Rate and time of boron application increase almond productivity and tissueboron concentration

Previous studies with tree species have demonstrated that foliar boron (B) promotes flowering, fruit set, and yield, However, for most species the opt imum time for foliar B application has not been determined. This investigat ion was undertaken to study the effects of time and rate of B application o n almond [Prunus dulcis (Mill D.A.Webb)] tissue B concentration, fruit set, and yield. Solubor (Na(2)B8O(13).4H(2)O), a commercial product containing 20.5% B, was applied with a handgun sprayer either in September (3 weeks po stharvest), December (dormancy), or February (budbreak) at rates of 0, 0.8, and 1.7 kg.ha(-1) B to almond cv. Butte at one site (Parlier, Fresno Count y, Calif.), and of 0, 0.8, 1.25, 1.7, and 2.1 kg . ha(-1) B on the same cul tivar in August, September, or February at a second site (Orland, Glenn Cou nty, Calif) using Borosol, a polyboronated commercial product containing 10 % B, At site 1, September application was more effective in increasing tiss ue B concentration, fruit set, and yield than were December or February app lications. The optimal rate was 0.8 to 1.7 kg . ha(-1) B when applied in Se ptember. At site 2, application in 1996 and 1997 increased tissue B concent ration almost linearly, especially when applied in August and February. App lication at the highest rate (2.1 kg . ha(-1) B) in September produced the greatest final fruit set and yield in 1996, February applications increased initial fruit set at both sites but were less effective than September app lications in increasing yield. Application of B did not affect any yield va riable in 1997. These results suggest that B should be applied immediately postharvest (September) for optimal effect on tissue B concentration, fruit set, and yield in almond.