Part 10

As with the other anthracnose diseases, no practical control is available for forest trees. Control of walnut anthracnose on ornamentals and nut trees is partially achieved by raking and burning of old leaves.

OAK ANTRHACNOSE

Oak anthracnose is caused by the fungus Gnomonia veneta. Trees of the white oak group, particularly white oak, are susceptible to this disease. Oaks throughout the entire eastern United States are affected by the disease, although it is less common in the Northeast.

Infection occurs in the early spring or mid-summer. Symptoms on leaves develop as irregular brown diseased areas (blotches) along the midrib and the major side veins. These blotches may grow together by late spring or early summer if infection occurs early. Blotches are usually confined to the areas bordered by the larger veins. Leaves on the lower branches are frequently killed and occasionally trees will be defoliated. However, a second crop of leaves soon develops and mortality is rare. Fruiting bodies of Gloeosporium, the imperfect fungus fruiting stage of anthracnose, are located on the midrib and veins of infected leaves. When the disease spreads to the twigs, cankers and crown dieback may occur. The anthracnose fungus overwinters on diseased twigs and in the leaf litter. Oak anthracnose has the same causal agent as sycamore anthracnose, and the weather conditions favoring the sycamore disease also increases the anthracnose on oak.

Control is similar to sycamore anthracnose and involves an integrated program of pruning disease tissue, fertilization, and burning of leaf litter. No practical control is available for forest trees.

DOGWOOD ANTHRACNOSE

Dogwood anthracnose, caused by the fungus Elsinoe corni, occurs in states bordering the Atlantic Ocean and has also been reported in Louisiana. Its primary host is flowering dogwood, Cornus florida L.

Anthracnose occurs in the spring and affects not only the leaves, but also the buds and “flowers”. The buds may fail to open or they may produce stunted flowers. These have many circular to elongated spots with light tan centers. Margins of these spots are purple to brown. Often the flowers abort before development. Foliage spots (1-2 mm. in diameter) are raised at the margins. They are purple at the edges and yellow-gray in the center. Later centers of spots may fall out causing a “shothole” effect. Dozens of spots may be present on a single leaf and may be scattered or concentrated at tip, margin, or midrib. Twisting and malformation of the leaves are common. In addition to floral and foliage spots, infected areas may also occur on petioles, stems, and fruit clusters. All three areas have spots similar to those on the foliage.

Other diseases which may be confused with anthracnose include Septoria and Ascochyta leaf spots. Septoria usually begins around July and unlike anthracnose has more angular lesions that are between the veins. Ascochyta spots may be larger (6 mm. in diameter) than anthracnose, and tissue discoloration may extend outside of their borders. Occasionally the leaves may totally blacken. This disease may occur as early as June.

Wet, humid weather at certain stages of plant development is required for infection. Homeowners may obtain effective control by removing and burning infected plant parts. Various fungicide sprays are recommended by authors of ornamental handbooks.

COTTONWOOD RUST

Cottonwood rust, caused by Melampsora medusae, is probably the most important leaf disease of cottonwoods wherever they are grown. In the Lower Mississippi Valley, all sizes of eastern cottonwood trees may be infected with this rust. However, the disease is probably of most importance in cottonwood nurseries.

In mid-summer, yellow to orange pustules containing spores of the fungus form on the under surface of the cottonwood leaves. In late summer and early fall, dark brown fungal growths replace the orange structures. Cottonwood may be prematurely defoliated or even killed by successive attacks. The rust may weaken trees and subject them to attack by other disease-causing organisms. Also, there is often a reduction in growth in these normally fast growing species. This is very important since there is presently a wide interest in the use of hybrid poplars for pulp and timber production.

There is generally no accepted control for cottonwood rust. Rust-resistant varieties of hybrid and exotic cottonwoods are being developed and may provide the best control of this disease.

BLACK KNOT

Black knot, caused by Dibotryon morbosum, is prevalent throughout the Southeast (with the exception of southern Florida) wherever black cherry grows, and in orchards on plums and domestic cherries.

The most prominent symptoms are the elongated black swellings which appear in summer on small twigs and branches. Heavily infected trees appear quite grotesque, with large swellings which may be several times the diameter of the twigs. Cankers occurring on black cherry trunks usually ruin the commercial tree value. Initial infection occurs on lateral branches and twigs in the spring, but the swellings do not become noticeable until the following spring.

The most practical control for black knot is removal of infected black cherry from the stand. Twigs and branches with knots should be burned.

NECTRIA CANKER

Nectria canker of hardwoods, caused by Nectria galligena and N. magnoliae, is frequently found on yellow birch and black walnut. Common hosts also include bigtooth aspen, sassafras, northern red oak, red maple, beech, Carolina poplar, paper birch, and sweet birch. A closely related canker disease is also found on yellow-poplar and magnolias. The range of this disease includes the Lake States, the Northeast, and the southern Appalachians.

Older Nectria cankers are easily recognized in forest stands because of their typical “target” shape. “Target” cankers have rings, each of which represent a year’s growth. Younger cankers tend to be grown over by bark and callus tissue attempting to heal the wound. Such cankers are difficult to recognize, but close examination of the affected area may reveal tiny red bodies, which are the fruiting bodies of the Nectria fungi. Mortality rarely occurs from this disease, but stems may break at canker locations during high winds.

Control of Nectria canker is the same as for Strumella canker—the removal of infected trees during thinning operations.

STRUMELLA CANKER

Strumella canker of hardwoods, caused by Strumella coryneoidia, most frequently attacks trees of the red oak group. Other hosts include species in the white oak group. Beech, basswood, blackgum, shagbark hickory and red maple are also occasionally affected. This disease is found in the East, from the southern Appalachians to northern New England.

Strumella cankers are of two types: diffuse, and the more common “target-shaped.” The first develops on smooth-barked saplings and rapidly girdles and kills the tree. Killing results because callus tissue, which tends to heal over cankers, does not have time to develop. Target-shaped cankers are more common. “Targets” are formed by the alternation of killing of bark by the fungus around the canker’s perimeter and the formation, in turn, of a callus ridge by the host tree. The fungus is active usually in the dormant season, while callusing occurs in the spring. As with most canker-causing fungi, Strumella usually enters the tree through a branch stub. Cankers are quite large and may reach several feet in circumference or length. The presence of the causal fungus is revealed by dark brown, cushion-like structures about one to three millimeters in diameter on the dead bark and surrounding tissue.

No feasible control method is available under forest conditions. However, the disease impact can be greatly reduced by removing cankered trees during thinning operations.

SPICULOSA CANKER

Spiculosa canker, caused by Poria spiculosa, is found on bottomland oaks in the South. Occasionally this disease will also degrade hickories and honeylocust.