October Pistachio Task List

By Robert Beede, U.C. Farm Advisor, Emeritus

Harvest: It was fast and brutally disappointing for many growers in areas with insufficient chilling. Many growers report harvesting 100-200 pounds, and some orchards were apparently not harvested at all. Orchards on the east side of the southern San Joaquin Valley performed much better where the chill accumulation was higher. I have heard of near-normal crops in this region, with split and blank nut percentages typical for the area. In contrast, a high elevation orchard near the base of the Tehachapi Mountains had only 100 pounds ACP weight. People keep asking what happened, so I guess we have to come up with a Martin story rather than the excessively simple answer of inadequate winter chilling!

Oil application did not help this year. It is my horticultural OPINION (that means I do NOT have research to back up my statement) that oil application actually made the poor chill condition worse this season. How come it backfired this year, and not during the 90s and early 2000s when I was studying it extensively? I hypothesize that the difference lies in the high temperatures experienced in January this past winter, one of the two most critical months for chill hour accumulation. Pistachio trees may be “at rest” in the winter, but they are NOT dead! Consequently, they are still consuming stored food to fuel the physiological process known as “respiration.” Plant respiration rates typically increase with temperature, so in addition to negating the accumulation of chilling hours, high January temperatures also consume larger quantities of limited carbohydrates in the fruit wood which are needed for bloom, fruit set, and shoot growth in early spring. Excessive depletion of this stored food could account for delayed leafing, erratic bloom, and poor fruit set. If dormant oil advances bud break in normal years by increasing plant respiration, which in turn renders the tree more responsive to favorable spring weather, then oil treatment this winter could have depleted the carbohydrate reserves more than in non-oiled trees. Even though they broke buds sooner than the non-oiled trees, they lacked the necessary growth substances to set fruit successfully. We are hopeful that research will be performed in the future to determine if this hypothesis is true.

Pest Management: It will be important to remove all the nuts left over from harvest this year in order to keep the navel orangeworm from overwintering in high numbers. It may also pay to apply a NOW spray postharvest in orchards with high insect damage in either their first or second shake. One grower reported NOW damage going from only a quarter of one percent during the first harvest to five percent in the second. This rapid increase in NOW infestation has been repeatedly shown in multiple studies over the past 30 years. I would also be inclined to put the mating disruption dispensers back up if you took them down for harvest, providing there is still pheromone left to disperse. The necessity of doing this depends upon your NOW pressure. A spray application may prove more cost effective in some situations.

This is a very short task list because we have typically written one covering September and October for the September PNP issue. I hope to have a more accurate summary of the season for the November issue.