Jim Chatfield
Stephen Nameth
Jim Chatfield
Leaf blight is a serious disease of a few very susceptible English hawthorn cultivars. Most other hawthorn species and cultivars are resistant or are not seriously affected.
The disease is first evident as small, angular, reddish-brown spots on the leaves. The spots have irregular margins and coalescence of spots often occurs resulting in larger irregular diseased areas. Diseased leaves yellow and fall prematurely. Sometimes, spots may be more prevalent near the margins of the leaves. When conditions are favorable for disease development, extensive defoliation occurs. It is usually mid-summer or after before the disease becomes prevalent. Small, black, raised dots develop in the center of the spots. These are the spore masses of the causal agent. They are especially evident when the leaves are wet.
Leaf blight of hawthorn is caused by the fungus, Entomosporium thuemenii (Diplocarpon maculatum). The fungus survives from one year to the next in fallen diseased leaves and in inconspicuous stem spots. During May and June, spores are produced on these overwintered leaves. These spores are spread by splashing rain and initiate the disease on the current season's foliage. As the leaf spots develop, new spores are formed on the spots and rapidly spread the disease. Wet weather is favorable for rapid development because splashing water carries spores, and persistent water drops favor spore germination and infection.
Washington hawthorn types are resistant to this disease. English hawthorn types are susceptible and are commonly infected. Although Washington types are resistant to leaf blight they are susceptible to rust diseases. If hawthorn rust is a serious problem in your area, it may be best to select plants other than hawthorns for landscape plantings.
Since the fungus overwinters in the fallen diseased leaves, raking and destroying all leaves will help to manage leaf blight, but will probably not result in complete control.
Leaf blight can be prevented by spraying with fungicides. Control through use of fungicides depends on proper timing of the sprays. Based on presently available information, spraying at 10 to 14 day intervals from bud break through early July has given good control. Additional applications may be necessary during rainy seasons or when good control was not achieved with the earlier sprays. Fungicides registered for use on leaf blight of hawthorns include: thiophanate-methyl (Cleary's 3336, Domain, Fungo Flo); chlorothalonil (Daconil*); and mancozeb (Fore, Dithane). Follow instructions on the fungicide label.
* Commonly available for homeowner use.
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