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Growth and Survival of Xanthomonads Under Conditions Nonconducive to Disease Development. L. W. Timmer, Professor, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, Lake Alfred 33850; J. J. Marois(2), and Diann Achor(3). (2)Assistant professor, University of California, Department of Plant Pathology, Davis 95616; (3)Electron microscope technician I, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, Lake Alfred 33850. Phytopathology 77:1341-1345. Accepted for publication 7 May 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1341.

Detached leaves or entire plants of host and nonhost species were sprayed with suspensions (103104 cfu/cm2 of leaf) of xanthomonads and held under different humidities. Leaf surface populations were determined by washing and dilution plating onto a semiselective Tween medium. Populations of Xanthomonas campestris pv. vesicatoria (X. c. pv. vesicatoria) increased 10- to 100-fold on tomato leaves at high relative humidity (RH) (> 90%), fluctuated erratically at moderate RH (5065%), and declined to low or nondetectable levels at low RH (1025%). Bacteria did not survive well on tomato plants sprayed with X. c. pv. vesicatoria suspensions and placed immediately under conditions unfavorable for disease development. However, if plants were held under conditions of high moisture for 48 hr and then placed under adverse conditions, surface populations removable by washing declined slowly over several weeks and populations in leaf homogenates remained high. X. c. pv. alfalfae, X. c. pv. campestris, X. c. pv. translucens, X. c. pv. pruni, and a saprophytic isolate of X. campestris multiplied as well on detached tomato leaves at high RH as did X. c. pv. vesicatoria. At high RH, X. c. pv. vesicatoria populations remained constant or increased on leaves of almond, plum, peach, walnut, and sweet orange. When X. c. pv. translucens was applied to tomato leaves and plants were maintained at high RH for 48 hr, surface populations declined slowly over several weeks and populations in leaf homogenates remained as high as they did for X. c. pv. vesicatoria on tomato. Scanning electron microscopy revealed that xanthomonads survived on tomato leaf surfaces in small aggregates in an extracellular matrix. Xanthomonads are capable of multiplication and survival for at least several weeks on the surfaces of host species in the absence of disease and can also survive and multiply on nonhost plants under certain conditions.