With exports raking in billions of rand annually, South Africa’s apple growers form an important part of the country’s economic development. However, their heavy dependence on water in a country facing growing water challenges, demands an enhanced understanding of the water requirements of top-producing orchards. International research shows that high crop loads are associated with high water demands. However, CSIR senior researcher Dr Sebinasi Dzikiti explains that no information currently exists on the water requirements of high-yielding apple orchards in South Africa and elsewhere. Thus, to support apple growers in a country saddled with water challenges, the CSIR has partnered with the Water Research Commission (WRC), the Agricultural Research Council, the South African Apple and Pear Producers Association represented by Hortgro Science, and the Universities of Stellenbosch and Pretoria to determine the water requirements of fruit trees with a particular focus on apple trees, from planting to full bearing age.
The four-year study, published in the Agricultural Water Management Journal, found that crop load has a very low impact on the orchard’s water requirements. “Instead, canopy size is the major driver of water use and how you manage the canopy is extremely important,” says Dzikiti. Given the industry’s importance in the country’s economic development, the researchers also deemed it important to determine how crop load affects fruit quality, which influences the fruit’s selling price.
The study was conducted in the Koue Bokkeveld and the Elgin/Grabouw/Vyeboom and Villiersdorp regions of the Western Cape – a prime apple producing province. The reliable availability of water is critical for the sustainability and growth of the country’s fruit industry as all commercially grown apples in South Africa are irrigated. Given the Western Cape’s volatile water supply with demand expected to exceed supply in the future, it is imperative to find a solution for the water needs of the deciduous fruit industry.
To determine the water productivity in full-bearing orchards expressed in Rands per cubic meter of water consumed, the researchers sought to obtain accurate quantitative information on the water use of unstressed high-performing apple orchards, from planting to full bearing, to improve irrigation scheduling and water allocation decision-making for water licensing and for the development of water-saving strategies to cope with water shortages induced by droughts. The study also provided insights on the income generated per unit volume of water used in the orchards.
The widely planted Golden Delicious and the blushed cultivars were studied. Both are high- yielding cultivars with yields exceeding 100 tons per hectare becoming common. All the orchards were irrigated using one micro-sprinkler system per tree, delivering between 30 – 35 litres of water per hour. The frequency of irrigation ranged from two to three times per week, lasting one to two hours early in the season. During the hot summer months, the frequency increased to daily or several times a day.
The study showed that the maximum unstressed seasonal total orchard water use in the high-yielding orchards range from a little under 8 000 to just over 10 500 m3/ha, depending on canopy size. Golden Delicious orchards, which tend to have larger canopies to protect the fruit from sunburn damage, used the most water. However, the smaller canopies of the red cultivars, normally maintained open to improve light penetration to promote the development of the red fruit colour, had significant water saving benefits.
The study has shown that high apple yields can be produced sustainably without using excessive amounts of water, provided the canopy is managed optimally. Dzikiti explains that high crop loads in this study did not necessarily have a negative effect on most fruit quality attributes in the high-yielding orchards. The study made several recommendations which have a direct impact on the deciduous fruit industry.
Firstly, it is important to carefully manage crop load in the Golden Delicious cultivar as high fruit numbers reduce fruit size and hence the quantity of export quality fruit, he says. Secondly, growers should consider using dwarfing rootstocks to reduce canopy size and hence minimise orchard water use. Dwarfing rootstocks control wood production in the tree, directing its energy into fruit production. Thirdly, cultivars that are susceptible to sunburn such as Golden Delicious and Granny Smith should be grown under shade nets where small canopies can be maintained to reduce orchard water use. Two new follow-on projects, funded by the deciduous fruit industry and the WRC have been commissioned to further investigate and quantify the water-saving benefits of various apple rootstocks as well as fixed and drape shade nets. Both projects are being led by Stellenbosch University, with the CSIR as a key partner.