Resistance of Almond (Prunus dulcis) to Xylella fastidiosa: A Comparative Study on Cultivars

Almond leaf scorch is a disease caused by Xylella fastidiosa, which is increasingly widespread globally in the main almond cultivation areas. Previously confined to America, in the last decade this disease has been reported in Iran and southern Europe. In this comparative study, the resistance to X. fastidiosa subsp. multiplex of 13 almond cultivars (Mamaei, Non-Pareil, Sefied, Rabie, Ferragnes, Shahrood21, Thompson, Merced, Marcona, Frudeul, Kapariel, Princess, and Tuono) grafted onto seedlings of Talkhe was evaluated in controlled conditions. Plants were artificially inoculated and maintained in greenhouse conditions. Approximately 3 months after inoculation, three times at 3-week intervals, disease incidence, disease severity, and disease index were determined based on scaling, and bacterial populations were estimated. The effect of winter survival of bacteria in outdoor potted seedlings was also investigated in all almond cultivars. Findings showed a great degree of variability in response to X. fastidiosa among cultivars considering symptom development and severity, as well as bacterial titer. Thompson and Rabie cultivars scored the best results from both a symptomatological and infectious point of view, indicating resistance against the pathogen compared with other tolerant cultivars (e.g., Ferragnes, Tuono, and Kapariel), thanks to the development of mild symptoms. Mamaei, Non-Pareil, and Sefied scored worst, suggesting a susceptible behavior when infected by X. fastidiosa. Given that the pathogen was not detected by culturing and PCR during the following summer, bacterial population in potted seedlings was reduced significantly by overwintering in outdoor conditions regardless of cultivar susceptibility. This suggests that cold treatment can be used as a preventive treatment to manage nursery almond seedlings.