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Title: Diversity of Xylella fastidiosa host suitability among siblings from a non-traditional almond X peach cross

item Ledbetter, Craig
item Lee, Steven

Submitted to: Euphytica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2018
Publication Date: 4/21/2018
Citation: Ledbetter, C.A., Lee, S.A. 2018. Diversity of Xylella fastidiosa host suitability among siblings from a non-traditional almond X peach cross. Euphytica. 214:84.

Interpretive Summary: Almond, a widely grown tree nut in California’s central valley and consistently among the top California exports in terms of tonnage and product value, is affected by a bacterial disease called Almond Leaf Scorch that reduces tree vigor and almond yields. Both the particular almond variety and the rootstock used in the orchard can have a dramatic effect on the Almond Leaf Scorch present in the orchard, with peach rootstock generally being considered resistant to the bacteria responsible for Almond Leaf Scorch disease. However, the almond industry is now moving away from peach as a rootstock in favor of higher yielding and more vigorous peach-almond hybrids. In our research, we inoculated diverse peach-almond hybrids with the bacteria responsible for Almond Leaf Scorch disease and followed tree development and disease expression during the growing season. At the end of the season we also determined the amount of bacteria present in each of the inoculated trees. While all inoculated trees developed measurable bacteria by the end of the growing season, only about half of the peach-almond hybrids developed Almond Leaf Scorch disease symptoms. Further, concentrations of the bacteria ranged widely among the trialed peach-almond hybrids, with some non-symptomatic accessions having very high bacterial concentrations. Our work demonstrated the diversity of symptoms expression and host suitability of peach-almond hybrid candidate rootstocks to the bacteria responsible for Almond Leaf Scorch disease. Almond Leaf Scorch has been a re-occurring problem throughout California’s 1.1+ million acres for at least the last 50 years. With the shift toward peach-almond hybrid rootstocks in the California almond industry, it is important to determine the resistance/tolerance profile of potential new rootstocks to the wide variety of biotic and abiotic problems facing almonds in California orchards.

Technical Abstract: Ten F2 clones from an initial hybridization of Prunus webbii X P. persica cv Harrow Blood were evaluated under greenhouse conditions for their reaction to Xylella fastidiosa subsp. fastidiosa strain M23 during two growing seasons. Clonal accessions used for the study were selected on the basis of horticultural diversity, and were a small subset of trees from a large F2 population. Foliar symptoms of M23-inoculated trees were monitored weekly throughout the 20-week growth period. Clones were then sampled for bacterial titer determinations. With the exception of parental accession Harrow Blood, all clones yielded measurable titer; however, almond leaf scorch disease symptoms were never observed in five of the ten sibling clonal accessions. Vegetative bud break and bloom phenology data collected from field-grown mother trees over a seven year period as well as leaf morphology characters of the clonal accessions were examined for associations with bacterial titers of inoculated clones using a principal component analysis. Higher bacterial titers were negatively associated with leaf blade size.