A requisite for development of bacterial phloem canker in walnuts (caused by Erwinia rubrifaciens) was the presence of the highly susceptible Hartley cultivar; although, when interplanted with Hartley, the Franquette and Payne cultivars were sometimes also attacked by the disease. The recently developed cultivars Gustine and Howe developed active cankers when inoculated, but not as extensive as those in Hartley. The age of the plant part was important to the development of the disease. The only parts of the tree developing the complete disease syndrome were the trunks and primary (scaffold) branches. Extension of the cankers in the tree was most rapid during the summer when the temperature was high. This was correlated with the effect of temperature on multiplication and growth of the bacterium in culture. Another requisite to development of the disease in these tests was the presence of openings in the thick phelloderm of the trunks and branches through which the pathogen can enter the inner bark. Of the several types of breaks commonly occurring, those produced by mechanical harvesting equipment and by sap-sucking birds were found to be infection sites. The pathogen occurred in large numbers in a slimy substance which exudes through cracks and accumulates on the bark of infected trees. They survived for at least 123 days in the exudate and were disseminated laterally as far as 20 ft in windblown rain. In addition, the exudate with viable bacteria was picked up on the pads of mechanical harvesting equipment.

This work was supported in part by the Walnut Control Board, the Diamond Walnut Growers Association, and USDA Grant No. 12-14-100-9930(34).