|Each year for the last two years, my Rudbecia (Gloriosa Daisy)plants will grow, bloom really nice, and then wilt, dry up and die within several weeks after full bloom. In previous years, the plants bloomed right up to, and even after the first heavy frost.
The leaves take on a greyish, spotted cast, and wilt, then the plant dies. I've tried culling out the sick plants as soon as I see the wilt, but the other plants soon succumb as well. I've fertilized with 10-10-10, but this has no effect. Watering also has no effect.
What is causing this, and what can I do to prevent it?
|The white powder 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.