material between the engineered endophyte and Xf or other microbes could occur. Genome-sequencing projects have revealed strong evidence for lateral transfer of genes to Xf. Examples include the tryptophan operon (Xie et al., 2003), 19 genes common to X. axonopodis pv. citri (da Silva et al., 2002), and a type II restriction–modification system similar to Nostoc (Van Sluys et al., 2003). A second problem concerns the inability to confine an altered organism once it is released into the environment. Finally, use of the approach could produce unintended effects on other microbes, particularly beneficial microbes or naturally occurring endophytic populations in nontarget insects and plants.
Because projects that have examined biological control of bacterial vascular pathogens, particularly of perennial crops, generally have shown limited success, the committee views them as Category 4 studies. Naturally occurring endophytes or attenuated strains of Xf have not been effective to date in control of PD in the field. However, better information about the Xf and endophyte genes required for colonization, establishment, and virulence or antagonism that could come from genome analysis could be used to identify target genes and allow development of effective biological-control agents. No biological-control organism, particularly one that is genetically modified, must ever be introduced to the environment without thorough evaluation of consequences. The current public concern over release of non-native or engineered organisms into the environment is significant, particularly in California.
Of the six general principles for disease control listed in the beginning of this chapter the most feasible for management of Xf are avoidance and disease resistance. The best prospects for research that will lead to management through interference with Xf–grapevine interactions, either in the short or in the long term, are projects that identify vulnerable points in the Xf life cycle or in its interactions with grapevines (attachment); those that evidence host plant resistance, either through identification and manipulation of genetic traits for resistance or through introduction of novel resistance through transgenic technologies; and those that promote avoidance of disease through cultural practices (vegetation management).