UC Davis

This thesis explores the use of hull split regulated deficit irrigation in combination with early harvest of almonds (Prunus dulcis (Mill.) D. A. Webb) for improved irrigation efficiency and control of navel orangeworm (NOW) (Amyelois transitella (Walker)) and Hull Rot (HR) (Rhizopus stolonifer (Ehrenb:Fr.) Vuill.; Monilinia fructicola (G. Wint.) Honey). Chapter 1 contains a review of the literature regarding California almond production, almond phenology, almond water status monitoring, and NOW and HR phenology and control. Chapter 2 describes research carried out during the 2020 growing season to test the efficacy of early almond harvest for NOW and HR control and water use efficiency. Standard Harvest (SH) and Early Harvest (EH) treatments were applied to trees in two almond orchards in Denair and Woodland, California using a randomized controlled block design; EH treatments were harvested three to four weeks prior to SH. Midday Stem Water Potential (SWP) measurements were used to monitor tree water status and manipulate irrigation to prepare trees for EH treatments. Trees were maintained at -14 to -18 bars SWP for the two to three weeks leading up to both EH and SH. There were significantly lower HR strikes in EH treatment trees than in SH treatment trees. No significant difference was observed in NOW incidence between treatments, although neither site had a history of high NOW pressure. No significant kernel yield, marketable yield, or shaker efficacy differences were seen between the two treatments. Although 3.28 and 1.61 inches of water less was applied to EH trees than SH trees at the Denair and Woodland sites respectively, the trees from both treatment groups at both sites returned to approximately the same water status after Standard Harvest and mostly recovered from the induced moderate water stress by October 12. With further development of complementary harvest and processing strategies, early harvest of almonds shows promise as a strategy for pest and disease control and improved water use efficiency in almond orchards.