Research rundown

A summary of Hortgro-funded research on water and irrigation. By Anna Mouton.

Water scarcity was already a recognised risk in 1998 when Dr Theresa Volschenk and colleagues of what is now the ARC Infruitec-Nietvoorbij began investigating the Soil Water Balance model in apple and pear orchards. The model uses inputs such as weather data, crop and soil characteristics, and irrigation quantities to simulate the soil water deficit.

The researchers compared model outputs to measured soil water deficits. They concluded the Soil Water Balance model could be used to improve irrigation management, but only by trained professionals, and more research was needed.

A 2003 project led by Prof. Piet Stassen of the ARC Infruitec-Nietvoorbij looked at whether more frequent water and nutrient application improved tree efficiency. He found significant improvements in yield efficiency in Brookfield Gala apple trees in the EGVV receiving drip irrigation compared with micro-sprinkler irrigation. The drip-irrigated trees required less water and had better root development.

Around the same time, Volschenk measured evaporative losses in a drip-irrigated Cripps Pink orchard in the EGVV. She found that soil evaporation and evapotranspiration were reduced under drip irrigation, especially once the tree canopies were fully developed.

Deficit irrigation in pome fruit

Volschenk researched the effect of deficit irrigation on apple-tree performance from 2005–2009. She evaluated the response of full-bearing Golden Delicious on M.793 in the Koue Bokkeveld to different deficit irrigation strategies consisting of treatment combinations of 50%, 75%, 90% and 100% reductions in irrigation during various phenological stages.

Deficit irrigation from bud break to 40 days after full bloom or from 40 days after full bloom to 31 January reduced shoot growth. Deficits from 40 days after full bloom to 31 January reduced fruit growth rate and final fruit size. Water depletion during one season reduced fruit growth from bud break to 40 days after full bloom in the following season.

Volschenk concluded that there is a direct relationship between reduced income and increased water deficit in apples.

She conducted similar research on Forelle pears on BP3 in the Warm Bokkeveld from 2008–2012. Water deficits reduced fruit size and numbers. Deficits from bud break until 58 days after full bloom or from 58 days after full bloom until 31 January accelerated fruit maturation.

Limiting irrigation after 31 January was detrimental to the following season’s bloom. Severe deficits resulted in earlier leaf fall and out-of-season bud break and bloom.

More about apples

Dr Elmi Lötze of the Department of Horticultural Science at Stellenbosch University investigated the effect of mulches in full-bearing Cripps Pink apples on M.793 in Somerset West. She compared a clean-cultivated control with a geotextile fabric, compost, wood chips, and vermicast mulch topped with wood chips. The treatments were assessed on a sandy and a heavy soil.

Trials ran from 2009–2012. Mulching did not appear to affect plant water status significantly but enhanced plant performance by increasing photochemistry and gas exchange. Mulching reduced fruit-surface temperatures and generally decreased sunburn compared to the control.

Dr Johan van Zyl and a team from the Department of Soil Science at Stellenbosch University looked at whether applying more water less frequently promotes deeper root development in young apple trees than less water applied more frequently. He tested three irrigation treatments in newly established Bigbucks on MM.109 with M.9 interstems in the EGVV from 2016–2020.

The irrigation cycles were twice weekly at a soil water potential of -15 kPa, weekly at a soil water potential of -30 kPa, and every two weeks at a soil water potential of -80 kPa.

Van Zyl found that roots reached their maximum depth of 1 metre during the second season. Larger water volumes applied less frequently promoted deep root penetration. Yields were similar for the different treatments at the first harvest, but fruit were larger in the short-cycle treatment.

Other completed projects

A 2015 project led by Prof. Wiehann Steyn, General Manager of Hortgro Science and extraordinary associate professor in Horticultural Science at Stellenbosch University, researched heat damage in Japanese plums.

Well-watered trees seemed to sustain higher transpiration rates and lower canopy temperatures, which helps to moderate fruit-surface temperatures. Maintaining sub-optimal soil moisture will increase sunburn, but irrigating beyond optimal soil moisture levels provides no additional sunburn protection. This corresponds with previous findings in apples.

