Irrigation, Water Deficit and Crop Load Effects on ‘Hass’ Avocado Fruit Size Under New Zealand Growing Conditions

The potential for ‘Hass’ avocado production is predicted to increase with climate warming in New Zealand, a country where avocado orchards often lack irrigation because of a cooler and wetter climate compared to most other major growing regions. However, intermittent summer droughts are also predicted to increase in frequency and intensity. This study assessed the effects of summer soil water deficits on fruit growth of ‘Hass’ avocado in the Bay of Plenty, New Zealand, by comparing irrigated and non-irrigated treatments. Rainfall was variable over the three years of the study (2016–17, 2017–18, and 2018–19), but each summer there was a dry period without any rainfall for 2–3 weeks that decreased soil water content in the non-irrigated treatment. Fruit number and final yields were highly variable between trees and years, an effect of variable fruit set during the spring flowering period, and were not affected by the irrigation treatments because soil water deficits did not occur until later, during the summer. Increasing tree crop load caused decreasing individual fruit weight and dry matter content at harvest. However, in the year with the highest average crop load a dry period occurred during early fruit development, and mean fruit weight at harvest was decreased by 26.4 g (10%) in the non-irrigated treatment, an effect that was only apparent after accounting for the effects of variable crop load. The trees responded to dry conditions by reducing stomatal conductance (gs) by 20%, preventing midday leaf water potential (Ψleaf) from decreasing below −0.25 MPa. Irrigation of avocado under the conditions at this site is therefore recommended when soil tension decreases below −30 kPa at 30 cm depth, and adverse effects on fruit growth are likely when tension decreases below −50 kPa. Irregular bearing of avocado under New Zealand growing conditions causes highly variable crop loads that obscure economically significant effects of mild to moderate water deficits on fruit growth. However, irrigation is still an important consideration for avocado production under current growing conditions and is likely to become more important under future climate scenarios as the risk of summer droughts increases.