By Nick Sakovich
This disease is caused by a bacteria, Pseudomonas syringae and likes cold, rainy weather; especially prolonged periods of rain. It is most prevalent from the Fresno area north.
In California, leaves and twigs of orange and grapefruit trees are most susceptible to infection. Whereas in lemons, it is the fruit which is most susceptible - especially following a puncture injury by thorns while the fruit is wet).
The disease is most active in the winter and spring. Lesions often start on the wing of the petiole, extend to the base of the leaf blade and on to the twig at the place of attachment. Callus tissue can later form, becoming dry and scabby, reddish-brown to chestnut in color. Leaves wither rapidly and either fall off, or may dry while still remaining attached to the tree. Whole twigs can be girdled and die back.
In severe cases, and in young trees, prune out the dead twigs and spray with copper bordeaux fungicide (10-10-100)around November 1. This is a high dosage of copper and with higher dosages in smoggy areas like Southern California, there is always the potential danger of copper injury to the fruit.