Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: TUESDAY, March 6, 1990 TAG: 9003061714
SECTION: EXTRA PAGE: E1 EDITION: METRO
SOURCE: JOHN K. ARBOGAST SPECIAL TO THE ROANOKE TIMES & WORLD-NEWS
DATELINE: LENGTH: Long
Dogwood anthracnose has devastated the native flowering dogwood in several Eastern states in the past few years and concern is now growing for the Roanoke area because dogwood anthracnose has been found in Virginia.
This disease is caused by a fungus that likes cool, wet conditions. The rainy conditions in the East last year account for the spread of the disease. This year, dogwoods in the the warmer central and coastal parts of Virginia will likely have a good chance of remaining vigorous, but dogwoods growing in the mountains are at great risk.
With a new growing season just beginning, there is be a tendency to worry that each spotted leaf or dying branch on a flowering dogwood signals the start of dogwood anthracnose. Not necessarily. Dogwoods will still get unrelated leaf spots, including the tiny leaf spot disease called spot anthracnose, which is not related to D.A. The trees may also produceundersized leaves because of poor, dry soils or show signs of overall weakening or even die because of lawn mower bumps or other injuries.
The symptoms of D.A. are fairly specific. They develop first on the lower branches of a flowering dogwood. Death of those leaves and branches gave this disease the earlier name of lower branch dieback. This pattern may be helpful for diagnosing D.A., since dogwoods suffering from site-related injuries may have scattered leaf and branch death or show a top-down pattern.
D.A. symptoms on the leaves include gray
Infections often progress down the leaf stems of blighted leaves into the shoots, resulting in cankers, which are localized areas of diseased, often discolored wood tissues. By carefully removing the bark from the trunk or main branch at the base of a shoot killed by D.A., one can see the brown canker-infected wood. Several cankers formed close together around a branch or trunk can girdle that branch and result in the death of parts above the canker area.
Dogwoods in woodland areas seem to be more prone to D.A. than landscape dogwoods, because close spacing and shade in woodlands can result in poor air circulation and increased humidity - which help the growth of the fungus. Increased incidence of the disease in the wild is a strong argument against transplanting native dogwoods from the wild into urban landscapes.
Flowering dogwoods planted in sunny, open lawn areas are virtually immune to the disease, but dry soil and other hazards in urban areas can spell trouble for dogwoods that are planted in less ideal spots. Drought injury weakens flowering dogwoods and increases susceptibility to disease.
The good news is that proper care of flowering dogwoods can go a long way in fighting D.A. Also, there are chemical fungicides recommended for control of D.A.
Dogwoods receiving good care will be better able to withstand D.A.
You can maintain the health of flowering dogwoods by watering them once a week during periods of drought, but avoid wetting the leaves from above to minimize the chance for leaf infections.
Mulching trees can help to reduce watering needs as well as protect trunks from mechanical injury.
Prune out and destroy all dead parts as well as the soft shoots and leaves along the trunk or branches. Winter seems to be a good time for this pruning.
In the spring, use low nitrogen fertilizer on dogwoods - high nitrogen fertilizer or turf fertilizer from the surrounding lawn can stimulate the soft shoots, which are more susceptible to the infection.
Improve air circulation around trees by removing nearby crowding or overhanging branches. This will also help keep the tree's foliage dry and help reduce infection. As always, avoid mechanical and chemical injuries to dogwoods.
If dogwood anthracnose is suspected, remember that effective control is possible only if the disease is detected before extensive dieback occurs. Prune and dispose of diseased twigs and branches to reduce potential sources of inoculum and improve tree appearance. Raking up fallen leaves may be of some benefit.
The Kousa dogwood, also known as the Japanese dogwood appears to be resistant to dogwood anthracnose.
The U.S. Department of Agriculture Forest Service warns that fungicides should be used only to supplement a cultural control program for dogwood anthracnose. According to the Forest Service, applications of chlorothalonil (Daconil 2787), mancozeb, or benomyl (Benlate) will protect against leaf infections. They suggest spraying three or four times during spring leaf expansion at 10- to 14-day intervals. If conditions are favorable for disease development later in the growing season, additional fungicide applications may be beneficial. As always, the chemical label must be strictly followed.
All this talk about dogwood disease may be alarming to homeowners and landscapers, but the trees can still be planted and maintained successfully with a little extra care. After all, it is our state flower.
by CNB