Several inquiries and samples have arrived at the Plant Clinic from growers concerned about dogwood anthracnose. I have not seen this disease yet on an Illinois sample. It is still early, but we might see it soon in southern Illinois. Here is some information on what to look for and why to be concerned.|
Anthracnose is found each year on many plant species; and several fungal species are involved in anthracnose diseases. For example, anthracnose that affects sycamore does not affect strawberry. A few anthracnose fungi commonly infect shade trees in Illinois but rarely cause lasting or serious injury. Dogwood anthracnose is an exception and may cause severe damage or even death to the host. Trees in a forest setting are more at risk. Landscape trees are usually in more exposed locations, where the disease cannot progress as rapidly and where it can be managed. It is important to be aware of 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 may kill shoots. Infected leaves may have brown spots (with purple borders), usually larger than those of spot anthracnose (1/2-inch versus 1/4-inch diameter). Leaf edges may be necrotic: This area may spread until the entire leaf dies. Brown leaves often remain attached for weeks. If you see leaf symptoms resembling Discula anthracnose, examine larger branches for cankers by peeling back the bark in infected areas. Look for twig dieback, especially in the lower crown. Under very humid conditions, infected leaves and twigs produce tiny fruiting bodies with spore masses, which are used to confirm the disease in the lab. The clinic is able to test for anthracnose with a one-day turn around.
Discula anthracnose is hard to control once it has caused significant dieback. Maintain optimal 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 (by pruning surrounding vegetation) to minimize infections and encourage drying of foliage. Prune and discard infected branches and shoots. Rake up fallen leaves. Avoid high nitrogen fertilization that encourages succulent, susceptible vegetation.
Discula anthracnose is prevalent in moist, humid areas in dense forests where plants do not dry out quickly. In Illinois, we tend to use dogwoods as specimen trees in the landscape; such sites are usually exposed to sun and good air movement, helping foliage dry quickly. Protective fungicides are available. Consult the 2003 Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide for some registered fungicides. Trade names, active ingredients, mobility information, and company names are listed in a table at the end of the appropriate chapter.
Recent dogwood samples received at the clinic have exhibited environmental scorch or frost injury. A very good publication with photos is available on the Web from the USDA Forest Service: How to Identify and Control Dogwood Anthracnose. Plug the title into a search engine for easy access.