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Homalodisca coagulata (Hemiptera, Cicadellidae) Transmission of Xylella fastidiosa to Almond

Almond leaf scorch (ALS) is caused by the bacterium Xylella fastidiosa, transmitted by sharpshooter leafhoppers and spittlebugs. The recent invasion of a X. fastidiosa vector, Homalodisca coagulata (Hemiptera, Cicadellidae), into California may have major consequences to the spread of ALS because this insect feeds readily on trees, including stone fruit species. We found that, under laboratory conditions, H. coagulata acquired X. fastidiosa from symptomatic almond plants with low efficiency relative to grape (3.3 to 10% per individual per day). Inoculation efficiency also was low, approximately 4% per insect per day. H. coagulata inoculated 1-year-old woody tissues of almond plants at similar rates as green shoots. H. coagulata transmitted two X. fastidiosa grape strains from grape source plants to grape and almond. We also observed X. fastidiosa transmission to dormant almond plants. X. fastidiosa populations in the petioles of field-collected symptomatic almond leaves were not higher than 10⁷ CFU/g of tissue, suggesting that low bacterial populations within almond are partially responsible for the lower acquisition rates observed from diseased almond compared with diseased grape, which are usually within the range of 10⁸ to 10⁹ CFU/g. The relevance of our findings to ALS epidemiology, considering H. coagulata as a vector, is discussed.