Xylophagous leafhopppers are common and abundant insects of tropical and subtropical environments and play important ecological roles in these ecosystems. The feeding biology of these insects is unique in terms of their high feeding rates and a digestive physiology that allows them to assimilate amino acids, organic acids, and sugars at approximately 99% efficiency. For those species well studied, fluctuations in plant xylem chemistry and tension appear to determine the diurnal and seasonal use of their host plants. Relatively few species of xylem fluid-feeding leafhoppers are considered important pests in commercial agriculture, as they transmit the bacterial plant pathogen Xylella fastidiosa. X. fastidiosa induces diseases of grapevines, citrus, coffee, almond, alfalfa, stone fruits, landscape ornamentals, and native hardwoods for which there is no cure. Two Xylella diseases, citrus variegated chlorosis (CVC) and Pierce's disease (PD) of grapevines, have emerged as important issues within the past decade. In Brazil, CVC became important in the early 1990s and has now expanded throughout many citrus-growing areas of South America and threatens to spread to North America. The recent establishment of the exotic glassy-winged sharpshooter (Homalodisca coagulata) in California now threatens much of the United States' wine grape, table grape, and almond production. The spread of H. coagulata throughout southern California and the spread of CVC northward from Argentina through Brazil exemplifies the biological risks from exotic species. The occurrence and epidemiology of leafhopper-vectored Xylella diseases are discussed.

Citrus variegated chlorosis (CVC), caused by the xylem-limited bacterium Xylella fastidiosa, has arisen from obscurity to become a major threat to the Brazilian citrus industry in less than 10 years. Since X. fastidiosa was first shown to cause CVC (3,20), serological (5) and molecular methods (1,21,25) have been developed to detect the bacterium in citrus. The pathogen is naturally transmitted to citrus by sharpshooter (Cicadellinae) leafhoppers (29). Transmission by natural root grafts and by top grafting with infected budsticks also were reported (10). Epidemiological studies suggest that most spread of CVC is from tree to tree within the citrus groves (19). Therefore, management of CVC is mainly based on planting healthy nursery trees, vector control, and removal of inoculum by roguing of symptomatic young plants (<2 years old) and pruning of branches with early foliar symptoms in older trees (8,30). The population behavior of X. fastidiosa within citrus is crucial to its pathology and control, but bacterial colonization of citrus has not been studied quantitatively and some characteristics of infection, movement of X. fastidiosa within citrus, and disease development remain unknown. He et al. (10) used polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) to detect movement of X. fastidiosa from scion graft to rootstock but did not quantify populations of the pathogen.A limiting factor to studies concerning X. fastidiosa is the slow growth of the pathogen on solid medium, leading to frequent contamination of culture attempts. This has hindered attempts to quantify populations of X. fastidiosa. Hopkins (14) developed a protocol to estimate population of viable X. fastidiosa cells from grapevines with Pierce's disease (PD) based on counting CFU of this bacterium on solid media by dilution plating. A modification of this technique (11) was used to study PD strains of X. fastidiosa in alternative hosts, with the bacterium reaching high populations (log 8 CFU/g of plant tissue) in grapevine, 100-fold lower populations in an asymptomatic host, Himalayan blackberry, and still lower, nonsystemic populations in water grass and mugwort. Populations of X. fastidiosa estimated by dilution plating were also correlated with vector transmission rates; bacterial populations lower than log 6 to log 7 CFU/g determined by culture in grapevine resulted in reduced transmission efficiency (13).The objective of our study was to follow the fate of a CVC strain of X. fastidiosa in citrus seedlings under greenhouse conditions and to estimate population levels of X. fastidiosa in symptomatic leaves of infected citrus trees in commercial orchards. We used culturing to quantify X. fastidiosa in leaves collected from the field and its growth and movement within citrus seedlings, following mechanical inoculation in the stem and leaf petioles with variable concentrations of live cells. We also tested different media for X. fastidiosa growth and evaluated citrus tissue homogenates for inhibition of bacterial growth i...