Dogwood anthracnose, caused by the fungus Discula sp. (Gloeosporium), is common on native western flowering dogwood (Cornus nuttallii) in western Washington. It is less common on other dogwoods and is not known to attack other tree species. The disease generally appears from late May to early July, but can be active during moist weather anytime in the growing season.
Large, brown irregularly-shaped blotches develop on the leaves (Figure 1). Blotches usually have well-defined, dark grayish green, purple, or brown margins. Often infections are located on the leaf mid-vein forming wedge-shaped blotches (Figure 2). Sometimes leaves have brown spots about 1/16-1/4 inches (2-7 mm) across, often with dark brown to purple margins. Spots and blotches may be on the same leaf (Figure 1). Infected leaves usually drop early. Defoliation may be severe.
Tiny brown dots are often present in the blotch or spot, and may be associated with leaf veins (Figure 2). These are fungus fruiting bodies containing spores which spread the disease.
Twigs are also infected. Sunken tan to brown spots with purple borders develop which eventually cause twig dieback. Tiny fungus fruiting bodies can be seen on dead twigs during winter and early spring. Dead leaves containing fruiting bodies often remain on infected twig tips all winter and spring (Figure 3). Severe twig infection kills leaf buds on the twig, so leaves may not emerge in spring. Depending on tree vigor and infection severity, new buds may form and produce leaves later in the season.
Cultural. Properly prune out and dispose of infected twigs and attached leaves. Wash shears in soapy water and swab in rubbing alcohol after cutting. Rake up fallen leaves during the growing season and in the fall, and burn them or place them in the garbage. Do not compost them. These procedures should eliminate much of the fungus, but control will probably be incomplete because on most trees it is usually not possible to remove all of the overwintering diseased tissues, and spores can also spread from nearby infected trees.
The eastern flowering dogwood (C. florida), commonly grown in this area as an ornamental, is only moderately susceptible to this disease compared to the highly susceptible native dogwood. Under high disease pressure, however, it will also sustain considerable damage. Kousa dogwoods (C. kousa) have good resistance to this disease and are suggested for planting in this area. Kousa varieties have a wide range of forms and flower habit but will not duplicate the large upright native trees.
Chemical. Several fungicides will control this disease when applied at the proper time and with thorough coverage of leaves and twigs. "WSU Hort Sense" has recommendations for home gardeners. Sprays should begin at bud break. Follow label directions and precautions. Applications later in the season may be needed if wet weather occurs and there are infected leaves and/or twigs on the trees. These late season sprays will not cure tissues already infected, but are aimed at protecting twigs and buds that will be important for tree growth the following year.
Many native dogwoods grow in natural situations where it may be impractical or impossible to practice control procedures. Diseases will be worse in some years than in others. Weather patterns are very important in influencing disease cycles. Prolonged dry weather tends to slow or stop disease spread. When springs and summers are moist, especially several years in a row, disease incidence and spread will become worse. Damage could be significant.
Dead trees have resulted directly and indirectly from heavy anthracnose infections. Weakened trees are subject to other diseases and insect infestation.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
For more information contact your local WSU Extension Office.