Etiology and management of trunk and scaffold canker diseases of almond in California

In 2016, the California almond industry accounted for about 5.9 billion dollars in farm value and the total almond acreage was estimated at 1.11 million acres. Trunk and scaffold canker diseases (TSCD) of almond can cause significant yield and tree losses within orchards, while also reducing orchard life spans. TSCD of almond have become a major concern in recent years in California as they can affect young, recently planted orchards, and usually become prevalent as orchards age. During the years 2015 and 2016, surveys were conducted throughout California to determine the main pathogens associated with TSCD of almond. Approximately 300 fungal isolates were isolated from cankers and identified for this study. Results revealed a broad diversity of fungi associated with cankers including Botryosphaeriaceae spp., Ceratocystis fimbriata, Eutypa lata, Cytospora spp., Collophora spp., and Diaporthe spp. Botryosphaeriaceae spp. accounted for the largest group of pathogens with Spencermartinsia viticola, Neofusicoccum vitifusiforme, Diplodia mutila being reported for the first time in California almond. Eutypa lata was detected mainly in the Northern San Joaquin Valley and Sacramento Valley. Collophora hispanica and C. paarla constituted first reports in California almond. Ceratocystis cankers were common in orchards that suffered bark injuries caused by mechanical shakers. Field observations suggested that pruning wounds made during primary and secondary scaffold selections serve as the main sites for infection with canker pathogens. Pathogenicity tests revealed that all species tested were pathogenic to almond with several Botryosphaeriaceae species being the most aggressive canker pathogens. Management strategies against TSCD rely for the most part on remedial surgery and prophylactic practices including removal of dead wood or trees from the orchard. Studies of the efficacy of fungicides, pastes, sealants and biocontrol agents are underway to determine the most effective products for pruning wound protection and management of TSCD.