Dry season management — apples
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 apple 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 apple trees during the dry season
In the growth cycle of apple trees, there are critical stages that require adequate soil moisture to produce a commercially viable crop:
- flowering and fruit set
- fruit growth
Root growth commences from late August onwards, so the soil must be moist and soft to establish an early root system before flowering.
Irrigate in late 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. Irrigation can be used to keep the surface soil moist but avoid over-irrigating and wasting water.
- 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.
- An aggressive blossom-thinning program is needed to maximise fruit size. If water availability for irrigation is very low, further thinning after flowering and fruit set may still be needed.
- Apple cultivars continue to size fruit during all of their development cycle to maturity. Moisture stress at any time during the season reduces fruit size.
- Fruit growth by cell division after flowering and the accumulation of sugars and size during the final 8 weeks before fruit harvest remain the more critical stages of fruit development.
- If moisture stress cannot be avoided during the season, thin trees again as early as possible after the decision is made to cut back on irrigation.
- 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) for some of the day. Minor wilting in the hottest part of the afternoon will not be too detrimental.
- Avoiding excessive water stress after harvest ensures nutrients are accumulating in the tree for budburst, fruit set and early shoot development in the following spring.
Dry season issues
Frosts are likely to be more severe and occur later into the season during dry seasons. Install a frost alarm system. Use the irrigation system, soil management and other frost reduction methods in the orchard.
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 using long-term irrigation requirements, and assuming 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 of the following strategies:
- Purchase water.
- Irrigate at a deficit and suffer a fruit size loss — applying a mild regulated deficit irrigation (RDI) early in the season (mid-November and December and again 2 weeks before harvest) will reduce fruit size. In this case, heavier and earlier thinning than normal plus RDI will result in lighter crops but similar fruit size compared to a full irrigation program.
- Do not irrigate lower productivity blocks and de-blossom or even pollard (main limbs cut in half or pruned even lower) to reduce water needs. Wound dressing must be applied to these larger pruning cuts. Alternatively, some older trees may be close to being non-viable. This may be a good time to pull them out earlier than planned and transfer the water to productive blocks of trees to ensure some commercially viable fruit can be harvested.
Saving water in orchards
You can incorporate the following orchard management strategies to save water:
- Prune the most valuable and productive blocks first. Leave the lower value blocks to be pruned until last. This gives more time to evaluate the seasonal rainfall and means decisions about abandoning poorer blocks to save water can be left until later.
- Thoroughly and aggressively apply blossom and post-flowering thinning sprays to reduce competition between fruit as soon as possible even though there is a frost risk. This is especially needed if flowering and weather conditions are conducive to high fruit set.
- Decide if more thinning is needed at 6 weeks after fruit set and complete the thinning as soon as possible. Secateurs thinning can be done in the top third of the tree (vase-shaped trees) to speed up thinning until late October.
- Strip all fruit off one-year-old trees and those just planted. Trees that are 2 or 3 years old should be thinned heavily or the fruit stripped if water is short.
- Measure soil moisture to schedule irrigations.
- Eliminate all weed competition. If micro-irrigated, spray weeds emerging in the irrigated zone during spring and summer.
- Slash the orchard more often and as close to the ground as possible. Spraying out all the understorey pasture and weeds may be needed under drastic water shortages.
- If possible, mulch the tree-line and irrigate the soil shaded by the tree and not out in the traffic row area.
Irrigation requirements for apples
- Irrigate to maintain good soil moisture from bud-swell to 4 weeks after flowering. Make sure the water penetrates to the full depth of the root zone — but not past the root zone.
- Soil moisture measuring instruments will indicate when and how much irrigation is needed. This will be of great benefit to guide irrigations while trees are leafing up and assess if there has been any effective rainfall that can save on irrigation water.
- RDI will reduce fruit size in apples. RDI should only be applied to apples if there is insufficient irrigation and trees are heavily thinned.
Requirements for specific cultivars:
- Assume 5 to 6 megalitres of water per hectare are needed to grow mid-season cultivars such as Gala.
- Assume 6 to 7 megalitres per hectare for late cultivars such as Granny Smith and Cripps Pink.