Water saving options for pears
Written by Ian Goodwin and Lexie McClymont
Horticulture production in a changing climate faces the prospect of continued severe water shortages at regular intervals.
During the Millennium drought, options for minimising irrigation applications to pear orchards were investigated by Agriculture Victoria. Parking trees and post-harvest irrigation cut-off were evaluated in terms of potential water savings and the impacts on current and future production and are discussed here. Regulated deficit irrigation (RDI) had previously been established as an effective technique to control vigour and maintain yield, with the added benefit of lower water use, and is also discussed.
Option 1: Parking trees
The aim of parking trees is to sacrifice the crop and apply the minimum amount of irrigation so that trees survive and return to full production the following year. Trees are managed to minimise the transpiring leaf area and avoid excessive water stress so that developing fruit buds are not desiccated.
The following are recommendations to park trees and minimise irrigation:
Withhold irrigation until the root-zone (defined as containing 80 per cent of the roots) dries out to a minimum soil water tension of at least 400 kPa. This occurs at the start of November in the Goulburn Valley after average winter and spring rainfall.
Use deficit irrigation by applying 30 per cent of orchard water use capability. This is conservative and based on RDI studies conducted at Tatura. Apply frequent irrigation with less water. In other words, cut irrigation run time back to 30 per cent of full irrigation but maintain irrigation interval similar to full irrigation.
Remove fruit from the tree. De-fruiting reduces competition from dry weight accumulation by the fruit and thus increases the amount of carbon available for the developing fruit buds for next season’s crop. Fruit also consume water for growth and lose water through their skin although the amount is small compared to foliage transpiration.
Remove water shoots. Shoot growth will be strong at the start of the season if there is adequate soil moisture from winter and spring rainfall. Summer pruning to remove water shoots will reduce transpiration, increase fruit initiation and remove a sink for dry weight accumulation.
De-branching (mid-season) is a radical approach to reduce transpiring leaf area and is generally not recommended. De-branching reduces tree size and water use, but part of the following season’s crop is removed. If branch removal is undertaken it should be done mid-season rather than in winter to avoid high re-growth in late spring and early summer which draws on stored resources and depletes the pool of dry weight for next season’s crop.
Option 2: Post-harvest cut-off
Post-harvest water application in Australian orchards is substantial. This creates an opportunity to cut-back on irrigation with limited impacts on the developing flower organs in the reproductive buds.
A study on post-harvest deficit irrigation of pear (Pyrus communis Williams’ bon chrétien) in a commercial orchard in the Goulburn Valley showed not irrigating during the postharvest period had no effect on subsequent yield or fruit quality. Five irrigation treatments (0, 50, 100, 160 and 200 per cent of the grower’s normal irrigation volume) were applied during three seasons. Irrigation of the 100 per cent treatment ranged from 141 to 290 mm depending on rainfall. Measurements showed no difference in yield between treatments, with mean yields of 45.3, 56.8, and 61.5 t/ha in the three subsequent seasons.
Option 3: Regulated deficit irrigation
RDI is a well-known strategy that saves water and reduces excessive vegetative growth with no effect on fruit size. Less irrigation than optimum crop water requirement is applied during the period when shoot growth is rapid and fruit growth is slow. Given the reduction in vegetative growth from RDI, it is likely that radiation levels reaching the lower parts of the canopy will increase, which will improve colour of blush pear cultivars, soluble solids concentration and firmness of the fruit.
There are distinct growth stages in fruit trees and the success of RDI depends on varying the application of irrigation during each of these stages:
Fig 1. Pear shoot and fruit growth and the timing of RDI
Fruit cell division: In most seasons in the Goulburn Valley, pears are not irrigated until reference crop evapotranspiration (ETo) exceeds rainfall by 125 mm. This allows the soil to dry out so that water stress can develop during the slow fruit growth period. However, in recent years there has been insufficient winter and early spring rain to wet up the root zone. Root zone soil moisture must be measured to avoid water stress. Moisture should be accounted for over the entire root zone i.e. to a depth of 600 to 800 mm. Soil should not be allowed to dry out beyond 60 kPa in clay loam soils.
Slow fruit growth: During this period, fruit growth is slow and shoot growth is rapid (Fig. 1). Water stress will significantly reduce shoot growth. Irrigation should commence when the soil has dried out in the fibrous root zone to 400 kPa in a clay loam. The trees are irrigated with greatly reduced volumes of water compared to that which would normally be applied. Irrigation replacements of less than 30 per cent of crop water requirement are recommended.
Rapid fruit growth: This stage commences in early December for Williams’ bon chrétien and mid to late December for Packham’s Triumph. The fruit is growing rapidly and the tree now needs ample water to maintain this growth. Water stress must not occur during this final period of fruit growth. Irrigation replacements of 100 per cent of crop water requirement are recommended. Soil moisture in the middle of the wetted fibrous root zone should not exceed 40 kPa in sandy soil or 60 kPa in a clay loam. A long irrigation event, or several irrigations at short intervals, may be required to re-wet the root zone in December, particularly if rainfall has been low.
Post-harvest. Many orchardists in the Goulburn Valley have implemented post-harvest irrigation deficits similar to the RDI strategy described above.
Water savings and avoiding excessive stress
In addition to the strategies outlined in this article, conversion to drip irrigation offers an opportunity to reduce water consumption. Use of drip irrigation from planting does not adversely affect growth or yield of pear trees. Irrigation of a mature orchard converted to drip should be managed conservatively in the first season to allow trees to adjust to the new wetting pattern.
Concerns regarding irrigation practices (deficit or otherwise) can be alleviated by equipping yourself with the knowledge to accurately calculate crop water requirement and auditing your irrigation system to ensure it is operating efficiently with a good distribution uniformity.
Experiments in the Goulburn Valley have shown that pear trees seldom reach high levels of stress due to soil waterholding capacity and summer rainfall patterns. In other environments, care needs to be taken to avoid excessive water stress. In Israel, severe water stress in pear appeared to slow flower organ development, leading to reduced flowering intensity and lower fruit set compared to a moderate level of water stress (Naor et al., 2006). Growers contemplating imposing a severe irrigation deficit should plan to monitor soil moisture or use a plant-based indicator of water status, such as stem water potential, and discuss these options with their agronomist or Agriculture Victoria staff.
Avoid mite stress: Monitoring of mite populations is particularly important during dry conditions, regardless of irrigation strategy. Pear trees with high mite populations are prone to flower in late summer and autumn if they become stressed and are then irrigated. Control of mite populations and avoidance of excessive water stress will minimise this risk.
Contact
Ian Goodwin
Research Leader, Tatura
Email: [email protected]
Phone: 03 5833 5240
Acknowledgements
Financial support was provided by DEDJTR project 102303 Managing disruption to water supply in perennial horticulture in a changing climate. Experiments were conducted at Boumbis Orchards and H.V. McNab and Son and we thank the families for their involvement.
