ANTHRACNOSE DISEASES OF LANDSCAPE TREES
Anthracnose diseases can infect a number of common landscape trees. The effect of Anthracnose infection is leaf discoloration and deformation. The disease causes dead areas, or blotches, on tree leaves.
Although there are several recognized species of Anthracnose fungi, the disease is host specific. The species of Anthracnose that attacks your Dogwood tree will not also attack your Sycamore or Maple tree.
Anthracnose diseases may infect leaves, twigs, buds, shoots, and even the fruit of various landscape trees. Repeated Anthracnose infections can weaken a tree and cause it to be more susceptible to attack by insect pests or to decline due to adverse environmental conditions.
Anthracnose spores overwinter on fallen leaves, on infected buds, and in cankers on infected twigs. In the spring these spores are blown or splashed onto newly emerged leaf tissue. If the weather is cool and moist the spores will germinate and infect the tender leaf tissue. Once warmer weather arrives, and the leaves mature, the incidence of Anthracnose infection diminishes.
TREES INFECTED BY ANTHRACNOSE
A number of shade and ornamental trees can be infected by Anthracnose including Ash, Basswood, Birch, Catalpa, Dogwood, Hickory, Horsechestnut, Tuliptree, and Black Walnut. Anthracnose can be particularly bad on American Sycamore and White Oak, as well as other oaks in the white oak group, such as English Oak or Swamp White Oak.
Although trees in the red oak group are less likely to be infected by Anthracnose, it can still be found to infect Black Oak, Bur Oak, Pin Oak, Red Oak, Scarlet Oak, and Swamp Chestnut Oak.
Maple species susceptible to Anthracnose include Boxelder, Japanese Maple, Norway Maple, Red Maple, Silver Maple, Striped Maple, and Sugar Maple.
The London Planetree, which is a cross between the American Sycamore and the Oriental Sycamore, is resistant to Anthracnose infection.
SYMPTOMS OF ANTHRACNOSE DISEASE
Evidence of Anthracnose varies depending on the tree species being infected. Tree leaves may have small dead spots or large irregular dead blotches. The spots, or blotches, on the leaves may be black, brown, or purple and infections are often found along the veins of tree leaves. Infected leaves often become distorted due to the unequal growth occurring from healthy and infected portions of the leaf.
Ash Anthracnose (Apiognomonia errabunda)
Ash leaves with brown blotches littering the ground in late spring is an indication the tree may be infected with Anthracnose. The infection often occurs at the tip of the leaflet or in the middle of the leaflet, causing the leaflet to have a bend in it from uneven growth. Infection is generally confined to the lower half of the tree canopy.
Dogwood Anthracnose (Discula destructiva)
Extensive loss of both native woodland dogwoods and ornamental dogwood trees has resulted from Dogwood Anthracnose infection. This disease infects both Dogwood leaves and twigs.
Leaf infections appear on the upper surface of the leaf as brown blotches with purple borders. The infected areas appear tan colored on the underside of the leaf. The infected areas on one side of a leaf usually stop when they reach the midvein of the leaf.
As the disease moves into the wood of the tree twig dieback will begin to occur. Elliptical cankers may form at the base of dead branches and fungal fruiting bodies may be seen on dead twigs. Epicormic sprouts may grow out from the main trunk below dead branches.
Maple Anthracnose (Gloeosporium spp., Discula spp., Kabatiella apocrypta)
Anthracnose on Maple trees is limited to leaf infections. Irregular tan, brown, or reddish-brown blotches may occur along the leaf veins or on the leaf margins. Anthracnose on Maple trees is rarely serious, but can cause leaf drop and unsightly leaves.
Oak Anthracnose (Apiognomonia quercina)
When prolonged cool, moist weather occurs during leaf development Anthracnose infection can be severe on oak trees, especially White Oaks. Large brown dead areas will develop between the leaf veins, usually on the lower half of the tree. Leaves may appear shriveled, cupped, and distorted. Twig infection can occur and twig or branch dieback may be evident next spring.
Sycamore Anthracnose (Apiognomonia veneta)
Sycamore trees can be seriously affected by Anthracnose infection. Anthracnose on Sycamore trees infects the leaves, buds, twigs, and shoots. When optimal conditions are present almost complete defoliation of the tree may occur.
Tan to reddish-brown blotches develop along the veins of infected leaves. Eventually, the disease moves into the petiole of the leaf and then into the twigs of the tree. Cankers develop on the twigs from which new spores are produced. Anthracnose is worse in coastal regions due to elevated moisture levels.
Trunk injection of fungicides may be an option to control Anthracnose in large Sycamore trees.
Walnut Anthracnose (Gnomonia leptostyla)
Anthracnose on walnut trees starts as a brown lesion surrounded by a yellow margin on the underside of the leaflet. Eventually, the lesion can be seen on the upper leaf surface as well. Leaves begin to turn yellow, curl, and fall prematurely.
Anthracnose on Black Walnuts can affect nut production. Lesions on Black Walnut fruit appears as sunken spots on the husk of the nut. The infected nuts will have reduced quality and may drop prematurely.
Anthracnose on walnuts may increase as the summer progresses if frequent rainfall occurs. Often, secondary spores, called summer spores, are produced. These spores are spread throughout the tree by wind or rain.
HOW TO CONTROL ANTHRACNOSE ON LANDSCAPE TREES
Cultural Control of Anthracnose
Raking and removing infected leaves will remove the main source of spores that could infect the tree next spring. The leaves should be burned or removed from the site.
Pruning of dead twigs and branches removes spores that may be produced from cankers. Pruning will also increase air circulation and light penetration, which will allow leaves to dry quicker in the spring.
Proper fertilization and watering will maintain the health of the trees and encourage secondary growth if the initial flush of growth is damaged by Anthracnose in the spring. Healthy trees are more likely to recover from an Anthracnose infection than stressed trees.
Plant resistant tree species or cultivars. The Oriental Planetree is resistant to Anthracnose disease. London Planetree and oaks in the red oak group are also less likely to be infected by Anthracnose.
Fungicide Control of Anthracnose
Several fungicides are labeled for control of Anthracnose diseases. The type of fungicide, the delivery method, and the timing of the applications depend on the species of Anthracnose being controlled and the prevailing weather conditions. Multiple applications will be necessary to achieve adequate control of Anthracnose.