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Craig A. Ledbetter and Elizabeth E. Rogers

Seedling peach (Prunus persica Batsch) and clonal peach–almond hybrids are popular rootstock choices for commercial almond growers in California. In this study, clonal replicates of peach and almond [P. dulcis (Mill.) D.A. Webb] rootstock germplasm and a first-generation peach–almond hybrid created from them were challenged with Xylella fastidiosa isolate M23. Clonal replicates were needle-inoculated with M23 and maintained in a greenhouse environment for a growing season. Typical almond leaf scorch disease symptoms began to develop on M23-inoculated almonds 11 weeks after inoculation. No leaf scorch symptoms were observed on M23-inoculated peach or peach–almond hybrids. Quantitative real-time polymerase chain reaction revealed consistent levels of X. fastidiosa DNA among inoculated almond replicates, whereas X. fastidiosa DNA was undetectable in replicates of peach–almond hybrids. A trace level of X. fastidiosa DNA was detected in a single peach replicate, and statistical analysis demonstrated that this level differed significantly (P < 0.001) from that detected in almond replicates. Selected almonds were further sampled sequentially along their meristematic axes to examine bacterial titer throughout the trees. Selected almond trees differed significantly (P = 0.036) in bacterial titer, but no significant differences were noted in levels of X. fastidiosa from different vertical sections of the main growth axes. The results suggest that peach and peach–almond hybrid rootstock germplasm used by commercial almond tree nurseries in California are not primary inoculum sources for X. fastidiosa-induced diseases.

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Elizabeth E. Rogers and Craig A. Ledbetter

To facilitate development of Prunus L. rootstocks with desirable agronomic traits, domesticated peach (Prunus persica) and almond (P. dulcis) were crossed with wild almond relatives. This work reports that a hybrid from a P. webbii × P. persica cv. Harrow Blood cross is susceptible to almond leaf scorch disease (ALSD). ALSD is caused by the fastidious, xylem-limited bacterium Xylella fastidiosa. The P. webbii × ‘Harrow Blood’ hybrid, along with its parents, was inoculated with two ALSD-inducing strains (X. fastidiosa subsp. fastidiosa strain M23 and subsp. multiplex strain Dixon). Both X. fastidiosa strains grew to high titer in the susceptible P. webbii parent and in the interspecific hybrid; defoliation was also observed. As expected, ‘Harrow Blood’ did not exhibit defoliation symptoms or support growth of X. fastidiosa. This result contrasts with earlier work demonstrating that a P. persica × P. dulcis hybrid is not a suitable host for X. fastidiosa subsp. fastidiosa M23. It appears that the genetic basis of resistance/susceptibility differs between a P. persica × P. dulcis cross and the P. webbii × P. persica cross reported here. Understanding the degree of susceptibility to X. fastidiosa in complex hybrids of subgenus Amygdalus should be an important part of rootstock development.