Wet weather during bloom starting in early April increased the threat of both Botrytis and Botryosphaeria in pistachios.
In an effort to head off another outbreak of Botryosphaeria, PCA Gary Weinberger, Weinberger Associates, Hanford, Calif., has begun treating his pistachio orchards in Tulare, Kings, Kern and Madera counties with fungicides.
Right now, he says, the trees show no sign of damage from either disease, but he is concerned that Botryosphaeria inoculum carried over from last year’s outbreak could be spread by rain and insects and could kill one-year-old shoots and nut clusters later in the season.
“Botryosphaeria infection begins in cankers in wood during winter and spreads to year-old wood and fruit in early spring,” Weinberger notes. “Hot weather triggers growth of the fungus during the summer. You have to control it early to mid-season to prevent the damage that occurs just before harvest in August and September.”
He plans to spray his pistachio orchards with fungicides from now through July. Treatments in April and May are preventive, to head off problems from persistent rains this spring. Moisture can spread disease that overwintered, and cankers can supply inoculum for as long as six years.
Buds that were partially infected the previous season produce fruit clusters and shoots from the end of May to June that develop blight. As temperatures rise May through July, the fungus causes blighting of fully developed clusters, and blighted shoots, leaves, and clusters turn brown.
“Fungicide applications in June and July are more preventive,” Weinberger says. “We want to stop growth of any infections that occurred earlier in the season.”
The total number of treatments over the four-month period could range from just two or three, if infections are light, to as many as six in heavily-infected orchards.
University of California Plant Pathologist Themis Michailides says 0.2 inch of rain and temperatures at or above 52 degrees can initiate Botryosphaeria infection (optimum temperature range for disease development is 80-86 degrees). The disease can become very severe during late summer when temperatures and relative humidity are high.
Twelve seasons ago, a warm, very wet spring and the lack of effective registered fungicides caused a severe outbreak of Botryosphaeria in many California pistachio orchards. Dry conditions since then have limited further Botryosphaeria problems until last year, when infections exploded.
Weinberger says two events triggered the 2009 outbreak — rain and ideal temperatures for growth of the fungus in May and large numbers of leaf-footed bugs, poking their needle mouth parts into nuts, created entry points for the disease.
Botryosphaeria has been more of a problem in the eastern areas of the San Joaquin Valley and Sacramento Valley than on the drier West Side.