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Discussion: Fastidious Prokaryotes as Plant Pathogens

Gram-Negative, Xylem-Limited Bacteria in Plant Disease. D. L. Hopkins, Professor of plant pathology, University of Florida, Agricultural Research Center, Leesburg 32748; Phytopathology 73:347-350. Accepted for publication 29 July 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-347.

Studies that led to the isolation in axenic culture of the Gram-negative, xylem-limited bacteria associated with Pierce's disease (PD) of grapevine, almond leaf scorch, alfalfa dwarf, periwinkle wilt, phony disease of peach, and plum leaf scald are reviewed and discussed. The PD bacterium, shown by serological techniques to be present in citrus, could not be isolated in pure cultures directly from citrus, but was isolated in pure culture by using grapevines as selective hosts to separate the PD bacterium from bacterial contaminants. Epidemiological studies in Florida and California have found at least six wild plant species that serve as alternate hosts of the PD bacterium. Many characteristics of the PD bacterium, such as colony type, aggregation of bacterial cells in H₂O suspension, and virulence, are unstable during maintenance of the bacterium in axenic culture for 6- 12 mo. Avirulent isolates that developed in culture were shown to infect grapevines without producing symptoms. Bacterial aggregates blocking leaf veins were correlated with leaf-necrosis symptoms of PD. Tolerance of grape cultivars to the PD bacterium is discussed.