Bacterial blast

Symptoms
Infections caused by Pseudomonas syringae usually start as black lesions in the leaf petiole and progress into the leaf axil. Once the petiole is girdled, leaves wither, curl, and eventually drop. Entire twigs may die back. The damage is most severe on the south side of the tree, which is exposed to the prevailing winter winds. Diseased areas are covered with a reddish brown scab. Infections result in small black spots on the fruit.
Bacterial blast, also known as citrus blast or black pit, is restricted mainly to citrus growing areas in the Sacramento Valley where wet, cool, and windy conditions during winter and spring favor development and spread of the blast bacterium. Leaves and twigs of oranges and grapefruit and the fruit of lemon are most susceptible to infection. The bacterium infects small injuries caused by thorn punctures, wind abrasions, or insect feeding.

The disease occurs on branches, twigs, buds, leaves, and fruit. The most conspicuous symptoms are the cankers that exude gum during late spring and summer. Gumming is common on stone fruit trees, whether on trunks, limbs, twigs, or fruit when injuries occur. Thus, the name gummosis does not define a cause, only a response. Cankers on the twigs are darkened areas often at the base of buds. On limbs or trunks they are often darker than the normal bark, sunken in their centers, and they may extend for a considerable distance. Leaves and shoot growth beyond the canker may wilt and die during the growing season. Sour sap often occurs following winter injury to the trunks or limbs.