If your Viburnum has angular shaped, water soaked, diseased areas on its leaves, the plant may have bacterial leaf spot, also known as bacterial blight. This disease is caused by Pseudomonas syringae pv. viburnum. A layer of exuded bacterial cells often found on the leaf surface can make the leaves shiny. If infection occurs early in the season, leaves can become distorted. In severe cases, entire shoots can die back.
Infected twigs and buds are potential places for the bacterium to survive the winter. In the spring, when conditions are cool and wet, P. syringae pv. viburnum becomes active, multiplying and infecting the young, sensitive plant tissue.
Management includes removing diseased plant material. This will help reduce the infection potential for the following year. Pruning and good spacing helps promote good air circulation around plants and aids in drying leaves and branches. Heavy fertilization enhances excessive growth that can be easily attacked by the bacterium. Avoiding this practice will help reduce the amount of material that has the ability to become diseased. Bactericides containing copper can be used protectively in areas where the disease is an annual problem.
This article originally appeared in the 8/22/2003 issue.