Mechanical and insect transmission of Xylella fastidiosa to Vitis vinifera
The bacterium Xylella fastidiosa Wells et al. induces Pierce's disease of grapevine. Four potential mechanisms of X. fastidiosa transmission to grapevines, Vitis vinifera L., were investigated: graft transmission by naturally occurring root grafts, mechanical transmission by pruning shears, and through the nymphs and adults of Homalodisca liturata Ball, the smoke tree sharpshooter, and the adults of Diceroprocta apache Davis, the apache cicada, two insects native to California. No naturally formed root grafts between X. fastidiosa-infected and noninfected grape plants were found and, consequently, no X. fastidiosa transmission by natural root grafts was detected (n = 5). One of 21 transmission attempts with pruning shears resulted in X. fastidosa transmission. Three of 24 smoke tree sharpshooter nymphs transmitted X. fastidiosa. Three of 14 attempts to transmit X. fastidiosa with smoke tree sharpshooter adults were successful, and 13 adults tested positive for X. fastidiosa as detected by polymerase chain reaction. One of 12 transmission attempts by the apache cicada resulted in X. fastidiosa transmission. Our findings of mechanical transmission by pruning shears from an infected shoot to a healthy shoot and insect transmission of X. fastidiosa by smoke tree sharpshooter nymphs and an apache cicada adult represent new means of transmission of the pathogen to grapevines. While transmission was rare via these mechanisms, results highlight the need to consider all means of transmission in the development of Pierce's disease management programs.