Symptoms and Signs
Infected buds fail to open, dry out, and die. Postbloom infections result in depressed, rather shiny, black spots on fruit and leaves. Unlike fire blight, infections seldom progress past the base of spurs and are usually concentrated in the lower portion of the tree's canopy. In young trees, symptoms may also develop on shoots when the outer bark separates from the underlying tissue, giving the bark a papery appearance.
Blossom blast can severely reduce crop yield, number of leaves, and number of fruit spurs in the trees. Asian pears seem to be more severely affected, probably because they bloom earlier and are more exposed to frost injury.
Comments on the Disease
Pseudomonas syringae seems to be universally present on plant parts throughout much of the season, although population levels vary. Cold, wet weather favors population and disease development, especially in low areas of the orchard. The bacteria serve as nuclei for ice crystal formation, and high populations induce freeze damage in fruit and foliage tissue at temperatures 3° to 6°F higher than would occur in their absence. The bacteria then invade the frozen tissue, causing fruit and foliage infections. In spring, plant tissue is most sensitive to freezing and incidence of frosts is high. Pear trees are most commonly affected by P. syringae in the bud stage in Mendocino County and from the cluster stage through early fruit set in other areas.
Bacterial blossom blast is difficult to control. Monitor temperatures and protect the orchard against frost, which may help prevent infection. In areas where blossom blast is common, copper sprays applied in the fall or dormant season are labeled for this use, but the efficacy of these treatments is questionable.
Use frost protection methods to reduce frost injury. Provide a firm, wet soil surface with a minimum of cover crop to keep orchards warmer.