Cultivar-Specific Interactions for Strains of Xanthomonas campestris from Florida that Cause Citrus Canker and Citrus Bacterial Spot. J. H. Graham, Associate Professor, University of Florida, IFAS, Citrus Research and Education Center, Lake Alfred 33850. T. R. Gottwald, and D. Fardelmann. Research Plant Pathologist, U.S. Department of Agriculture, ARS, Orlando, FL 32803; and Assistant in Plant Pathology, University of Florida, Lake Alfred 33850. Plant Dis. 74:753-756. Accepted for publication 19 March 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0753.

Lesion expansion and development of bacterial populations in leaves after pin-prick inoculation were used to characterize the host interaction of eight citrus cultivars with an Asiatic citrus canker strain of Xanthomonas campestris pv. citri (group A) and with three strains of X. campestris pv. citrumelo that differ in aggressiveness and cause citrus bacterial spot in Florida nurseries. Populations of X. c. pv. citri increased or were maintained in all cultivars, and differences among cultivars in rates of lesion expansion up to 40 days were slight except for trifoliate orange, which is a citrus relative. In contrast, populations of the aggressive strain of X. c. pv. citrumelo were maintained in trifoliate orange and its hybrids, Swingle citrumelo and Carrizo citrange, but declined in Duncan grapefruit, sweet orange, and other citrus cultivars. In general, the size of lesions caused by the aggressive strain on trifoliate orange, Swingle citrumelo, and Carrizo citrange was greater in the greenhouse and field than on other cultivars where lesion expansion ceased after 20–30 days. Less aggressive strains of X. c. pv. citrumelo caused smaller lesions than the aggressive strain, and populations declined to 10² or less in lesions after 40 days on all cultivars. Bacterial population in lesions and lesions size were correlated when all cultivars and strains were considered. We conclude that the aggressive strain of X. c. pv. citrumelo has a host range different from the Asiatic strain of X. c. pv. citri. The less aggressive strains, even though they are isolated from citrus, are apparently incompatible with citrus and perhaps should not be classified in X. c. pv. citrumelo.