Plant growth regulators for reducing pre-harvest fruit abscission in rainfed avocados

Abstract

Severe fruit drop events occurring after flowering and during early summer strongly limit rainfed avocado (Persea americana Mill.) production. This study aimed to evaluate the effect of late-spring and early-summer sprayings of plant growth regulators (PGRs) and monopotassium phosphite (PF), KH₂PO₃, on fruit abscission, yield and quality of non-irrigated ‘Quintal’ and ‘Margarida’ avocado trees. The following treatments were applied over two growing seasons: water; 25 mg L^-1 of (2,4-dichlorophenoxy) acetic acid (2,4-D); and 45 mg L^-1 of (E)-L-2-[2-(2-aminoethoxy)vinyl] glycine hydrochloride (AVG), separately applied as a single spraying in November; and three PF-sprayings, bi-weekly applied from mid-December to mid-January. The treatments did not affect fruit yield or quality of ‘Quintal’ avocados. Nevertheless, the PF sprayings significantly increased cumulative fruit yield of ‘Margarida’ trees (944 Kg tree¹, P = 0.0028). In both cultivars, the mean pre-harvest fruit drop in the studied biennium was not affected by the treatments, although AVG significantly reduced abscission of ‘Quintal’ avocados only in 2015 (3.88 fruits m^-2, P = 0.0121), under non-stressful environmental conditions during fruit growth. In this cultivar, a single spraying, either with AVG or 2,4-D, significantly reduced fruit size and modified fruit shape. In the ‘Margarida’ cultivar, those treatments did not affect fruit shape, but a single 2,4-D spraying significantly increased diameter and length, by 2.7% and 1.9%, respectively. Large variability of the responses to the treatments among cultivars confirms that fruit abscission is a complex process regulated by several factors and demanding a multidisciplinary approach to develop efficient methods for its chemical manipulation.