Symptoms and Signs
Bacterial cankers appear as slightly sunken, dark areas on the bark and vary in size from about 1 to 4 inches in diameter. Bark around cankers may crack. Fluid often oozes and dries, leaving a white powder around or over the lesion. Usually cankers appear and spread upward in a line on one side of the trunk or branch. (see UC IPM video, Monitoring for Root and Crown Diseases in Avocado Orchards, for more information). Cutting under the bark surface reveals a decayed, reddish brown necrotic pocket, which may contain liquid. Dark streaks in the wood radiate out both above and below from the lesions. These necrotic streaks are usually in the bark cortex or xylem, but sometimes extend deeper into the center of branches or trunks. Often the disease will become inactive and canker wounds will close, except that a bark flap over the wound will remain.
Severely affected trees may have pale, sparse foliage and low yields on one branch or on the entire tree, but this is rare. Sometimes newly planted trees become stunted with many lesions; new branches may grow from buds below the affected part.
Comments on the Disease
Bacterial canker is widespread but is a relatively unimportant disease. In some groves the bacterium infects over 60% of the trees, but most of these trees will perform well if otherwise cared for appropriately. The pathogen can also be introduced through nursery practices.
Xanthomonas campestris is a common bacterium on avocado leaves and green twigs, where it apparently is harmless. Its reproduction and spread is favored by wet plants and humid conditions. It can infect through wounds and branch stubs and spread within the plant's vascular system. Drought stress and boron deficiency may promote development of disease symptoms. The disease most typically shows up in drought years, at the end of irrigation lines, or at points where irrigation system water pressure is lowest.
Normally the disease is a minor problem. Usually no control is necessary on established trees. If the disease is severe and yield is affected, remove the tree. Keep trees healthy and provide good cultural care. Provide appropriate amounts and frequency of irrigation and good uniformity of water distribution among trees. Use certified, disease-free nursery stock if available. Regularly inspect young trees and remove and dispose of young trees if they are infected. Nurseries should use stringent sanitation, regularly screen stock for disease, and dispose of affected trees so they are not planted.