In the summers of 2000 and 2001, shoot blight was observed in pistachios (Pistacia vera L.) grown in Kern County, California. Black, necrotic lesions developed at the base of shoots originating from contaminated or partially infected buds. Infection moved upward resulting in a progressive wilting and blighting of leaves. Leaf blades on infected shoots withered, and petioles became necrotic. Symptoms have been considered characteristic of infection by Botryosphaeria dothidea (Moug.:Fr.) Ces. & de Not., but this pathogen causes panicle and shoot blight of pistachio (1). However, there were no symptoms of any fruit panicle infections on trees we observed. Isolations on acidified potato dextrose agar from the base of blighted shoots in both years revealed a fast-growing fungus producing pycnidia which was identified as the anamorph Lasiodiplodia theobromae (Pat.) Griffon & Maubl. of B. rhodina Berk. & Curt. Arx. Identification of the pathogen was based on characteristic dark brown, oval pycnidiospores with striations on the surface of the spore along the long axis. Pathogenicity tests were performed on 12 Kerman pistachio trees grown at Kearney Agricultural Center, in Parlier, CA, using three isolates recovered from pistachios grown in two locations. Six to 16 current season shoots of pistachio trees (1 to 2 shoots per tree) were wounded with a 5-mm-diameter cork borer, and a mycelial plug of 5-day-old cultures of B. rhodina was inserted in each wound. Shoots were wrapped with Parafilm to prevent desiccation of inoculum. Six other shoots (one per tree) were inoculated similarly with mycelial agar plugs of a pistachio isolate of B. dothidea and served as positive controls, while six similar shoots were inoculated with only agar plugs and served as negative controls. Wilting of lower leaves in the majority of inoculated shoots started within 4 days for B. rhodina and 7 days for B. dothidea. Depending on the isolate of B. rhodina, 1 to 5 shoots and 50 to 80% of leaves were blighted within 7 days after inoculation. All inoculated shoots were left on the trees until 3 to 4 months after inoculation, pruned and assessed again. For inoculations done in September 2001, 33 to 71% of shoots were blighted, and the rest had cankers ranging from 22.5 to 28 mm long and 13.5 to 23.5 mm wide. A majority (67 to 100%) of shoots had pycnidia of the pathogen present. For inoculations done in October 2001, none of the shoots was blighted, but cankers ranged from 5 to 55.4 mm long and 6 to 22 mm wide and 33.3 to 100% developed pycnidia. B. rhodina was isolated from all inoculated shoots but not from negative controls or those inoculated with B. dothidea. Inoculations of shoots with B. dothidea produced similar symptoms as those of B. rhodina. Shoots that served as negative controls did not develop symptoms. Because panicle and shoot blight of pistachio caused by B. dothidea has developed to epidemic levels in commercial pistachio orchards and is of concern to the pistachio industry in California, it would be of interest to monitor how much shoot blight caused by B. rhodina would eventually develop over the years in commercial pistachio orchards. A survey was initiated in 2002 to determine how widespread B. rhodina is in California pistachios. To our knowledge, this is the first report worldwide of B. rhodina causing shoot blight of pistachio.
Reference: (1) T. Michailides. Panicle and shoot blight. Page 68 in: Compendium of Nut Crop Diseases in Temperate Zones. B. L. Teviotdale, T. J. Michailides, and J. W. Pscheidt, eds. American Phytopathological Society, St. Paul, MN 2002.
