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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office
Anthracnose of Shade Trees
Anthracnose is a fungal disease common throughout Utah on Maple, Sycamore, Oak and Ash. Although in most years it does not cause significant damage, it can be very destructive during years when extended cool wet spring conditions occur. Consecutive years with conditions conducive to disease can seriously weaken trees and may cause death if the conditions persist and control measures are not implemented.
Causal Organisms The group of fungi that cause anthracnose produce spore bearing structures called acervuli. The acervuli erupt through the plant tissue and are evident as small black dots on twigs. The fungi that cause anthracnose are Kabatiella apocrypta in Maple, Apiognomonia veneta in Sycamore, A. quercina in Oak and A. errabunda in Ash. Symptoms
The first symptoms occur on leaves as small water soaked lesions. They are usually found along main veins of leaves, but can also occur between the veins. The spots enlarge and eventually turn tan, reddish-brown or black, depending on the species and cultivar of tree affected. Acervuli are then produced by the fungus in the necrotic tissue. Sycamore leaves can become infected as they emerge from buds resulting in blighting of the entire leaf cluster.
Infections on twigs and branches initially appear as discolored depressed areas in the bark followed by splitting bark. Severe infections may even result in death of small branches. Cankers usually develop raised margins resulting from the healing process in the tree. Sycamore branch infections result in multiple lateral shoots called witches brooms.
Disease Cycle The fungi overwinter in fallen leaves, petioles, twigs or branches. Under cool moist spring conditions the fungi mature and produce spores that are dispersed by wind and rain. Spores that contact susceptible host tissue infect and grow throughout the adjacent tissue leading to the characteristic leaf spot symptoms.
Control Cultural controls are only marginally effective for anthracnose and are mostly aimed at sanitation by reducing overwintering sites of the fungi. The effectiveness of sanitation is often minimal because there are many external sources of spores that are blown into the area. Cultural controls should be considered if the tree or adjacent trees have a history of anthracnose. Recommended cultural controls include: 1. Rake and destroy fallen leaves. 2. Prune out and destroy infected twigs and branches. 3. Maintain tree vigor with adequate water and fertilization. 4. Plant resistant cultivars. Sycamore cultivars Bloodgood, Columbia and Liberty are reported to be resistant. Check with your local nursery for resistance information regarding other tree species.
It is usually not necessary to use chemicals to control anthracnose because the fungus only infects when wet conditions persist. Most trees can withstand occasional infections without any serious damage. However Chemical controls should be implemented in addition to the cultural controls when cool, wet spring weather occurs year after year. Chemical applications should begin at bud swell and continue at labeled rates and intervals during wet weather. Table 1 lists registered chemicals for control of anthracnose on Maple, Sycamore, Oak and Ash. Be sure to check the label for specific information about labeled uses and rates.
Sherman V. Thomson/Extension Plant Pathologist Scott C. Ockey/Plant Disease Diagnostician
Listing of commercial products implies no endorsement by the authors or the Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Persons using such products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its proper use. Always read and follow the label.