Identification of an Almond Cultivar Resistant to <i>Xylella fastidiosa</i> subsp. <i>fastidiosa</i> and Insights into Host-Specific Bacterial Behavior

Xylella fastidiosa (Xf) is a xylem-limited bacterial pathogen causing disease in a variety of important agricultural crops, including almonds. Almond leaf scorch (ALS) disease was first identified in Israel in 2017 and is associated with the subspecies X. fastidiosa subsp. fastidiosa (Xff) sequence type 1. Currently, there is no effective means to cure infected almond trees, and ALS management relies primarily on uprooting infected trees and reducing vector populations, yet these methods are only moderately effective. Thus, identifying resistant almond genotypes is a critical goal for developing durable ALS management strategies. In this study, we utilized three complementary methods to evaluate resistance among various almond genotypes: 1) grafting onto Xff-infected trees; (2) mechanically inoculating young trees’ stems; and (3) mechanically inoculating individual leaves. In a field-based grafting experiment involving seven genotypes, ‘Lauranne’ exhibited strong resistance to Xff infection compared to the other tested cultivars. In subsequent two experiments, we monitored Xff migration and colonization in ‘Lauranne’, a susceptible almond cultivar (‘Um ElFahem’), and a non-host plant, plum. Although Xff was able to migrate and replicate within ‘Lauranne’, it did so at significantly lower levels and with slower progression rate. Histological analysis at the leaf level further confirmed that the number of Xff-colonized xylem vessels was substantially reduced in ‘Lauranne’ relative to the susceptible cultivar. These findings highlight ‘Lauranne’ as a promising resistant cultivar and underscore the importance of identifying resistance mechanisms in almonds. Incorporating resistant genotypes (such as ‘Lauranne’) into breeding programs may offer a durable and sustainable solution to manage ALS in the future.