Differential Susceptibility of Prunus Germplasm (Subgenus Amygdalus) to a California Isolate of Xylella fastidiosa
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 I I 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.