Answer: If the powdery coating is white, it is most likely powdery mildew, a fungal disease. Powdery mildew fungi spread quickest when the humidity is high and temperatures are between 50F and 90F. To discourage the disease, plant on sites with good air drainage, give the plants plenty of space, water during the morning only, and allow the soil to dry out between waterings. Remove any affected leaves, and if your plant is heavily covered with powdery mildew you should probably remove the entire plant so the infection doesn't spread. For heavy infestations of powdery mildew, sulfur or copper sprays can be applied weekly, starting in July in your area. To obtain these treatments, check your local garden center, and follow the directions carefully.
Gerbera daisies are fussy about watering. They need a thorough soaking and then the soil should dry out before they're watered again. Try not to splash water on the crown of the plant. Gerbera daisies like the nightime temperature to be about 60F degrees, and the daytime temperature about 75F, so move yours if it's too warm where you have it. The plant will take full morning sunshine, but appreciates some afternoon shade. If the flowers continue to droop, or when they begin to shrivel, cut them off and allow the plant to produce new flowers.
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