Sharka, an Important Quarantine Disease for California: Risk Assessment through Almond Cultivars and Rootstocks Phenotyping

Sharka is among the most important diseases of stone fruits globally. It is caused by the Potyvirus plumpoxi [plum pox virus (PPV)] and affects mainly apricot, plum, prune, and peach trees. In 1999, PPV was detected for the first time in the United States, and it was declared eradicated by the US Department of Agriculture in Oct 2019. Despite the official declaration of eradication, Sharka is still of great concern, mainly because of the huge quarantine efforts and millions of dollars spent on eradicating it. Studies of vulnerability of Californian almond cultivars and rootstocks to Sharka are scarce, and the reactions of different cultivars and rootstocks to Sharka were never thoroughly analyzed. The main objective of this study was to assess the risk of the potential damage of PPV to almond orchards in California and the subsequent dispersion of the virus to other Prunus tree crops by evaluating the susceptibility and tolerance of the most cultivated almond cultivars and rootstocks. A group of cultivars and rootstocks were evaluated to determine their vulnerability to PPV-Dideron. After 4 years of analyses with more than 1100 tests performed, a total of 11 almond cultivars and 17 rootstocks were infected. None of the almond cultivar replicates that tested positive (21) displayed Sharka symptoms. However, most of the 19 rootstocks assays displayed significant Sharka symptoms and tested positive (327). These results indicate the importance of rootstocks in a potential Sharka disease outbreak in California. In addition, almond tree infection was detected for the first time with our isolate, but only in a small number of trees.