Several diseases of the ash tree include anthracnose, ganoderma root rot, laetiporus root rot, rust, yellows and decline, according to the Penn State Extension. Other diseases include verticulum wilt, powdery mildew, cankers and butt rot. Pests such as the emerald ash borer, banded ash clearwing and ash flowergall mite also infest ash trees. Many of these maladies weaken ash trees and lead to death if diseases and pests go untreated.
Symptoms of anthracnose include leaves with brown spots at their tips, leaves that fall prematurely and young twigs that are girdled and killed. Anthracnose is caused by a fungus and affects the lower part of ash trees the most.
Ganoderma root rot causes branch dieback as the fungus grows. In later stages, reddish-brown and brown fungus shelves are topped with white and may grow up to 14 inches across on ash trees. The fungus may grow by itself or in overlapping layers.
Laetiporus root rot creates cracked bark, and the limbs are prone to wind breakage. Bright yellow or orange clusters of fruiting structures form on ash trees before falling off in winter. These splotches occur long after the most severe damage to the tree has already been done.
Anthracnose is treatable by removing infected limbs. Both forms of root rot require the infected tree to be removed immediately upon detection of the disease, as those fungi can spread to other ash trees very quickly.