Fungal Pathogens Associated With Canker Diseases of Almond in California

Abstract

Almond canker diseases are destructive and can reduce the yield as well as the lifespan of almond orchards. These diseases may affect the trunk and branches of both young and mature trees, and in severe cases can result in tree death soon after orchard establishment. Between 2015 and 2018, 70 almond orchards were visited throughout the Central Valley of California upon requests from farm advisors for canker disease diagnosis. Two major canker diseases were identified including Botryosphaeriaceae cankers and Ceratocystis canker. In addition, five, less prevalent canker diseases were identified including, Cytospora-, Eutypa-, Diaporthe-, Collophorina and Pallidophorina canker. Seventy-four fungal isolates were selected for multi-locus phylogenetic analyses of ITS1-5.8S-ITS2 (ITS), and part of the translation elongation factor 1-α (TEF1- α), β-tubulin (TUB2), and glyceraldehyde 3-phosphate dehydrogenase (GPD) gene sequences, which identified 27 species including 12 Botryosphaeriaceae species, Ceratocystis destructans, five Cytospora species, Collophorina hispanica, four Diaporthe species, two Diatrype species, Eutypa lata, and Pallidophorina paarla. The most frequently isolated species were C. destructans, Neoscytalidium dimidiatum and C. californica. Pathogenicity experiments on almond cv. Nonpareil revealed that N. parvum, N. arbuti and N. mediterraneum were the most virulent. Botryosphaeriaceae cankers were predominantly found in young orchards and symptoms were most prevalent on the trunks of trees. Ceratocystis canker was most commonly found in mature orchards and associated with symptoms found on trunks or large scaffold branches. This study provides a thorough examination of the diversity and pathogenicity of fungal pathogens associated with branch and trunk cankers of almond in California.