2019 ASHS Annual Conference

Pistachio yields in California have recently been harmed by low-chill winters. Low-chill winters can cause delayed and uneven dormancy release, asynchrony between male and female bloom, and greater exposure to late-season pest pressure from navel orangeworm. We tested putative dormancy-breaking agents (DBAs) for late-winter application to pistachio, hoping to advance or compress the bloom window without adverse yield effects. We compared ethephon, GA3, and AVG sprays near endodormancy completion with grower standard treatments of horticultural oil and water. In 2018, 0.2% GA3 advanced bloom by 5 +/- 1 days, and 500 ppm ethephon and 6% oil both advanced bloom by 2 +/- 1 days. GA3 additionally compressed the bloom window from 11 to 9 days by regularizing the differences between shoots with different facings. 125 ppm AVG did not advance bloom and may interfere with the accumulation of heat. DBAs' effects on bud respiration immediately after application were often different from their effects on the endogenous increase in respiration in the pre-bloom period. Some DBAs increased dormant bud respiration rates from ~7 to ~10 mg CO2/(min-g FW), but these changes were overshadowed by endogenous pre-bloom bud respiration increases to ~50 mg CO2/(min-g FW) or more. DBA enhancement of endogenous pre-bloom respiration concorded with DBA-induced bloom order; in contrast, DBA-induced increases of respiration immediately after application were uncorrelated with effects on bloom and should be considered a side effect unless part of a known mode of action. Planned analyses of non-structural carbohydrates will supplement respiration and phenology data to elucidate the interplay between energy stocks, energy consumption, and chemically assisted dormancy release in pistachio.