A plum fruit is a drupe, in other words, the fruit has a thin outer skin, a soft pulpy inside with a hard stony central part which encloses the seed.
CHOOSING A CULTIVAR
When deciding on a cultivar, pay attention to climatic variations throughout the year, susceptibility to diseases, the cultivar’s chilling requirements and time of blossoming, the slope of the land, soil characteristics and pollination requirements.
The general rule is to plant early-maturing cultivars in warmer areas and to reserve late-maturing, high-chilling cultivars for areas that get warm later. Cultivars sensitive to frost shouldn’t be planted in low-lying areas or where temperatures are unusually low (cold pockets).
Avoid areas prone to frost in late spring, but you could choose cultivars that blossom after the last date on which spring frost occurs.
- A market should be established before selecting a cultivar.
- Plum trees should not be propagated from seed as the offspring won’t be true to type.
- Buds of the required cultivar are either budded or short shoots are grafted onto compatible rootstocks.
- Certified disease-free and true-to-type vegetative material of the cultivars is budded onto rootstocks.
- Over the past 4 decades, more than 40 plum rootstocks were imported and evaluated in South Africa, using a number of commercial scion cultivars.
- Several local crosses were evaluated, as well as trees propagated on their own roots.
- The clonal rootstocks primarily used are Maridon and Marianna.
Yellow-fleshed peach seedlings and Marianna cuttings have long been used in the deciduous fruit industry. Marianna, though, is susceptible to bacterial canker which can shorten the lifespan of trees, and peach seedlings are susceptible to crown gall and nematodes.
Marianna is recommended for prunes on shallower and poorly drained soils, but peach seedlings (as rootstocks) should be used for gravelly and lighter sandy soils. Maridon roots readily and is compatible with all Japanese plums, as well as those European plums for which it was tested.
Most plum trees have Plant Breeders Rights and legislation protects it against illegal propagation. It’s advisable to purchase trees from a reputable registered nursery to ensure true-to-type as well as pest- and disease-free trees.
Plum trees can grow in a variety of soil types but prefer deep, well-drained soils, ranging from sandy loam to sandy clay loam, with an effective depth of at least 600 mm.
- Plums are more tolerant of heavy or waterlogged soils than most other stone fruit types are.
- The soil should have a pH (KCl) of between 5.5 and 6.5.
- Avoid areas where nematodes are problematic, even though plum rootstocks are reasonably resistant to root-knot nematodes.
- Dig soil profile pits to inspect the soil and determine the best preparation method.
- Identify any limiting soil layers, such as compacted or very clayey ones, as they will hinder water drainage and limit root development.
- Take at least 6 soil samples to determine the nematode status.
- Because of their shallow horizontal root system, Marianna rootstocks do well in soils that are well-aerated to a depth of at least 200 mm.
- Peach seedlings used as rootstock need about 600 mm well-aerated, uncompacted soil to function efficiently.
When working out a fertilisation programme, farmers should consider:
- The removal of nutrients
- Effective application of fertiliser.
- Expected yield.
The size of the crop, the vigour of trees and the concentration of nutrients in the fruit and the rest of the tree will determine how much of the nutrients is removed during harvest and in autumn.
Some fertilisers don’t reach the roots and can be inaccessible to the tree, while others, such as nitrogen, leach from soils, and less is absorbed by the trees’ roots. Larger quantities of fertiliser are usually applied to compensate for these factors.
Take soil and leaf samples (no later than January of each year) to determine the nutrient status of the soil as well as of the trees. Based on the results of these analyses, you can devise your fertilising programme.
- It’s important to prune trees so that they are trained correctly from the start. This usually occurs during July and August.
- Cut off dead and diseased branches throughout the year and remove them from the orchard. Start corrective pruning during the third year after establishment.
- Summer pruning entails the removal of all water (wild) shoots. Seal large pruning wounds with a sealing compound.
How much water is needed will depend on factors such as soil type, water quality, climate, season, type of fruit, the age and size of trees, the tree’s growth phase and the type of irrigation system used, as well as mulching with organic material such as straw.
Sandy soils consist of coarse particles, resulting in poor water-holding capacity. Such soil needs a little water at relatively short intervals. Clay soils consist of finer particles and their water-holding capacity is much greater. More water should therefore be applied, with longer intervals in between.
See Table 1 for the estimated water consumption for plum trees based on Class A-pan evaporation (a method used to determine water requirements of plants) and the use of micro-sprinklers for irrigation. These estimates apply to mature trees and should only be used as a guideline. The quantity of supplementary irrigation needed can be derived by subtracting rainfall figures from the water requirements of the trees.
PESTS AND DISEASES
Dieback and necrosis in plum trees are widely known in the Western Cape Province in SA, and were noted as early as 1905 by Harry Pickstone.
Surveys indicate that there are essentially two causes:
Bacterial canker (Pseudomonas syringae) and waterlogging.
- The even growth of shoots of Marianna cuttings decreases dramatically if waterlogging includes September – the time when budding occurs.
- The incidence of bacterial canker is increased by excessive wind and rainfall, and affected branches or trees cannot recover.
- Infection of small shoots and buds occurs during autumn rains, but can also be caused by pruning wounds.
- Marianna is regarded as being resistant, or immune, to most root-knot nematodes.
- Ensure there is a demand for your produce before planting plums.
- Try to find a gap in the market, but make sure that the sustainable need for the product is large enough.
It would be unwise to grow and sell produce to fulfill the needs of only a few consumers. So, do your market research, and ensure that your produce or service is fulfilling a need.
- This article was written by Odette Beukes and Chris Smith and first appeared in Farming SA.