Dry season management — stone fruit

As the dry weather conditions continue in Victoria, fruit growers must consider all available options to get through the dry summer conditions and minimise losses.

Managing young stone fruit trees during the dry season

• Delay planting as long as possible to assess the season and water allocations. If water is in short supply, it might be easier to hold new trees in a nursery site for the season or in a cool room for 12 months and plant next year.
• Make sure the irrigation system is in place and fully operational by the time trees are planted.
• Water new trees in thoroughly at planting time — especially smaller trees with finer, more easily dried out root systems.
• Mulch newly planted trees after the frost period if straw or other material is available. This reduces water losses from soil evaporation.
• Irrigate close to the tree, not the whole paddock (for example, invert micro-jets so the surface wetting pattern is reduced or replace existing micro-sprinklers with smaller wetting pattern micro-jets).
• Eliminate weed competition early and widen the herbicide strip.
• Remove any fruit on trees planted this season or last year.
• Summer prune and train trees more frequently.

Managing mature stone fruit trees during the dry season

In the growth cycle of deciduous stone fruit, there are critical stages that require adequate soil moisture to produce a commercially viable crop:

1. pre-flowering
2. flowering and fruit set
3. fruit growth — early maturing cultivars
4. fruit growth — mid- to late-maturing cultivars
5. post-harvest

Pre-flowering

Root growth commences from August onwards so the soil must be moist and soft to establish an early root system before flowering.

Irrigate in August if soil is dry (if there's insufficient rain).

Flowering and fruit set

• Bare or closely slashed tree rows with moist soil are needed to absorb heat during the day to minimise the impact of frosts.
• Adequate soil moisture until approximately 4 weeks after full bloom is critical for root growth, fruit set and to maximise cell division during the early stages of fruitlet growth.
• Thin early and aggressively to maximise fruit size if water availability for irrigation is low and flowering and fruit set have been strong.

Fruit growth — early maturing cultivars

Very early maturing stone fruit cultivars (apricots, early plums, nectarines and peaches maturing up to mid-January) must be irrigated fully from flowering until harvest to ensure commercially viable fruit size.

Fruit growth — mid- to late-maturing cultivars

• Mid- to late-season maturing stone fruit cultivars don't continue to size fruit during November to early to mid-January. Moisture stress in this time reduces the growth of leaves and shoots but not the fruit. This presents an opportunity to use regulated deficit irrigation (RDI). A water saving of 1 to 1.5 megalitres per hectare is possible in this 6 to 8 week period.
• Fruit growth by cell expansion and the accumulation of sugars by the fruit depends heavily on optimum soil moisture and full irrigation during the final rapid fruit growth stage in the last 6 to 8 weeks to harvest.

Post-harvest

• Deficit irrigation at 30 per cent of full irrigation during post-harvest period. Irrigations after harvest can be reduced but not to the level that causes significant leaf loss.
• Continue post-harvest irrigations to water in nitrogen fertilisers and maintain leaf activity (photosynthesis). Some minor wilting in the hottest part of the afternoon will not be too detrimental.
• Irrigate as long as possible after harvest to ensure nutrients are accumulating in the tree for budburst, fruit set and early shoot development the following spring.

Dry season issues

Other issues you're likely to face because of dry seasons include:

• severe frosts
• fruit shedding

Frosts are likely to be more severe and occur later in the season during droughts. Set up a frost alarm system and have a frost protection strategy in place.

If there is no frost damage, heavy fruit set is possible because of fine weather during flowering. Remove excess fruit under high crop load conditions.

There may be excessive fruit shedding in November from dry soil during the first 4 weeks from budswell to flowering and fruit set.

Irrigation strategies in a dry season

The following strategy is suggested if irrigation water entitlements are low.

