By Sean Facey
Anthracnose is a family of fungi that winters on twig tissue on the tree. In the spring, spores are transported to new buds and shoots. Anthracnose is also called leaf, shoot, or twig blight, depending on the host species and method of disease attack.
In many hosts, infected leaves develop tan to reddish-brown lesions that extend along the veins of the leaf. Considerable defoliation, sometimes with complete leaf loss, occurs on many trees by late spring in cool, wet years.
What does it look like?
The primary signs of anthracnose are tan to red-brown lesions that extend along the veins and edges of the leaf, as well as considerable defoliation, sometimes with complete leaf loss. Anthracnose may also present as twig cankers, stunted growth, and a number of other issues, depending on the host species.
According to the U.S. Forest Service, “The symptoms vary somewhat, depending on the host. Under certain conditions, the whole leaf dies and falls prematurely. On some tree species, the diseases may also damage twigs, shoots, buds and fruits. Repeated defoliation reduces growth, weakens the tree, and increases its susceptibility to attack by other pests and to winter injury.”
Host material and range
Different species of anthracnose impact a variety of tree species, including oak, ash, maple, elm, hickory, walnut, birch, linden, sycamore and dogwood. Sycamore, white oak and dogwood are particularly susceptible to anthracnose.
In sycamore and dogwood trees, the particular strains of anthracnose may cause eventual tree death, especially if the tree is attacked annually. Most strains cause unsightly damage but cause no lasting harm to the tree.
This disease is seen from coast to coast in the United States, in nearly all growing zones. Essentially, wherever there are trees, there is the risk of anthracnose in that population.
Effect of weather
In cool, wet, rainy spring weather, anthracnose flourishes. This is especially true when weather of that nature persists for a long period of time. Anthracnose needs rain or wind to spread spores and germinate into a full infection. When the weather is consistently over 80 degrees Fahrenheit and trees are subject to more sunshine than rain, the effects of the disease are lessened. If a cool, rainy year is followed by a warm, sunny year, the year-over-year effects of anthracnose are greatly mitigated.
Pruning can help slow the spread of anthracnose and prevent year-over-year attacks. Be sure to destroy pruned plant material by burning to kill any spores. It’s important to disinfect pruning tools to ensure the disease is not communicated to an uninfected tree or property.
When planting new trees, consider the growing environment. If the area is prone to cool, wet, rainy springs, choosing tree species that aren’t as susceptible to anthracnose is wise. When possible, plant tree types native to the area — these trees will be healthier overall. The U.S. Forest Service states that “Pin oak, swamp chestnut oak, bur oak, and London planetree are only occasionally infected by the fungi.”
Treating with trunk injection
Arborjet recommends a trunk injection with a systemic fungicide like PHOSPHO-jet, Alamo fungicide, or another propiconazole product. PHOSPHO-jet inhibits fungal cells while eliciting a plant health response from the tree. Alamo will have more direct and aggressive activity against the fungus itself and is recommended if infection is chronic or particularly severe.
Tree health will affect treatment efficacy, so assess tree health prior to treating. For example, a declining tree (greater than 50 percent canopy dieback) is a poor candidate for treatment.
Generally, the best seasons for injection are fall and spring, as uptake occurs when trees are actively transpiring. The environmental conditions that favor uptake are adequate soil moisture and relatively high humidity. Soil temperature should be above 40 degrees Fahrenheit for trunk injection.
Trees may still defoliate despite our best efforts; however, we recommend treatments that enhance tree health. Avoid high-nitrogen fertilizers, which may exacerbate fungal infection.
Sean Facey is the strategic accounts and support services manager at Arborjet, with 30 years of experience in various facets of the green industry. Applying his skills in arboriculture, entrepreneurship, public policy, and international sales management, Facey dedicates himself to Arborjet and the preservation of the natural and urban forests.