Irrigation Savings Crucial to Finish Commercial Quality Fruit

By Henry Schneider, FruitCheque Officer, DPI Cobram

Orchardists can save between 1 to 1.5 megalitres of water per hectare by using “Regulated Deficit Irrigation” or RDI on their pears and peaches during November and December. This water can then be used to produce commercial size and quality fruit during the crucial final fruit growth phase in the final weeks before harvest. Pear and peach trees in the Goulburn – Murray Valleys have two stages of actual fruit growth. The first is from flowering to about the end of October. The second stage of fruit growth, commonly called the ‘fruit fill’ stage, occurs in William pears from early December until harvest in mid January. For Packham pears, this is delayed about 2 weeks, with the ‘fruit fill’ stage from mid December until harvest in the first week of February. In peaches the critical ‘fruit fill’ period is the last 6 to 8 weeks before harvest with the slow fruit growth period starting in November. It is during the ‘fruit fill’ stage that pears and peaches will need constant, optimum irrigation management to produce market-quality fruit.

RDI – irrigate at the same times (ie weekly) but half the run time during the RDI period

However in the period between November until early or mid December, the main growth in pears and peaches is the shoots. Fruit growth is at best very slow and moisture stress will only suppress vigour in the shoots – the fruit will not be affected in size if full irrigation is resumed at the appropriate time in early December (Williams) or mid December (Packham). This period of slow fruit growth is when the process of Regulated Deficit Irrigation (RDI) can save between 1 to 1.5 megalitres per hectare.

A simple way of using RDI is to apply the irrigations at the same time (ie weekly), but halve the amount applied. As an example, if the irrigations were 8 hours in late October, then only apply a 4 hour irrigation each week during the RDI period. See RDI table for peaches and pears.

The irrigation water saved during the RDI period can then be best used for the rapid ‘fruit fill’ period during the critical 6 weeks before harvest. This is the most efficient way to use the limited irrigation water resources currently in the Northern Irrigation districts of Victoria.

Canning peaches have the same growth habit as pears and the RDI period and approximate full irrigation periods are listed for key varieties in the table below.

If market peach and nectarine varieties mature in the same time period as those in the table, RDI can also be used.

Regulated Deficit Irrigation (RDI) Guide For Peaches and Pears

Canning Peaches	R. D. I.	Full Irrigation till harvest.
Tatura 204	Nov.>> 5th Dec.	5th Dec.>>20th Jan.
Tatura211	Nov.>> 15th Dec.	15th Dec.>>5th Jan.
G. Queen	Nov.>> 14th Jan.	14th Jan.>>1st Mar.
T. Queen	Nov.>> 18th Jan.	18th Jan.>>15th Mar.
Pears	R. D. I.	Full Irrigation till harvest.
WBC	Nov.>> 1st Dec.	1st Dec.>>15th Jan.
Packham	Nov.>> 15th Dec.	15th Dec.>> 8th Feb.

Note: RDI usually falls in the period from the start of November until about 6 to 8 weeks before harvest. This means varieties maturing by mid to late December do not have an effective RDI period - they must be fully irrigated from spring through to harvest to ensure good fruit size. However, RDI can be used after harvest to gain some vigour control for these early varieties.
Later maturing varieties of peaches, nectarines and pears have a progressively longer RDI period. The table above gives periods for RDI and full irrigation for some of the important varieties during the mid to late part of the fruit season.

The important thing is to resume full irrigations from 6 to 8 weeks before harvest to maximise the trees’ ability to size and mature the fruit in that last ‘fruit fill’ stage. If water savings are desperately needed for flood irrigated pear orchards, only watering the same alternate rows or only watering in furrows either side of the tree will get trees through the season, particularly if there is only a light crop. This is harder to control and water savings are not as efficient as with sprinklers, micro-jets or drippers. Trees with very light crops, where fruit size may become too large, orchardists could extend the RDI period by 2 to 3 weeks. However, this should be done carefully and with the help of good irrigation monitoring equipment in the last 3 to 4 weeks to harvest to make sure there is no stress in these later stages of the ‘fruit fill' period. NOTE: Under normal crop loads, do not use RDI on apples. However if apples have a very light crop and there are fears of fruit getting too large, the best time to use RDI is in the November to December period, again irrigating at the same time, but halving the amount of water applied. Strategies for this season: Do an irrigation budget and buy extra water if needed. It is likely to be more water than was needed in the 2002-03 low water allocation season. Assess each block for crop load and irrigate accordingly, especially if trees are showing strong vigour. Some blocks will need thinning to ensure fruit is of commercial size and quality. Where all fruit has been lost to frost, extend RDI (same irrigation frequency, but half the run time) for most of the season. Only irrigate longer during hot summer weather if trees wilt late in the day after only 3 or 4 days after the last irrigation. Keep shoot growth shorter…. Do not fertilise ANY trees now until after harvest. Keep all grass or weeds clear of tree lines and slash the orchard regularly to keep the tree row grass very short… if this is still needed. If mulching with straw is possible, especially for young trees, irrigation emitters are best put under the straw next to each tree. Removing water shoots as they grow will reduce water used by trees – especially from trees with light crops due to frost damage. Summer pruning will also help young trees.

Check and maintain irrigation monitoring equipment Pears will still get to full size if using RDI (in November to early December) and then fully irrigating 6 weeks before harvest.

Monitor irrigations with a probe, tensiometers or gypsum blocks or get a consultant to help. Some orchards may be able to save water (especially during the critical fruit fill period) by only watering as deep as the effective root zone of the tree. Make sure some water is available after harvest to boost bud wood for next season. Enquire about ‘anti-transpirants’ from chemical resellers. They protect the leaves (and the fruit) from hot sun and can reduce both water losses from leaves and reduce fruit sunburn. Check bores for salinity if these are essential to meet irrigation needs…. But well before they are used - just in case “shandying” is needed, especially in pears.

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