These spots may enlarge and coalesce, causing leaves to become misshaped. Eventually leaves may be killed. When the infection spreads around a twig, it becomes girdled and dies. This phase of the disease is evident as new shoots develop in the spring. The shoots turn a black color, droop over, and die.
Control consists of pruning out blighted twigs as soon as they occur.
Cut several inches below the margin between healthy and diseased tissue. Prune in dry weather only. As a preventative measure, thin plants to provide good air circulation. Prepared by Paula Flynn , Department of Plant Pathology Iowa State University , Ames, Iowa. This information subject to a usage policy .