Leaves and shoots of aspen and poplars may be infected by species of the fungus Venturia. Disease development is favored by wet spring conditions.
Leaf symptoms are evident in May. Irregular brown or black spots form, causing leaves to appear deformed. Infected shoots turn black, brittle, and bend over to resemble a shepherd's crook. Only young shoots and leaves are susceptible. As the season progresses and tissues mature, they become resistant.
Death of shoots reduces height growth and can deform small trees by causing bends in the stems. Repeated attacks by the fungus can weaken trees and allow attack by other organisms.
The fungus overwinters in infected shoots. Fungal spores are splashed by rain from shoots blighted the previous season to the newly emerging shoots and leaves. Trees in crowded plantings are more often severely diseased than those planted on wider spacings. Disease severity differs among hybrids.
Although fungicides are available for leaf disease control on poplar, none are labeled specifically for this disease. If feasible, it may be helpful to remove blighted shoots, pruning at a junction well below the margin between healthy and diseased tissue. Trees less than 10 feet tall are at greatest risk. Once trees are over 15 feet in height the damage becomes negligible.
This article originally appeared in the June 13, 1997 issue, p. 89.