By Diana Alfuth, UW-Extension Pierce County Horticulture Educator
With the cool, wet spring we’ve had, foliar fungal diseases are having a banner year! Expect to see lots of leaf spots, blotches, and blights on trees and shrubs. Fortunately, leaf blotches and spots are usually only cosmetic, and they bother us much more than they bother the tree.
The most common leaf disease we’ve seen so far is anthracnose on maples, ash and oaks. Symptoms of anthracnose vary depending on the tree species, but usually include irregular spots and dead areas of leaves that often follow the veins of the leaves. The areas can be tan, brown, dark brown or even blackish. Severely affected leaves often curl and may fall off, but don’t worry if you have some affected leaves on the ground—there are still plenty of leaves in the tree and the ones that drop won’t be missed.
Anthracnose is caused by several fungi that overwinter in leaf debris. In warm, drier springs, we don’t see many symptoms on trees. Cool, wet springs, when the tender leaves are just coming out of the buds, favor a variety of fungal leaf diseases.
Treatment for leaf diseases is rarely recommended, as these diseases really don’t hurt the tree. And, by the time you see the symptoms on your tree, it’s too late to treat anyway. The actual infection occurred several weeks before symptoms appear. Fungicides typically do not cure an existing disease; they have to be applied before the infection occurs as a preventative.
To prevent problems in the future, rake and remove leaves in fall. The fungi overwinter on leaf litter and that will be the source of spores to affect next spring’s young leaves. Those leaves should be buried, burned (where allowed), or properly composted to kill the fungal spores.
For more information about anthracnose or other tree diseases, contact Diana Alfuth, UW-Extension Pierce County Horticulture Educator at 715-273-6781.