Regulated deficit irrigation in peach and nectarine

Regulated deficit irrigation (RDI) is the controlled imposition of water stress on peach at one very specific point in the season: while the fruit is not growing. Applied correctly it saves water, curbs vegetative vigour and improves quality. Applied badly it costs fruit size, and sometimes trees.

The curve that explains everything

Peach fruit does not grow at an even rate. It follows a double sigmoid curve, that is, three distinct stages:

The logic is simple: during stage II the water applied does not go into the fruit — it goes into water sprouts. So it is withheld there, and restored in full as soon as cell expansion begins.

What is gained

• Vigour control without the secateurs. Less vegetative growth means better light penetration and air movement through the canopy.
• Less fruit rot and therefore lower plant protection product use — a direct consequence of the improved air movement.
• Substantial water saving. The international literature reports improvements in water use efficiency of up to 60% with no yield penalty; in trials, total irrigation water fell from 3,000 to 1,400 cubic metres per hectare with no effect on yield.
• Better quality. Moderate stress triggers osmotic adjustment — a concentration of solutes in the fruit — which favours sugars and flavour.
• Compensatory growth. Once irrigation is restored the fruit growth rate accelerates, and that is where the large size advantage over permanently full irrigation comes from.

Where it fits and where it does not

This is the point that decides whether it works, and it is the one most often left out.

Late cultivars — this is where it works

In late-maturing cultivars stage II is long. There is time to impose the stress, let it act on shoot growth, and then allow the tree to recover fully before cell expansion begins. The literature consistently shows reduced vegetative growth with no loss of crop.

Early cultivars — here, caution

In early cultivars stage II is very short. Withholding water during pit hardening alone does not have time to work, while extending it into stage III — and that is the temptation — costs fruit size immediately. For early cultivars classical RDI is not the right strategy; other approaches are being examined, such as mild continuous deficit irrigation or post-thinning stress.

How it is measured — do not do this by eye

The margin between useful stress and damaging stress is narrow. It cannot be judged from the appearance of the foliage, because by the time wilting is visible it is already too late.

• Pressure chamber (Scholander chamber) for midday stem water potential on a shaded leaf. This is the most reliable indicator.
• Tensiometers or soil moisture sensors at two depths at least.
• Shoot growth monitoring: length of new shoots per week.
• Fruit measurement with callipers on permanently marked trees — to confirm that stage II has in fact begun.

Sources

• Chalmers D.J., Mitchell P.D., van Heek L. (1981) — the foundational paper that introduced the concept of regulated deficit irrigation in peach.
• Girona J. et al. (1993, 2005) — a series of papers on RDI in peach under Mediterranean conditions, showing increased water use efficiency with no effect on yield or quality.
• Sustainability of regulated deficit irrigation in a mid-maturing peach cultivar. Irrigation Science (2016).
• From water scarcity to fruit quality: continuous deficit irrigation in early-maturing peaches. Agricultural Water Management — on the limits of classical RDI in early cultivars.
• Scheduling deficit irrigation of fruit trees for optimizing water use efficiency. FAO.
• Yield and fruit quality of almond, peach and plum under regulated deficit irrigation. Frontiers of Agricultural Science and Engineering.

Diagram: Pastopoulos Agronomics, based on material from Good Fruit Grower.