EFFICACY OF FOLIAR-APPLIED CYTOKININS AND NITROGEN TO INCREASE FLORAL BUD RETENTION AND TO REDUCE ALTERNATE BEARING OF PISTACHIO

Alternate bearing in pistachio (Pistacia vera L., cv. Kerman) is due to excessive abscission of floral buds for next year’s crop during the heavy on-crop year. Floral bud abscission begins with the initiation of embryo development (June) and proceeds through the period of rapid embryo growth (July). During this period, zeatinriboside and isopentyladenosine each decreased 40% in floral buds on shoots of on-crop trees (more than 70 fruit per shoot). Foliar-applied 6-benzyladenine (BA) (25 or 50 mg L^-1) with low-biuret urea (0.25% N) per application in June and July or May (early fruit set), June and July increased bud retention on shoots of on-crop trees ≥ 1.6-fold compared to bud retention on untreated shoots (P ≤ 0.05). Three applications of an algal extract from Ascophyllum nodosum (0.08%) with low-biuret urea (0.25% N) or N-P-K (11-8-5) liquid fertilizer (0.17%) in May, June and July were equally effective, increasing bud retention 1.9- and 2.0-fold, respectively. In commercial orchards, the efficacy of 69 g BA plus 6.9 kg N as low-biuret urea per ha per application and 2.4 L algal extract plus 6.9 kg N as low-biuret urea or N-P-K fertilizer at 4.7 L per ha per application to increase bud retention during the on-crop year and increase yield the following (off-crop) year was determined. Treatments were initiated during the on-crop year and applied annually in June and July or in May, June and July. All treatments increased 2-year cumulative yield (kg split nuts [dry wt] per tree) (P = 0.0018). In a subsequent 5-year experiment, the algal extract at 4.7 L plus 28 kg N as low-biuret urea per ha per application in June and July increased the cumulative yield of kg split nuts (dry wt) for the three off-crop years (P = 0.10), but not the 5-year cumulative yield (P = 0.12). The algal extract combined with low-biuret urea had a significant but variable effect on alternate bearing index. No treatment affected nut quality. There is great interest in improving the efficacy of the algal extract paired with an appropriate nitrogen source for use in organic pistachio production.