bulk pistachio Thinkstock Photo Credit: Olaf Bender/Thinkstock

Pistachio growers: Winter sanitation critical for NOW, keep tab on chilling hours

“By the time the later maturing nuts were ready, many of the hulls had already begun deteriorating, allowing NOW to infest the nuts and increasing their susceptibility to aflatoxin-producing molds.”

As California pistachio orchards enter dormancy this year, growers should keep their eyes peeled on two major developments which could impact next year’s production practices – the unusually high level of Navel orangeworm (NOW) damage to the 2016 crop and the number of chilling hours this winter.

Mae Culumber, University of California Cooperative Extension nut crop advisor for Fresno County, says the pistachio industry this year lost up to 2.5 percent of the crop to NOW damage.

She says much of the damage occurred late in the season and could be tied to lower split percentages this season. Waiting for the optimum number of splits may have caused growers to delay harvest and leave early-maturing nuts on the tree too long.

“By the time the later maturing nuts were ready, many of the hulls had already begun deteriorating, allowing NOW to infest the nuts and increasing their susceptibility to aflatoxin-producing molds,” says Culumber.

Early splitting hulls, which typically are 2 to 5 percent of harvested pistachios in an average year, can provide an opening to NOW infestations.

Winter rains can help degrade the nut and the mummies inside. However, current forecasts predict a relatively dry winter for much of the pistachio-growing region, heightening the importance of proper winter sanitation to minimize the NOW threat to next year’s crop.

“Winter sanitation is a must,” Culumber says. “It’s the most effective way to control NOW.”

She adds, “Shake the trees and blow the mummies out of the crotches of trees so nuts don’t settle in (the) canopy. Then sweep them into the row middles and shred them with a flail mower or disc them to destroy the nuts and help reduce the number of mummies.”

Monitor chilling hours

This winter, Culumber also advises growers to monitor the number of chilling hours.

“We estimate pistachio requires about 800 chill hours at or below 45º F. for normal growth of flower and shoot buds the following season,” she says. “Otherwise, blooming could be erratic, resulting in reduced fruit set and uneven maturity at harvest, which can exacerbate NOW and aflatoxin contamination in pistachios.”

Weather models suggest a warmer winter. She says frequent warming events during the winter can also impact bloom timing, even with an adequate number of chilling hours.

From Sept. 1 through Nov. 15, chilling hours at the Five Points station in Fresno County totaled five. This compares to the 117 chilling hours accumulated during this same period last year.

“While it’s still early in the chilling season, the number of chilling hours this winter could be a big concern for next year’s crop,” Culumber says.

More key winter management issues

Correct high soil salinity level - Although pistachios are considered more tolerant of saline soil conditions than other nut crops, growers should still monitor salt accumulation in the soil.

“I’ve seen a lot of pistachio orchards in Fresno County with visible indications of salt stress this season,” she says.” High sodium, chloride, and boron soil levels can reduce a tree’s ability to take up water and photosynthesize and, ultimately, decreasing growth and yield potential.”

Culumber recommends sampling soils in one-inch increments to a depth of five feet in different places in the orchard in the fall to determine the need to leach salts with water. Leaching during the winter is more effective than during the growing season since low evapotranspiration rates reduce salt accumulation in the shallow surface of the soil where tree roots are concentrated.

“Depending on soil texture and water holding capacity, growers should apply three-to-six inches of water to recharge the soil profile,” she says. “Then, allow the soil to drain completely before beginning leaching applications. Repeat this schedule until the targeted water application is reached.”

She encourages growers to allow enough time between leaching events for the soil to drain so water is always moving and doesn’t become stagnate.

Soft scale damage – Soft scales are becoming more prevalent as natural enemy populations decline with the increased use of permethrin treatments to control true bugs in pistachios, UCCE researchers report.

Any live and parasitized scales likely to be found on first-year fruit wood should be treated before the rubber stage, which usually occurs by the third week of February, says Culumber.

An average of one to five live scales per inch of fruiting wood in early February is considered a light-to-moderate population. A heavy population would be average 10 or more live scales per inch.

The recommended treatment is four quarts of Sevin XLR plus four-to-six gallons of oil, depending on the scale severity. An alternative to Sevin plus oil is Seize 35W which eliminates the rest-breaking effect of the oil.

Monitor for weeds - Tracking the type and location of weed stands this fall and winter can help growers select the most effective pre-emergence herbicides to apply next fall. 

“It’s easier to control perennial weeds going into dormancy because they’re not as vigorous as they are earlier in the season,” Culumber says. “Also, because annuals are smaller at this time they’re easier to control.”

Botryosphaeria-infected wood - When pruning, look for signs of Botryosphaeria panicle and shoot blight, and prune out cankers two inches past blighted margins. This fungal disease, which kills shoots, leaves and nut clusters, is very difficult to control, especially if allowed to increase over several years, says Culumber.

Sher recommends randomly collecting about a hundred fruit buds and cutting them in half. Blackened buds are likely infected with the fungal disease. Also, cut into dead one-year old shoots and black fruit rachises which don’t knock off the tree easily and look for signs of infection — a black streak in the limb extending beyond the base.

Keep in mind that wood damaged by cold temperatures also has a black zone between the live and dead wood. But, its margin is very sharp and the black area does not run into the limb.

Cankers can release inoculum for as long as six years, University of California researchers report. After harvest, remove and destroy unharvested nuts and mummies to reduce sources of inoculum.

Orchard winter sanitation practices to control NOW will also help reduce the source of Botryopshaeria inoculum, Culumber adds.

“Fungicides are available for treating this disease during the growing season. However, the main way to control Botryopshaeria is to remove any over-wintering cankers and prevent inoculum levels from building up in the first place.”

She notes to keep the inoculum from spreading don’t cut out infected areas of the tree if rain is expected soon.

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