Development of resistance traits in agricultural crops is often a critical component of integrated disease management strategies. In contrast to resistance traits, tolerance traits—traits that reduce disease symptom severity without reducing pathogen populations in a host plant—may actually increase the risk of pathogen spread through a host population. At the same time, vector preference for hosts based on host infection status can have strong impacts on disease spread. Theoretical work from our group predicts that the precise form of defense—whether resistance or tolerance—will influence the spread of a vector-borne pathogen depending on whether vectors prefer or avoid feeding on diseased hosts.
The vector-borne bacterial pathogen Xylella fastidiosa causes Pierce’s Disease in grapevines and has led to reduced productivity and increased insecticide use in California vineyards. In response, a new defensive trait from native Vitis spp., conferred by the PdR1 gene, has been incorporated into commercial cultivars. At the same time, the insect vectors of X. fastidiosa avoid feeding on diseased grapevines, increasing the risk of enhanced pathogen spread among tolerant grapevines. We sought to assess the nature of PdR1 defense and the potential for vectors to transmit X. fastidiosa in small-scale experiments.
While the new PdR1 trait reduces disease severity in grapevines, our preliminary results indicate that it may confer a complex mix of tolerance and resistance against X. fastidiosa. Moreover, the nature of defense appears not to be constant over time but varies as the disease progresses. Specifically, PdR1 plants appear to have a long incubation period, or tolerant phase, prior to the onset of disease symptoms. Finally, we use our experimental results to parameterize a compartmental SIR epidemic model; we predict significant spread of X. fastidiosa within PdR1 vineyards, though lower disease prevalence, compared to a wild-type vineyard. Our work has implications for Pierce’s Disease management specifically and the evaluation of host plant defenses against vector-borne pathogens more generally; distinguishing between host plant resistance and tolerance appears to be critical when evaluating defensive traits.