to eight days.
Powdery mildew is an obligate parasite, meaning it needs a live plant host to survive. It overwinters as mycelium or cleistothecia on dormant or dead plant tissue. Thorough sanitation and removal of weeds between crop productions can reduce future infections.
Begin scouting for symptoms as soon as new shipments of plant material arrive, and continue to scout on a weekly basis. When possible, carefully remove and bag infected leaves so they can be disposed of off-site to limit the spread. Scouting should include inspection of leaf surfaces, as well as stems and buds.
Excess nitrogen can encourage the development of powdery mildew and certain other diseases. Monitor crop nutrition to keep fertilizer levels in the optimum range for each crop. Record fertilization applications and review regularly to avoid excessively high or low nitrogen levels in plant tissue. Cultural practices that reduce powdery mildew infection and spread include good air circulation, timely and proper plant spacing, and managing relative humidity. Avoid rapid temperature changes that may increase relative humidity.
For susceptible plants, preventative foliar applications of properly labeled fungicides are recommended. Growers have a variety of products from which to choose, some of which are OMRI-listed for organic production. Some perform better with a surfactant. Consult your supplier for detailed information, and always read and follow all label directions.