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Diseases of the dogwood tree

POSTED: May 10, 2013 1:00 a.m.

Just the other day I was looking at a dogwood tree at the house and saw it was in pretty good shape overall. However, things can change and it is worth keeping an eye on the tree to see if it becomes infected.

Three main foliar diseases tend to affect dogwoods this time of year.

Powdery mildew

Powdery mildew is a fungus that shows up not only the tree’s leaves but on its tender shoots. New growth becomes covered in a white coating usually on the upper surface of the leaves. As the disease progresses, the leaves tend to look scorched along the edges or have dead patches. They also turn yellow and drop off the tree.

The disease is very common in a dense, shady area with poor air circulation.

Usually powdery mildews come on late in the year and generally do not pose a big threat to the plant’s health. But when they are present in the spring, treat the disease with a fungicide.

Attack plan

Before going straight to chemical warfare, try something to keep it from returning later. Start by raking up and getting rid of all fallen leaves. Prune out dead and infected branches. If you can, try to improve air circulation and sunlight by removing overhanging branches.

If the disease is severe enough to warrant the use of fungicides, be sure the spray equipment can provide good coverage. Effective fungicides for dogwood powdery mildew control include myclobutanil and propiconazole. Some control can be obtained with thiophanate methyl. Product labels will provide information on how often to spray.

When powdery mildew persists and sprays are repeated, it is recommended to alternate fungicides to decrease the chance of the fungi developing a resistance.

Resistant plants

Resistant species and cultivars are available and should be considered for new plantings. Varities generally resistant to powdery mildew are:

Cultivars of the oriental dogwood (Cornus kousa) such as Milky Way, Milky Way select and national.

* Cornus florida x Cornus kousa hybrids such as Aurora, constellation, celestial and stellar pink.

* Flowering dogwood (Cornus florida) cultivars Appalachian joy, Appalachian blush, Appalachian snow and Appalachian mist.

Spot anthracnose

Spot anthracnose is one of the most common leaf diseases of flowering dogwoods. Flower bracts are usually attacked first and then the leaves, young shoots and fruit of dogwoods, primarily during wet spring weather. Symptoms are small 1/8-inch tan spots with reddish-purple borders.

Infections

When the infection is severe, spots can cause flower bracts and leaves to become wrinkled and distorted. This fungus survives from year to year on infected twigs, fruits and other tissues.

Frequent rains or extended periods of high humidity are needed for the disease to develop. When dry weather follows bud swell and bloom, the symptoms are rarely seen on the flower bracts.

In most cases the disease does not result in significant damage, but severe and repeat infections each year can significantly weaken a tree.

Proactive measures

To halt the damage, thin the canopy to increase air movement. And planting species and cultivars with some degree of disease resistance is an excellent option for managing this problem.

The disease-tolerant and resistant varieties include:

* Kousa dogwood (Cornus kousa) national, Milky Way select.

* Flowering dogwood (C. florida) Cherokee brave, Cherokee chief, Welch’s Bay beauty, Cherokee princess and springtime.

* Rutger’s hybrid — stellar pink.

The worst spot anthracnose has been reported on Cornus florida rainbow and Cherokee daybreak.

Spotting problems

If spotting becomes severe, fungicides can be used in the spring starting at bud break and continuing until leaves are fully expanded. Fungicides available for use include chlorothalonil, mancozeb, maneb, propiconazole, thiophanate-methyl or copper fungicides. Fungicides for spot anthracnose will also control dogwood anthracnose (canker anthracnose). Apply all chemicals according to directions on the label.

Dogwood anthracnose

Dogwood anthracnose is a relatively new disease. It is most severe only in areas of the state above 2,000 feet. A few cases have been reported at lower elevations where dogwoods are grown in very cool, moist and shady locations.

It is a serious disease capable of killing several trees and most dogwood species can become infected.

Symptoms

The first symptoms appear in the spring with spots on the leaves and flower bracts. Infected leaves have tan spots with purple edges, dry brown margins or large blotches on them. Blighted gray or drooping leaves hang on the twigs and are often the first symptoms noticed during cool, wet weather.

Infection spreads into the shoots, main branches and trunk causing brown sunken areas (cankers) to occur. Cankers can girdle and kill individual branches or twigs. Multiple cankers can girdle the main trunk and eventually kill the tree.

Disease management

Planting resistant species and cultivars is a better way to manage the disease.

Kousa dogwood (Cornus kousa) is generally resistant to dogwood anthracnose and is a better choice for replanting in sites where dogwoods have died. Other dogwoods resistant to dogwood anthracnose include:

* Crosses between Cornus kousa and Cornus florida (Rutger’s hybrids) such as aurora, celestial, galaxy, Ruth Ellen, star dust, stellar pink and constellation.

* Cornus florida such as Appalachian spring.

Combative measures

A combination of cultural and chemical measures is necessary to control the disease. Effective control may be possible if the disease is detected before branch dieback begins.

During hot, dry summer weather, prune and dispose of dead or cankered twigs and limbs. Remove all water sprouts. Rake and remove fallen leaves. Do not leave dead leaves attached to the tree. Improve air circulation and light penetration by removing understory plants and crowding vegetation.

Avoid high applications of nitrogen fertilizer, since this can promote very succulent (susceptible) new shoots. Maintain healthy dogwoods by following recommended cultural practices.

Avoid transplanting dogwood seedlings from the woods as these plants may harbor the fungus.

Chemical warfare

Fungicide sprays to protect new leaves and shoots need to begin at bud break in early spring. Fungicides for spot anthracnose will help to control dogwood anthracnose. These include chlorothalonil, mancozeb, maneb, propiconazole, thiophanate-methyl or copper fungicides. Maintain a protective covering of fungicide when new growth is present. Apply all chemicals according to directions on the label.

Healthy environment

Dogwood diseases can really be a problem in keeping trees from looking good, but in general they can be kept to a minimum if we create an environment unfavorable to disease. And if all else fails, using fungicides is always a good management option.

Source: Clemson University, Cooperative Extension, “Dogwood Diseases and Insect Pests,” HGIC 2003.

Michael Wheeler is county extension coordinator for the UGA Cooperative Extension office in Hall County. You can contact him at 770-535-8293, www.hallcounty.org/extension. His column appears weekly and on gainesvilletimes.com/life.


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