The evaluation of winter kaolin clay and dormant oil applications on chill accumulation and yield in the California pistachio (Pistacia vera) cultivars ‘Kerman’ and ‘Peters’

Abstract

The California pistachio cultivars 'Kerman' and 'Peters' have an estimated requirement of 58 and 60 chill portions, respectively. In California's Central San Joaquin Valley, the recent combination of warmer winter temperatures and sharply decreased fogs have resulted in a reduction of winter chill accumulation. In 2015 and 2016, kaolin clay was applied to dormant pistachio trees attempting to increase chill accumulation by reflecting radiant heat. Four treatments including kaolin clay, kaolin clay plus 470 dormant oil, 470 dormant oil, and an untreated control were made to nine year old pistachio trees. Kaolin clay applications (33 kg ha-1) were applied twice with the first application occurring after leaf drop in mid-December and the second occurring in early-January. A 6% 470 dormant oil solution was applied at 150 L ha-1 on February 6th, 2016. Treatments were approximately 1.9 ha in size and were replicated four times utilizing a randomized complete block design. Temperature sensors measuring external wood temperature on the north and south side were installed on a single tree in multiple blocks. Yield was determined for each plot and evaluated for size and quality. Local weather station measurements indicate that adequate chill was received in 2015-2016. Never-the-less, wood surface temperatures in kaolin treated trees were lower than the untreated control, producing an increase in chill portion accumulation by ~15%. Yield, however, was higher in the 470 dormant oil than in the kaolin clay or untreated control (3,330 kg ha-1 vs. 2,796 and 2,774 kg ha-1, respectively). Results suggest that kaolin clay may be useful in increasing chill portion accumulation, but only if applied early in the dormant period. Our results also confirmed earlier reports that dormant oil increases pistachio yield in marginal to adequate chill years. Both techniques should be utilized as tools to help mitigate variable climate within pistachio orchards.