Pistachio Growers on High Alert for Botryosphaeria Disease
April 16, 2024 
Symptoms of Botryosphaeria panicle shoot blight on pistachio.
Photo by Gerald Holmes
Having dodged frequent raindrops for the second consecutive winter, California pistachio growers should remain wary of a precipitation-induced disease spread by the Botryosphaeriaceae family of fungi, according to University of California, Davis Plant Pathologist Themis Michailides.
Some of the worst outbreaks of Botryosphaeria panicle and shoot blight have followed wet winters and springs. Michailides, speaking on behalf of the UC IPM Program, cites several rainy seasons over the last 45 years, starting with 1982-1983 (before the disease was even diagnosed), when 25.61 inches of rain fell in California, nearly 14 inches above the average.
“In 1984 we discovered the first Botryosphaeria disease up in some orchards in Northern California,” Michailides says. “Initially the industry thought, ‘This is not very important.’ They supported some research. We discontinued the research. But as a plant pathologist I tried to monitor the pathogen, which, at that time, was Botryosphaeria, and eventually we showed that the pathogen was spreading throughout California where the pistachios were growing.”
The worst outbreak of Botryosphaeria panicle and shoot blight — 100% yield loss in many pistachio orchards — occurred in 1998 and ’99 after 31.28 inches of rain had fallen during the 1997-98 season. The yield-reducing fungal disease, while allowing trees to survive, kills the trees’ fruit clusters, current growth shoots, and fruiting buds.
“The industry was in an emergency at that time. Even in 1997, some orchards showed excessive disease,” Michailides says. “This epidemic really opened our eyes and led the industry to find solutions because the disease was all over the pistachio acreage.”
Fungicide Efficacy
Aside from some cultural practices, pistachio growers had no control methods in 1998. That is no longer the case 26 years later.
“Chemical control is very important and very effective against this disease,” Michailides says. “The strobilurins — FRAC 11 and FRAC 7 and the combinations — are really excellent for controlling Botryosphaeria. The biologicals show very low efficacy against this disease.”
Michailides cites a study in which the five most successful fungicides — each reducing blighted panicles from 43% to less than 5% — were Pristine (boscalid/pyraclostrobin, BASF), Merivon (fluxapyroxad/pyraclostrobin, BASF) during season, Fontelis (penthiopyrad, Corteva) plus Abound (azoxystrobin, Syngenta), Luna Sensation (trifloxystrobin/fluopyram, Bayer), and Viathon (tebuconazole/phosphite, Helena Agri-Enterprises) 4.1 FS.
“The best time for one spray is early June,” Michailides says, “but the sprays that are most effective for Botryosphaeria control start in April and end at the end of July.”
If fungicide sprays or cultural practices — irrigation management and sanitation (winter/summer pruning) — are not implemented, Michailides warns that some San Joaquin Valley orchards had already felt the wrath of Botryosphaeria panicle and shoot blight last summer after the August arrival of Hurricane Hilary.
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