Powdery mildew infection on crape myrtle.
The best method of control is prevention. Avoiding the most susceptible cultivars, placing plants in full sun, and following good cultural practices will adequately control powdery mildew in many situations.
Cultivars resistant to powdery mildew are available for some susceptible plants including rose, crape myrtle, euonymus, and sycamore. Choose resistant varieties to reduce the likelihood of having to apply sprays.
those with Native American names, e.g., Catawba, Cherokee, Hopi
variegated varieties more resistant than nonvariegated types
Yarwood, Columbia, Liberty
Marshall’s Delight, Blaustrumph, Colrain Red
Phlox maculata Natasha, P. glaberrima MorrisBerd, P. paniculata Robert Poore, David
Simplicity and Meidiland roses, Rosa rugosa varieties
R. yakushimanum, R. macrophyllum, R. Nova Zembla, R. Palestrina
Pulcino and African zinnias
Some ornamentals do require protection with fungicide sprays if mildew conditions are more favorable, especially susceptible varieties of rose and crape myrtle.
Fungus species: Golovinomyces cichoracearum
Controls: water sprays; fungicides if necessary
Fungus species: Erysiphe lagerstroemiae
Controls: resistant cultivars
Fungus species: Sphaerotheca pannosa
Controls: resistant cultivars; fungicides if necessary
Shade and moderate temperatures favor most powdery mildews. Locate plants in sunny areas as much as possible, provide good air circulation, and avoid excess fertilizing or use a slow-release fertilizer. Overhead sprinkling actually may reduce the spread of powdery mildew, because it washes spores off the plant. Also, if spores land in water, they die. The best time to irrigate is mid-morning, so plants dry rapidly, reducing the likelihood of infections by other fungi, such as ones that cause rust or black spot on roses. As new shoots begin to develop on perennial plants, watch closely for signs of powdery mildew.
In some situations, especially when growing roses, you may need to use fungicides, which function as protectants, eradicants, or both. A protectant fungicide prevents new infections from occurring, whereas an eradicant can kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Use eradicants at the earliest signs of the disease. Once mildew growth is extensive, controlling the situation with any fungicide becomes more difficult.