Marssonina spp. (fungus)

 

HOST Aspen, cottonwood, willow

DAMAGE/SYMPTOMS In spring, small dark-brown spots, often with yellow margins, develop on young leaves. The spots may coalesce to form larger lesions on leaves. Leaves may be stunted and drop prematurely. Young shoots are also susceptible to infection and may show lesions. Infections over several consecutive years will weaken the tree.

DISEASE CYCLE The fungus overwinters in infected leaf debris on the ground and infected shoots. In spring, spores are produced on previously infected tissues and dispersed by wind and rain, infecting new leaves. Infection is most severe under wet conditions. A second set of spores is produced on the newly infected leaf spots in late summer which infect adjacent leaves. This secondary infection is more severe under warm and moist conditions.

MANAGEMENT This disease may be unsightly, but it typically does not cause severe damage. Remove infected leaves that fall onto the ground and prune out affected shoots to reduce the level of infection. Disinfect pruning tools with 70% ethyl alcohol or a standard household disinfectant spray between cuts. Avoid wetting the foliage for prolonged times. Fungicides are not usually recommended unless the disease is a continuing problem. High value trees can be treated with a fungicide application in early spring when the buds start to swell. Applications may have to be repeated at 10- to 14-day intervals in spring as long as wet weather conditions continue. Products with the active ingredient chlorothalonil are effective in controlling this disease. Strictly follow instructions on the pesticide labels.

 

A Brown to tan spots on aspen leaves. B Brown leaf spots with yellow margins. C Close-up ofMarssonina spp. leaf spots with distinct yellow margin. D Large necrotic areas on leaves due toMarssonina spp. infection.

 

Brown to tan spots are visible on aspen leaves due to marssonina leaf blight infection.    

Young, small, brown leaf spots with yellow margins are visible on a poplar leaf.

Close-up of Marssonina sp. leaf spots with distinct yellow margin.    

Large, brown necrotic areas on leaves are visible due to Marssonina sp. infection.