Before the season starts, set out a water budget for each irrigation block on a monthly basis that:

• uses long-term irrigation requirements
• incorporates RDI and post-harvest deficit irrigation strategies
• assumes no effective summer rainfall

On a regular basis review:

• water allocation
• long-term rain forecasts
• market potential for each cultivar

If there's not enough irrigation water available, decide on one or a combination of the following strategies:

• Irrigate mid- and late-season cultivars using RDI.
• Purchase water.
• Irrigate at a severe deficit and suffer a fruit size loss (heavier fruit thinning may offset some of the loss in fruit size because of water stress).
• Do not irrigate — remove the crop and cut back branches to restrict tree size. Alternatively, some older trees may be close to being non-viable. This may be a good time to pull them out earlier than planned — especially if they are a late maturing cultivar that would require more irrigation to get them to harvest.

Saving water in orchards

You can incorporate the following orchard management strategies to save water:

• Measure soil moisture to schedule irrigations, particularly from flowering to the end of October and during 'fruit fill' in the last 6 to 8 weeks before harvest.
• Irrigate to a depth of 45 to 60cm only (check with probe or auger or sensor).
• Eliminate tree-line weeds early and widen the herbicide strip. If micro-irrigated, then spray weeds in the wetted area during the growing season.
• Slash the orchard more often and as close to the ground as possible. Under drastic water shortages you might need to spray out all the understorey pasture and weeds.
• If possible, mulch the tree-line and irrigate the soil shaded by the tree and not out in the traffic row area.
• Summer prune water shoots and cut back leaders to reduce the tree leaf area.

Irrigation requirements for early maturing stone fruit varieties (November to mid-January)

Early maturing stone fruit cultivars (harvested November to mid-January) must have good soil moisture from flowering to harvest to size the crop.

Early in the season:

• Irrigate carefully to prevent leaching and only irrigate to the depth of the root zone (45 to 60cm, depending on soil type).
• Summer prune early to relieve pressure from a larger than needed leaf area.
• Flower bud thinning, flower thinning or fruitlet thinning (immediately after shuck fall) is critical for good fruit size at harvest for early maturing cultivars. But early thinning increases split stones on susceptible cultivars. Unfortunately, early thinning also puts at risk money spent on thinning if subsequent frosts or hail severely reduce the crop available at harvest.

After harvest:

• Reduce irrigations after harvest. This can be done by reducing irrigation run times. Aim to irrigate to a depth of 30 to 40cm when irrigations are applied. It's like RDI, only after harvest.
• In orchards with tile drains and prone to salt accumulation in the root zone, a leaching irrigation may be needed after harvest.

Irrigation requirements for mid- and late-season cultivars (mid-January to March)

Mid- to late-maturing cultivars of stone fruit need optimum soil moisture from budswell to stone tip change (late October to early November). Adequate soil moisture at this time enables the tree to grow its root system, set fruit and develop early fruit size by cell division.

The following tips can help you manage water in mid- to late-maturing cultivars.

Regulated deficit irrigation

From November to early December or mid-January (depending on harvest date) is a period of slow fruit growth.

RDI is a water stress strategy that can be applied during this period. RDI reduces leaf and shoot growth but does not affect final fruit size. It can save up to 50 per cent of the water applied during this low fruit growth period (1 to 1.5 megalitres per hectare).

As an example, the RDI period for Tatura 204 is from November to the first week in December in a 'normal' year. For Taylor Queen peaches, the RDI period is from November until mid-January.

Water requirements

• Full irrigations must be applied during the last 6 to 8 weeks before harvest to maximise fruit size — an average of about 2 megalitres per hectare of water will be needed for the final 6 to 8 weeks of the fruit-fill growth stage in southern Victoria and 3 megalitres per hectare in northern Victoria.
• After the RDI period, at the start of the fruit fill growth stage, ensure irrigation fully wets up as much of the root zone as possible down to a depth of at least 45cm — soil moisture levels during the fruit fill stage should be kept in the range of 10 to 40 kilopascal until harvest.

Thinning

Fruitlet thinning of canning peaches at the 6mm seed length stage with ethephon will lead to better fruit size by seed tip change stage. Blossom thinning, by hand or chemically, will have an even better effect on final fruit size because it's done earlier in the season.

Ensure any follow-up hand or stick thinning is completed and done thoroughly soon after tip change for all mid- to late-season cultivars of stone fruit.

Peaches and nectarines highly susceptible to split stones will often have more split-stones if thinned early (at flowering or before tip change).