Establishing quantitative relationships between water relations, growth, yield and quality of high performing commercial apple orchards

The objective was to assess the effects and interaction of varying crop load and irrigation volume in ‘Golden Delicious’ on tree physiology, fruit, shoot and root growth, fruit yield and quality. This will provide information on the yield-water-growth relationships which can be used to optimise water use in apple production.

We established treatments of two crop loads and three irrigation volumes in a ‘Golden Delicious’ orchard in 2015-2016 and 2016-2017. Monthly measurements were performed of soil volumetric water content, midday stem water potential, leaf gas exchange, and fruit/shoot/root growth. Fruit number, yield, and fruit quality and maturity were determined at harvest.

Physiological measurements showed very few effects. Crop load effects on shoot and fruit growth dominated. High crop load yielded smaller fruit, not mitigated by high irrigation, and moderate shoot growth. Moderate crop load yielded larger fruit irrespective of irrigation. Moderate crop load with high irrigation stimulated shoot growth.

The use of very high irrigation rates to grow an excessively large crop is futile; it will not produce a high export quality (fruit size) packout. In years with moderate crop load, irrigation should be reduced to avoid excessive shoot growth, without forfeiting fruit size. In years with severe water supply shortages, crop load should be reduced to a level that will still produce fruit of marketable size.

Efforts to increase the economic water productivity of apple production should consider setting a ceiling on very high yield targets and introduce a degree of deficit irrigation.