Citrus Fruit

Citrus Improvement Programme and nursery practices

South African citrus producers purchase their trees from nurseries accredited by the South African Citrus Improvement Scheme (SACIS). The SACIS serves the citrus Industry via a centralised budwood supply farmblock – Citrus Foundation Block (CFB) – near Uitenhage, close to Port Elizabeth. Currently the CFB supplies all certified propagation material to accredited citrus nurseries.

A nuclear block of virus-free material of all cultivars is maintained at the Agricultural Research Council-Tropical and Subtropical Crops (ARC-TSC) and at Citrus Research International (CRI) in Nelspruit. All the propagation material, except lemons, has been inoculated with a mild strain of Citrus Tristeza Virus (CTV) to protect the trees against the more aggressive CTV strains that is commonly found in southern Africa.

Rootstocks

Since the 1980s, rootstocks rendering high internal quality fruit (such as Troyer and Carrizo, X639 and MXT citranges, and Swingle citrumelo) have become the rootstocks of choice. This aspect is important for the future competitiveness of the Southern African citrus industry to produce fruit of high eating quality to compete with citrus production regions such as Argentina, Uruguay and Australia where trifoliate orange rootstock is preferentially used due to the cold tolerance it imparts on the scion. Rough lemon rootstock is still the preferred rootstock for lemons. Where Eureka lemons are used, producers are aware of incompatibility with certain citrange rootstocks.

Planting time and procedure

Since most nurseries are producing trees in containers, time of orchard establishment is not critical. However, in the colder, windy areas the preferred planting time is early spring (September/October).

Nursery trees are commonly topped at 60 to 70 cm height to allow scaffold development to occur at a height of 40 to 60 cm. Growers ensure that trees with goose neck root systems are not planted.

Spacing trends

Citrus tended to be ranched in certain areas, especially the hot climatic regions. Due to increased establishment costs and the need for earlier economic break-even, and the need to have sunlight-, spray- and picker-friendly trees, there has been a move towards increasingly higher planting densities. Also, the new wave of technology development in tree size maintenance, particularly pruning, provide citrus producers with more confidence to plant at higher densities.

In the hotter regions, where Valencia oranges and grapefruit are produced, spacings of 7×3 meters or 6×3 meters are commonly used, whereas in the cooler regions, where Navel oranges, and Clementine and Satsuma mandarins are produced, spacings as wide as 6×3 meters and as close as 4.5 or 5×2 meters are used.

Preplant soil preparation

The high potential soils of the northern areas (with little or no need for pH correction) are usually only ripped and land preparation costs are thus quite low. In the Western Cape region a lot of money is spent on proper ripping and ploughing. Soil pH correction and other ameliorants (phosphorus, sometimes micro-elements) are added in a double ploughing action. Expensive subsoil drainage systems are often required. In addition, in many cases ridging is considered to provide for added drainage or where the soil is high in clay content.

In the case of replant soils the previous trees must be removed with their root systems, the soils must be ploughed and dried out properly to allow the incidence of soil borne pathogens such as Phytophthora to diminish, and replanting should only take place two years after the proper removal of the trees if the soils are not fumigated before replanting.

Windbreaks

In certain citrus producing areas windbreaks are necessary to reduce wind blemishes. Many windbreak types have been tested or are commonly used. The most well-adapted windbreak tree throughout Southern Africa is beefwood or Casuarina (Casuarina cunninghamiana Miq.). Pinus radiata D. Don and silky oak (Grevillea robusta A. Cunn) are sometimes used. Deciduous type windbreaks are often used as secondary windbreaks in conjunction with beefwood in the Western Cape, e.g. Dutch alder (Alnus cordata) and Chinese poplar (Populus simonii [syn. P. obtusa]).

Row orientation

Whereas it is not so critical to plant in north-south row directions in the northern regions (lower latitude, dry winters), it is still commonly done. In the more southern latitudes, with the more extreme angle of the sun and where rain or dew can keep the tree wet for extended periods during harvest, it is essential to plant in a north-south row direction, especially with Alternaria susceptible cultivars.

Irrigation and fertigation

Under-tree microsprinkler irrigation systems are most commonly used, while a few orchards still use overhead sprinkler irrigation. More recently, however, drip irrigation systems have become increasingly common, with an increased use of drip fertigation where pH and electrical conductivity are controlled in a balanced nutrient solution which is provided daily to restrict root system development in a bid to control tree phenology. To attain good eating quality, pre-harvest water stress (limited or deficit irrigation) has become an accepted practice, for example with Satsuma mandarin.

Fertilisation

Fertilisation of bearing trees is exclusively based on annual leaf analysis data from leaves from fruiting terminals and the previous history of the orchard with respect to yield, fruit size, quality and previous fertilisation record. Nitrogen phosphorus and potassium are most commonly applied as soil applications, whereas magnesium and the micro-elements (copper, boron, zinc, manganese and molybdenum) are applied as foliar applications, when required. Soil pH correction is achieved by the addition of calcitic or dolomitic lime, and water penetration or salinity problems are addressed by the application of gypsum.

Pruning

In some production regions there is a shift towards selective pruning by hand or with pneumatic pruning equipment. Most large orchards are, however, hedged and topped mechanically. Though higher density plantings are the norm, growers should guard against too dense plantings which could result in crops being pruned away. Managing post prune re-growth is critical.

Source: Citrus Research International. Visit www.citrusres.com.