The horse chestnut tree in front of my house looks worse than ever. Its remaining leaves are brown, and many fell early. Is my tree in trouble? What can I do to help it?
— Peggy Schwartz, Libertyville
I do not see many horse chestnuts in home landscapes and your tree is a beautiful specimen. It appears that your tree is suffering from a common fungal disease called horse chestnut leaf blotch. Several of the Garden’s horse chestnuts show signs of this same disease. This year’s spring weather created ideal conditions for many foliar diseases that have contributed to your tree’s current condition. Fortunately, the damage from this disease is mostly cosmetic. More severe symptoms like defoliation often happen late in the season when the tree’s annual growth is complete. Bad infections year after year can put extra stress on a tree.
Symptoms of horse chestnut leaf blotch begin as irregularly shaped, water-soaked areas that grow quickly. These areas (known as lesions) turn reddish brown with a yellow halo that merges with the surrounding healthy green tissue. Symptoms of this disease are similar to those of environmental leaf scorch. The leaves affected by scorch are usually on the sunny or windy side of the tree, while leaf blotch affects most leaves with tiny black fruiting bodies present on the lesions. Damage is most severe when canopies remain wet for an extended period.
The problem with your tree this fall looks much worse than it actually is. The management of this disease should focus on cultural practices. Consider pruning the tree to maintain an open canopy that will dry more quickly. All fallen leaves should be collected and disposed of at the end of the season to help reduce infection for the following season. The horse chestnut leaf blotch pathogen overwinters as fruiting bodies in leaves infected during the previous season. In the spring, these fruiting structures release spores into the air. Spores can land on developing horse chestnut leaves. When this is combined with an extended period of leaf wetness, germination and infection result. About 10 to 20 days after infection, leaves can produce new fruiting structures and initiate secondary infection cycles. Keep the tree well-watered during dry periods like we have had this fall to help reduce stress on the tree.
Fungicide sprays may also be applied beginning at budbreak in the spring and repeated at 10- to 14-day intervals while wet conditions continue. This will help maintain the appearance of the tree but is not so important to tree health. The chemical controls may be very expensive due to the size of your affected trees. Your tree may be far less affected next year depending on the weather. Fungicide sprays must be applied annually, as they only protect the tree for one season.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.