Black Spot Fungus on Aspens

Foliage diseases can reduce the aesthetic value of aspen, cottonwood and poplar trees. Occasionally, a severe disease outbreak causes premature defoliation or dieback of parts of the tree.

Facts:
Fungi cause most foliage diseases on aspen, cottonwood and other poplar species.
Foliage diseases develop readily in wet, cool weather.
Foliage diseases decrease a tree's aesthetic value and can cause premature defoliation.

Severe outbreaks can affect the general health of the tree.

To reduce future disease problems, rake up and dispose of leaves and prune out branches with cankers.Timely fungicide application can prevent severe outbreaks.

If a tree loses its leaves early in the season, it may grow new ones and its health is not seriously affected. If it loses them in midsummer, however growing new leaves may prevent the tree from fully hardening off before cold weather or reduce the amount of stored food. This leads to increased danger of frost damage, reduced growth, and increased risk to other diseases or insects.

The fungus Marssonina causes the most common foliage disease on aspen. Marssonina leaf spots are dark brown flecks, often with yellow halos. On severely infected leaves, in wet weather, several spots may fuse to form large black dead patches. Spots also may develop on leaf petioles and succulent new roots.

Marssonina survives the winter on fallen leaves that were infected the previous year. With spring and warmer, wet weather, the fungus produces microscopic "seeds" or spores that are carried by the wind and infect emerging leaves. Early infections are rarely serious, but if the weather remains favorable, spores from these infections can cause widespread secondary infection. Heavy secondary infections become visible later in the growing season and cause premature leaf loss on infected trees.

Fungicides, if applied early enough can prevent foliage disease. Spraying will prevent only new infections; it will not cure leaves already infected. Trees should be sprayed with a fungicide at bud break and then two or three times during the growing season at 12 to 14 day intervals.