Anthracnose has been common this year on shade trees. Details are provided in issue no. 5 of this newsletter. Most of those fungi are not a threat to plant health, and affected trees often recover by late June. Dogwood anthracnose, however, is a more damaging disease with the potential to cause dieback, decline, and possibly death if untreated. It is a potential threat to dogwoods in Illinois, so it is important to be aware of disease symptoms to make an early diagnosis and initiate the proper controls.
Dogwood anthracnose is caused by the Discula fungus and is usually referred to as Discula anthracnose to avoid confusion with a minor disease called spot anthracnose. The two are easily confused. The spot anthracnose fungus infects leaves, young shoots, and fruit, causing small spots with purple borders. Discula anthracnose causes leaf spots and stem cankers, and it may kill shoots. Infected leaves may have brown spots with purple borders that are usually larger than those of spot anthracnose (1/2-inch diameter versus 1/4-inch diameter). The edges of the leaves may be necrotic; the dead area may spread over the entire leaf. Brown leaves often remain attached for a month or more. If you suspect that leaf symptoms resemble Discula anthracnose, examine larger branches for cankers by peeling back the bark in infected areas. Look for twigs dying back, especially in the lower crown. Under very humid conditions, infected leaves and twigs produce tiny fruiting bodies with masses of spores, which are used to confirm the disease in the lab.
Spot anthracnose usually does not require fungicide sprays for control unless conditions stay favorable for prolonged periods. Discula anthracnose is difficult to control once it has caused significant dieback. Maintain optimum conditions for growth and recovery: water during drought stress of 2 weeks, avoid overhead irrigation, apply a mulch over the root system, and improve air movement around trees (through pruning of surrounding vegetation) to minimize infections and encourage drying. Prune and discard infected branches and shoots. Rake up fallen leaves. Avoid high nitrogen fertilization, which encourages succulent, susceptible vegetation.
We have not seen many Discula anthracnose problems. The disease is prevalent in moist, humid areas in dense forests where plants do not dry out quickly. In Illinois, we tend to use our dogwoods as specimen trees in the landscape. Such sites are usually exposed to sun and good air flow, so foliage dries quickly. In fact, positive cases of this disease have been found more often in wooded sites than in the landscape.
For pictures and more information, consult the University of Tennessee's Web site at http://dogwood.ag.utk.edu/. Work on this disease has been ongoing for many years.