Disease Cycle

The bacteria overwinter in the twigs, buds, and symptomless plant tissue. In the spring, the bacteria are spread by rain to leaves, shoots, and fruit. Spring infections can occur anytime after the leaves begin to unfold. Temperatures above 65°F and warm rains are needed for the bacteria to multiply, become exposed, and be disseminated. After these first infections, which are rarely noticed but do initiate the disease each year, the severity of the secondary infections depends entirely on the weather. A moderately warm season--with light, frequent rains accompanied by heavy winds--favors severe outbreaks of bacterial spot. Any recent injury to the leaves or fruit, such as wind-blown soil particles and hail, may result in severe outbreaks.

Secondary spread of the bacteria can occur from oozing summer cankers and leaf and fruit lesions during warm, wet weather. The systemic movement of the bacteria from leaves and shoots contributes to the formation of cankers. These cankers can be spread by budding to healthy nursery trees.