Biology and control of Neofabraea leaf spot and twig lesion of oil olives in California

Abstract: California produces more than 95 percent of the olives grown in the United States. In 2014, the bearing acreage for olives was 37,000 acres with a total of 94,000 tons of olives produced at a value of $72.9 million. While these figures indicate a growing industry, olive oil production could be challenged by the recent discovery of an emerging disease in olive orchards. This new disease, namely Neofabraea leaf spot and twig lesion, was first detected during the winter of 2016 in super-high-density oil olive orchards in San Joaquin and Glenn Counties in California. Affected tree canopies revealed numerous leaf spots and cankers on shoots and twigs, and developing at wounds caused by mechanical harvester. As the disease progressed into the spring, trees suffered severe defoliation causing substantial yield loss. Two species, namely Phlyctema vagabunda and Neofabraea kienholzii, were found to be consistently associated with the disease and Koch’s postulates were completed. Based on field observations and pathogenicity studies, the cultivar Arbosana was highly susceptible to the disease, whereas cultivars Arbequina and Koroneiki appeared to be fairly tolerant. Fungicides trials were conducted in the field to determine the efficacy of various chemical products to mitigate the disease. Several fungicides reduced the incidence of the disease by 60 to 80 percent. Management strategy guidelines are being developed and implemented to limit further spread of the disease.