The status of Neofabraea leaf and shoot lesions, Pleurostoma decline and Anthracnose in super-high-density oil olive orchards in California

Abstract: California produces more than 95 percent of the olives grown in the United States. In 2019, the bearing acreage for olives was 14,700 hectares with a total of 164,650 tons of olives produced at a value of nearly $130 million. While these figures indicate a growing industry, olive oil production could be challenged by new and emerging diseases in olive orchards. During the winter of 2016, Neofabraea leaf and twig lesions was first detected in super-high-density (SHD) oil olive orchards in California. Affected trees revealed numerous leaf and shoot lesions, and cankers in branches, which developed at wounds caused by mechanical harvester. Two species, namely Phlyctema vagabunda and Neofabraea kienholzii, were found to be consistently associated with the disease and Koch’s postulates were completed. The cultivar ‘Arbosana’ was highly susceptible to the disease, whereas cultivars ‘Arbequina’ and ‘Koroneiki’ appeared to be tolerant. Field trials indicated that several fungicides could reduce disease incidence and management strategy guidelines were implemented to limit further spread of the disease. Pleurostoma decline of olive trees caused by the fungal pathogen Pleurostoma richardsiae was also recently detected in SHD olive orchards. Symptoms of Pleurostoma decline in olive trees included leaf yellowing and browning, leaf drop as well as wilting and dieback of twigs and branches, as well as brown to dark discoloration of the wood, while severely affected trees died. Pleurostoma richardsiae was also isolated from collar rot symptoms in olive trees. Field observations suggested that infections by P. richardsiae initiate at wounds in trunks and branches caused by field equipment and large insect borers. Following a state-wide survey of olive orchards in California, olive anthracnose was not detected in SHD olive orchards. However, the disease was observed in an orchard of Gordal-Sevillana olives located at the Kearney Agricultural Research and Extension Center. Main symptoms in this orchard included fruit rot characterized by circular sunken lesions with acervuli forming in concentric rings. Fungal isolates obtained from the olive fruits in this orchard were identified as Colletotrichum fioriniae in the C. acutatum species complex based on DNA phylogenetic analyses. These analyses also indicated that Colletotrichum isolates from olive were genetically similar to those obtained from pistachio and almond, suggesting the olive anthracnose inoculum is present in California. However, the absence of olive anthracnose in commercial SHD oil olive orchards in California suggests that the disease does not thrive in olive under the environmental conditions of California’s Central valley.