Daniël Viljoen of ExperiCo Agri-Research Solutions recently concluded a project on the effect of water stress on bud break, flower quality, fruit yields, and storability of Cripps Pink, Fuji, and Golden Delicious apples in the Ceres area.

Preharvest water stress was created through a 40% irrigation deficit. Postharvest water stress was created by stopping irrigation for about five weeks after harvest. The researchers covered the soil with plastic to prevent rainwater infiltration in the water-stressed treatments.

Deficit irrigation before harvest resulted in smaller and firmer fruit with higher total soluble sugar levels at harvest and after storage. Postharvest deficits did not affect fruit quality in the following season.

Viljoen concluded that water stress after harvest is less detrimental than deficit irrigation during the growing season. At the moderate water deficits applied in this trial, postharvest quality was generally not negatively affected, except for a possible increase in internal browning in Cripps Pink. The researchers also did not find effects on bud-break progression and flower quality.

The following projects were covered in previous articles. Please click the links to read more or view the researchers presenting their results at the 2023 Hortgro Science Research Showcase.

2017. Steyn W. Tree water relations and sunburn in pome fruit.
2018. Dzikiti S. Quantifying water use of high-performing commercial apple orchards in the winter rainfall areas of South Africa.
2019. Jarmain C. Water use of pome and stone fruit: knowledge status, relevance, and gap analysis.
2020. Midgley SJE. Scientific and practical guide to climate change and pome- and stone-fruit production in South Africa.
2021. Dzikiti S. Water use under shade nets. Watch on YouTube.
2022. Midgley SJE. Sensitivity of various apple rootstocks to water stress. Watch on YouTube.

Current projects

Determining the water use of pome and stone fruit is important for planning at all scales, from farm-level planting and irrigation to provincial and national water infrastructure and management.

Prof. Nebo Jovanovic of the Department of Earth Science at the University of the Western Cape is determining the water use of high-performing full-bearing African Delight and Fortune plums in Wellington and Robertson. The project is set for completion in 2024. Click to watch Jovanovic present his preliminary results at the 2023 Hortgro Science Research Showcase.

Dr Sebinasi Dzikiti of the Department of Horticultural Science at Stellenbosch University previously quantified water use in apples. He now has funding from Hortgro and the Water Research Commission to study water use in pears.

In addition, a new project fully funded by the Water Research Commission is assessing water use in apricots. Hortgro is involved in the apricot project in an advisory capacity.

The response of apple trees to low-flow drip irrigation is under investigation by Dr Johan van Zyl in Golden Delicious on MM.109 in the EGVV. The trial orchard is on free-draining soil with 70%–80% gravel. Three drip-irrigation treatments (Table 1) are being compared to a micro-sprinkler control.

Table 1: Treatments in a trial comparing drip to micro-sprinkler irrigation in apples.

Emitters	Lines	Delivery rate in litres per hour	Spacing
Drippers	Double	0.4	50 cm
Drippers	Single	0.7	50 cm
Drippers	Double	1.6	50 cm
Micro-sprinklers	Single	32	Between every two trees

Data collection includes soil water distribution, vegetative and root growth, leaf area index, and fruit yield and quality. The project started in 2022, and drip-irrigated treatments used 68% less water than the micro-sprinkler control. There was no difference in fruit yield or quality.

Although these results are encouraging, the drip-irrigated treatments may have benefitted from the high rainfall during the 2022–23 season. More data is needed before any conclusions can be made. The project is set to continue until 2024.

New projects

Three new Hortgro-funded projects all focus on the bigger water picture. As mentioned in Keeping Clear of Day Zero in this issue, Dr James Cullis of Zutari will conduct a water status quo assessment and develop a water response strategy for the deciduous-fruit industry.

Hortgro is also funding a groundwater availability and use study by Lizanne Smit of specialist groundwater and earth-sciences consultancy GEOSS.

Prof. Adriaan van Niekerk of the Department of Geography and Environmental Studies at Stellenbosch University will mine existing FruitLook data to analyse spatial and temporal patterns of evapotranspiration, crop yield, and water productivity in the major fruit production regions of the Western Cape.