Our moist climate is ideal for fungus diseases on trees and other landscape plants. Cherry, Dogwood, Rose, and Photinia are particularly susceptible to leaf diseases such as anthracnose, mildew, black spot, or shot hole, during the spring and early summer. If you have plants which have been severely infected by one of these diseases in previous years, you may want to consider preventative measures.
Most of these diseases infect new leaves by producing spores from old infected leaves. This includes old, dead leaves which have fallen onto the ground. Raking up and disposing of these old leaves is the most important preventative measure you can take. I also usually remove black spot infected leaves on rose plants.
Several different pesticides (including natural ones) are available for preventing infection of new leaves. Three systemic chemicals are available which will actually stop the fungus from spreading in already infected leaves.
The three systemic fungicides are Propiconazole, Myclobutanil, and Tebuconazole. Propiconazole is available in Ferti-lome Systemic Fungicide. Myclobutanil is in Spectracide Immunex Fungicide. Tebuconazole is in Bayer All-in-One Rose and Flower Care.
Daconil and Chlorothalonil are also widely available and effective, but are not systemic and therefore protect only new uninfected leaves. In most cases you will have to look at the list of ingredients to find the technical, chemical names.
The three organic or natural fungicides are Messenger, Neem Oil, and Calcium Polysulfide. None of these are systemic and only protect uninfected leaves.
Spraying should begin as new leaves are emerging and continue every 2 to 3 weeks until weather becomes dryer in May.