Investigating canker and soil borne diseases of pistachio in California

Abstract

During the year 2015 and 2016, we surveyed the main pistachio growing counties in California to assess the health of pistachio trees and investigate the occurrence of canker and soil borne diseases. Numerous orchards were visited throughout the San Joaquin Valley to diagnose unusual symptoms and investigate the causes of decline of pistachio trees. Canker diseases causing branch and scaffold dieback were observed commonly in mature pistachio orchards. New fungal pathogens associated with cankers included Cytospora species, Diaporthe ambigua (=Phomopsis ambigua), and Colletotrichum karstii. A pathogenicity test using fungi isolated from cankers was conducted in the field to determine the ability of these fungi to produce cankers. Results of this study showed that all species tested were pathogenic to pistachio, causing extensive lesions and vascular discolorations just a few months after infection. Among them, Cytospora species were the most aggressive fungal pathogens and are considered new threats to pistachio in California. Other disease symptoms observed commonly in our surveys included declining trees characterized by chlorotic foliage as well as wilting, defoliation and eventual tree death. Trunks often expressed gumming together with crown rot symptoms. Investigations of the causes of pistachio tree decline revealed the occurrence in orchards of several Phytophthora species including P. niederhauserii, P. cinnamomi and P. taxon walnut. Other fungi isolated from declining pistachio trees and rootstocks included Fusarium oxysporum, F. solani, F. equiseti and F. proliferatum and Macrophomina phaseolina. Pathogenicity in pistachio of the various soil borne pathogens were conducted in potted UCB-1 plants maintained in a greenhouse. Pathogenicity studies showed that all the various fungi can colonize and damage the vascular tissues of UCB-1 seedling rootstocks, causing substantial lesions and vascular discolorations. To date, the present project allowed to identify several new pathogens currently affecting the health of pistachio trees in California.