AILING DOGWOODS

By Eve Keener and Steve Vandever

UCCE/El Dorado Master Gardeners

Q. "My dogwoods appear to be dying. The leaves curled up and turned brown and within two weeks the trees appeared dead. The bark on the lower main trunks looks black and has split and is peeling off. What is killing my trees and how can I protect my remaining dogwoods?

A. A species of anthracnose, which is a fungus, is probably responsible for the death of the trees. The fungus appears to have been introduced simultaneously in both the Eastern and Western United States in the mid-1970s, although the origin is unknown, since the disease has not been identified on any species of Dogwood occurring outside the U.S. According to the Cornell University Dogwood Anthracnose Fact Sheet, the infection starts in the leaves after they have expanded in late spring. Spots and blotches with tan centers and a reddish margin appear on leaves and brown or black spots may appear on the undersides. The flower “petals” may also show reddish or brown blotches. Entire leaves become infected and die but may remain on the tree throughout the winter. The fungus goes on to infect stems and shoots causing tan colored cankers which, over time, kill branches, usually beginning low on the tree. The tree often tries to send up sprouts from the trunk, which are quickly infected and die. The cankers quickly spread to the trunk and cause split and buckled bark. Affected trees tend to die within one to three years, with small saplings dying in the same year in which they became infected.

Healthy trees are much more capable of coping with the disease. Keep trees free of stress by applying a three to four inch mulch over the roots (do not allow the mulch to touch the trunk). If your tree is a Cornus florida, (an eastern dogwood used to summer water), water deeply during dry spells and fertilize moderately. These trees should be planted in a sunny location and overhead watering avoided.

For Cornus nuttallii, the California native dogwood found in the foothills and higher elevations as understory trees in forested situations, however, no summer water should be applied and they should not be fertilized, but care should be taken to leave their root systems undisturbed and not to change the soil elevation anywhere around them. Local dispersal of the fungus spores occur during rains and the disease favors moist locations. Unfortunately, these are exactly the conditions in which many of our native dogwoods grow and management is next to impossible in native forest conditions.

To help control the disease, avoid mechanical damage to any part of the tree. Rake and remove diseased leaves in the fall. Prune diseased branches and remove diseased suckers when the weather is hot and dry. Be sure to disinfect pruning instruments afterwards. Remove dead leaves from the tree. For trees planted in a garden setting fungicides can be applied starting at budbreak in spring and continuing every 10-14 days until the leaves are fully open. Also consider planting resistant varieties such as Kousa dogwood.