24 April, 2019
CSIR informs water requirements of SA's multi-billion rand deciduous fruit industry
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
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.