Spot Anthracnose on Dogwood in FULL BLOOM

From Dr. Kelly Ivors:

In case you haven’t noticed, we're having a good year for spot anthracnose on flowering dogwood, caused by Elsinoe corni, which can be attributed to a mild wet spring. The most prevalent foliage and flower disease of dogwood is indeed spot anthracnose. Dogwood anthracnose, caused by Discula species, occurs in the western part of North Carolina and causes similar foliar symptoms as spot anthracnose, but also causes lower limb dieback.

In most situations, spot anthracnose does not cause permanent damage to the tree; however, it can be unsightly and interfere with the beauty of flowering dogwood. Severe infections, especially if they occur in consecutive years, eventually weaken the tree.

Symptoms

Tissues of the flower petals are usually infected first; eventually infection spreads to other flower bracts, leaves, young shoots, and fruit. The initial symptoms are small (less than 1 or 2 mm), circular to elongated, reddish-purple spots which are first noticed in early spring. As additional infection occurs, the spots become numerous and eventually merge together forming larger leafspots. Therefore “spot” size cannot be used as the criterion for disease diagnosis. The centers of these spots are yellowish in color with margins a much darker color – brown to black. Severely infected flower bracts usually fall prematurely from the plant.

Preventative cultural practices

A healthy vigorous dogwood is better able to withstand infection from spot anthracnose than a weakened tree growing under stress conditions. Maintain tree health through proper watering, mulching, and fertilization. If possible, do not use overhead irrigation since this may increase the potential for disease infection and spread. Mulching to a depth of 2 to 4 inches can help maintain uniform soil moisture as well as help protect trunks from mechanical injury; however, be sure to keep the mulch away from the tree trunk. Fertilize as needed, using a balanced fertilizer with fairly low nitrogen content for moderate growth. Rapidly growing, succulent twigs which have been stimulated by excessive fertility are more susceptible to anthracnose infection.

Good sanitation is especially important for trees infected with spot anthracnose. Prune out and destroy dead and dying twigs and branches and rake up fallen leaves to help reduce potential sources of inoculum and improve tree appearance. It is also advisable to prune out water sprouts which develop on the trunk or main scaffolding limbs since they are very susceptible to infection from anthracnose. Prune only under dry conditions and sterilize pruners between pruning cuts.

Chemical control

In most years control is not necessary; however, if disease was severe the previous year or if a cool, wet spring is “predicted,” fungicides may be warranted. It’s a little bit too late to be thinking about preventative fungicide applications now in North Carolina, but keep this in mind next year in late winter. Since weather conditions cannot be anticipated, it may be wise to follow a regular spray schedule if disease control is desired. Spot anthracnose can be controlled preventatively with chlorothalonil (e.g., Daconil), mancozeb (e.g., Fore, Dithane), or thiophanate methyl + mancozeb. Spraying should begin when buds begin to open and be repeated when bracts have fallen, four weeks after bract fall, and in late summer after flower buds have formed. The recommended interval between sprays will vary depending on the fungicide and the rate of application. Make sure the fungicide you use has dogwood listed on the label. Follow all label instructions regarding amounts of pesticide to use, method of application and safety warnings.


For more information contact your local Cooperative Extension Center and ask for the Commercial Horticulture Agent.