Submitted to: HortScience
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/3/2009
Publication Date: 12/16/2009
Citation: Ledbetter, C.A., Rogers, E.E. 2009. Differential Susceptibility of Prunus Germplasm (Subgenus Amygdalus) to a California Strain of Xylella fastidiosa. HortScience. 44(7):1928-1931. Interpretive Summary: Almond is a widely grown tree nut in California’s Central Valley and is consistently among the top California exports in terms of tonnage and product value. Almond trees are affected by a bacterial disease called Almond Leaf Scorch that reduces tree vigor and almond yields. Almond Leaf Scorch has been a re-occuring problem throughout California's 700,000+ acres for at least the last fifty years. Almond Leaf Scorch-affected trees are observed in a random distribution in orchards, and the primary source of the bacterial disease is not well understood. In our research, we inoculated peach and peach-almond hybrids (common rootstocks in California almond orchards), and an almond variety with the bacteria responsible for Almond Leaf Scorch disease and allowed the trees to develop for a full growing season. All inoculated almond trees developed Almond Leaf Scorch disease, whereas peach and peach-almond hybrids did not. The work demonstrated that peach and peach-almond hybrid rootstocks are not the primary source of Almond Leaf Scorch disease in California’s almond orchards. Therefore, the most common rootstocks for almond trees can be ruled out as primary disease sources for this important disease of almond.
Technical Abstract: Seedling peach and clonal peach-almond hybrids are popular rootstock choices for commercial almond growers in California. In this study, we challenged clonal replicates of rootstock accessions of peach, almond, and a first generation peach-almond hybrid with Xylella fastidiosa biotype 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 almond after 11 weeks of growth. No leaf scorch symptoms were observed on M23 inoculated peach or peach-almond hybrids. Quantitative real time PCR revealed consistent levels of X. fastidiosa DNA among inoculated almond clonal 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 almond clones were further sampled along meristematic axes to examine bacterial titer throughout the plants. Selected almond clones differed significantly (P = 0.036) in bacterial titer, but no significant differences were noted in levels of X. fastidiosa from different quadrants of the main growth axes. The results suggest that peach and peach-almond hybrid rootstocks used by commercial almond tree nurseries in California are not primary inoculum sources for X. fastidiosa induced diseases.