In 2008, scientists with Great Britain’s Foresty Commission reported that nearly half of the nation’s 1 million horse chestnut trees had fallen victim to bleeding canker, a bacterial disease that causes red liquid to ooze from trunks and limbs. That potentially fatal disease is one of several that affect horse chestnut trees worldwide. Others include leaf scorch, leaf blotch and anthracnose. The same diseases that attack the horse chestnut tree (Aesculus hippocastanum) can also harm the Ohio buckeye tree (Aesculus glabra), a closely related species.
Before 2008, scientists had detected a type of fungus-caused bleeding canker that affected relatively few trees, according to the Forestry Commission. When infection rates rose sharply, researchers discovered a new type of bleeding canker linked to the bacterium Pseudomonas syringae. Symptoms include the oozing lesions as well as dead branches and cracked trunks and branches. Although some trees recover from the disease, many die. The best precaution involves removing diseased and weakened trees.
This fungal disease appears in the spring. Young leaves infected with the fungus Guignardia aesculi develop reddish-brown spots ringed with yellow. Small black structures known as fruiting bodies may appear within the spots. The leaves may eventually turn brown and fall off. Leaf blotch is unsightly, but it usually does not threaten the tree’s health. Gardeners can control it by gathering and disposing of infected leaves.
Poor weather and soil conditions can cause leaf scorch. The horse chestnut leaves will turn brown and paper-thin along the edges and may curl up. Horse chestnut trees that are subjected to drought or long rainy seasons may develop leaf scorch. The disease also occurs in trees planted in hard-packed soil or near sidewalks, pavement or buildings, which can reflect heat to the tree. Proper watering, soil conditioning or aeration and good placement can keep trees from developing leaf scorch.
Anthracnose is a common fungal disease linked to the fungus Glomerella cingulata. Like leaf blotch, it causes leaves to turn brown, but the brown areas cluster along the veins, petioles and midribs rather than appearing at random across the leaf’s surface. Trees infected with anthracnose can lose their leaves, but the disease rarely kills a tree. Prune affected trees and dispose of leaves to keep the fungal spores from spreading, or apply a fungicide made to treat anthracnose.
Powdery mildew, another fungus, leaves a white or gray moldy residue on horse chestnut leaves. The mildew can also cause leaves to shrivel, turn brown and fall off. Trees that have proper light and air circulation are less susceptible to powdery mildew. Apply fungicide to treat affected trees.