Vector transmission of Xylella fastidiosa to plants

Studies of vector transmission of Xylella fastidiosa to plants, emphasizing the sharpshooter leafhopper Homalodisca coagulata as a vector, and grape and almond as host plants, focused on factors influencing X. fastidiosa transmission. H. coagulata transmitted X. fastidiosa to grapevines with the same observed characteristics as other vectors: (i) no requirement for a latent period, (ii) persistence of vector infectiousness over time, and (iii) the elimination of vector infectiousness with insect molting. H. coagulata transmitted X. fastidiosa to two-year-old woody tissues of grapes and grape seedlings at a similar efficiency, and there was no correlation between numbers of bacteria detected by culture in the insect and transmission to plants. In comparison to other vectors previously studied, H. coagulata had a lower and more variable transmission efficiency. Transmission of X. fastidiosa to almonds by H. coagulata occurred with a low efficiency: approximately 4% inoculation efficiency per insect per day. The acquisition efficiency by individuals occurred at similar rates. X. fastidiosa populations were estimated in natural infections of almond at various locations in California. In general, bacterial numbers were approximately 107 colony forming units per gram of plant tissue, which is about 10-fold lower than that observed within grapevines, and may be one of the factors restricting vector transmission of X. fastidiosa from almond. H. coagulata transmitted X. fastidiosa to dormant grape and almond plants in the laboratory, as well as to dormant grapevines in the field during February (2002). Inoculation rates in the field during the winter were similar to those obtained during the spring (∼20% per group of 4 H. coagulata per week). Grape strains mechanically inoculated into grapes and almonds caused disease in both hosts, but most almond strains multiplied but did not cause disease in grapes. Most of these almond strains also did not grow on PD3 solid medium. Overall, bacterial populations for the same strain in almond were 10-fold lower than in grape. Probing behaviors of another leafhopper vector, Graphocephala atropunctata, on grape were electronically monitored, and the waveforms obtained characterized and quantified. Various behaviors associated with stylet penetration and ingestion phases were identified and waveforms designated. Insects fed during 68% of the time spent on plants, with little change in behavior after stylet penetration of plant tissue.