Beating BOT Blight In Pistachios

Many of California’s pistachio growers were taken by surprise when an outbreak of Botryosphaeria blight, or BOT blight, struck their orchards this past season. Boytryosphaeria is spread primarily through moving water in orchards, and late rains in the state last year helped spread Botryosphaeria dothidea spores, initiating multiple infections, says Themis Michailides, a plant pathologist at the University of California’s Kearney Agricultural Center.

Researchers have discovered that more species of Botryosphaeriaceae fungi are involved in BOT blight than were initially thought. According to Michailides, these species are abundant among nut crops and other hosts in California, and they accumulate in the bark of older trees, even healthy branches. When dead clusters, cankers, buds, and shoots accumulate on the trees, pathogens can survive and produce spores in these cankers for six years or longer. If present, the disease can be spread via insect transmission, pollen, and pruning equipment, too.

Monitoring And Management

Michailides says growers should do a bud monitoring (BUDMON) test to determine how much Botryosphaeria, if any, is present in orchards before pistachio shoots and blooms emerge, usually in February or March. “And watch the weather carefully,” he adds. “If the prognosis is for rains that coincide with full bloom, growers will need to apply a bloom spray before the rains.” Michailides recommends monitoring leaf wetness and temperature to determine the best times and how much to spray.

Another option is overnight freezing incubation (ONFIT), which is usually done in June.

Growers should remain vigilant and look out for symptoms like blighted shoots and clusters. Blackening of the main shoot and rachis of clusters is reason for concern. “Learn how to distinguish Botryosphaeria from Botrytis shoot and blossom blight,” Michailides says. “Beige or buff color powdery spores are present at the base of shoots in the case of Botrytis infections, while Botryosphaeria will have pycnidia inside the plant tissues.”

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