Multiplication and movement of a citrus strain of Xylella fastidiosa within sweet orange
Populations of cultivable cells of a citrus variegated chlorosis (CVC) disease strain of Xylella fastidiosa in stems and leaf veins of sweet orange (Citrus sinensis (L.) Osbeck) seedlings were estimated by dilution plating at 1, 2, 4, 8, and 16 weeks after needle inoculation. Cell populations ranged from log 4 to log 5 CFU/g of tissue after 1 week and increased to log 5 to log 7 CFU/g (median log 6) after 8 to 16 weeks. Recovery of greater than log 5 CFU/g from stem nodes distal to the inoculation site indicated systemic movement of the bacteria. Foliar symptoms in inoculated seedlings first appeared after 8 weeks. Population estimates from leaf veins of CVC-affected trees in citrus graves were in the same range but slightly lower (average log 5.8 CFU/g). X. fastidiosa was isolated from citrus more efficiently in periwinkle wilt-GelRite (PWG) and periwinkle wilt (PW) media than in charcoal-yeast extract with AGES buffer (BCYE) medium The relatively lower populations of cultivable cells of X. fastidiosa in citrus with CVC symptoms, compared with those reported in grapevines with Pierce's disease, suggest that most cells of X. fastidiosa within symptomatic citrus may be dead, explaining in part the low rates of vector transmission from citrus to citrus.