Several approaches to interfering with the interaction between the vector and the pathogen have been proposed for controlling the spread of Pierce’s disease (PD). In theory, interference could be targeted at one or more of three stages: at the acquisition of the bacteria by the vector, during attachment and replication of the bacteria in the vector, or during the inoculation of the bacteria to a healthy host. Most of those approaches are experimental, and their effectiveness has not been demonstrated for other insect-vectored disease systems. However, several strategies are worth considering in the context of long-term management of PD: feeding disruption, inhibition by other bacteria of Xf attachment in the vector, and inhibition of transmission of PD strains of Xf by other strains by paratransgenesis.
The characteristics of Xf transmission by GWSS are similar to those of other known leafhopper vectors of Xf, although GWSS is a less efficient vector than is BGSS, a primary vector in Northern California. Leafhopper vectors acquire Xf through feeding on the xylem of host plants, and the bacteria replicate in the mouthparts (Purcell et al., 1979). Although Xf can be acquired by adults and nymphs, it is lost during the molting process—a fact that suggests that the bacteria attach to the foregut, because the foregut lining is shed in molting. Scanning-electron microscopy of leafhopper vectors shows bacteria attached to the cibarial pump and the lining of the esophagus in the foregut (Purcell et al., 1979). Adults that acquire the bacteria can continue to transmit throughout their lifetime (Severin, 1979). Although there is currently no strong evidence of gender differences in transmission (Redak et al., 2003), differences have been shown for other leafhopper-transmitted pathogens and could have significant effects on epidemiology (Beanland et al., 1999).
Adult GWSS can acquire Xf from infected plants and inoculate healthy plants in less than an hour of access time on a plant. There is no evidence for latent period between acquisition of the bacteria and the ability to transmit it (Almeida and Purcell, 2003). The rate of successful inoculation increases with increased time on the plant, but acquisition efficiency does not increase after 6 hours. Nymphs and newly molted adults transmit more efficiently than older adults do. Almeida and Purcell (2003) reported, in their experiments, that a maximum of 20% of individual leafhoppers acquired and transmitted Xf. In contrast, experiments with BGSS indicated an average transmission efficiency of 90% (Purcell and Finlay, 1979). More work demonstrated 68% efficiency of Xf acquisition from infected grape and 56–99% inoculation efficiency to grape, depending on the number of days post acquisition (Hill and Purcell, 1995). One striking difference between the two major vectors is that GWSS feeds on woody tissues throughout the year and can transmit Xf to stems that are more than 2 years old, but BGSS transmits only to green shoots (Almeida and Purcell, 2003).