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 x 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 X '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 x 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 x P. dulcis cross and the P. webbii X 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.