Owners should address Ash tree problems

Ash trees are one of Minnesota’s most commonly planted trees. They are used in lawns as shade trees and as boulevard trees along roadways. Ash trees are popular because of their fast growth, attractive branching habit and toughness. However, this recent cool, wet weather has been favorable for disease development and even ash trees are having problems. Ash throughout the area have been dropping leaves in large numbers, due to a fungus called anthracnose. Anthracnose spores overwinter on tree branches and infect new developing leaves during periods of cool wet spring weather. Usually it’s the lower or interior leaves that become infected. But in bad years, an entire tree can be defoliated. When observed closely, fallen leaves may have brownish spots or puckered areas, or may still be green since they fell so early. Leaves infected late in spring usually remain on the tree and have dark, irregular blotches on them. When fully expanded leaves are attacked by anthracnose, damage is limited to small brown spots. Anthracnose is rarely a serious problem for established ash trees. Trees often have enough leaves remaining to maintain a full canopy, or if severely attacked, will releaf. However, newly planted trees or those growing stressful conditions (i.e. restricted root zone or several years in a row of disease or insect attack) may require fungicide treatments. Chlorothalonil is a common fungicide effective against anthracnose. (Use all chemicals according to label directions.) Keep in mind that fungicides prevent new infections from occurring, so already infected leaves will still develop disease symptoms. When looking at these ash trees, many homeowners notice dark, bumpy, ball-like growths hanging from the branches. While it may look like a fungus problem, mites are actually the cause. These insect-like pests feed on the male flowers of ash trees resulting in growths called ash flower galls. While unsightly, they are not harmful to the tree and will fall off after a couple years. Another pest of ash trees is the ash plant bug. These insects feed on the undersides of leaves and cause small whitish spots to appear on the upper leaf surface. Sometimes tiny dark round spots are found underneath the leaves, which are simply the ash plant bug’s droppings. Heavily attacked leaves may have a brownish cast. Again, this is not a serious pest to healthy established trees, but new or stressed trees may benefit from additional watering and fertilization.