Plums (Prunus spp)
Plums are placed within the Prunoideae subfamily of the Rosaceae family which contains all the stone fruits such as peach, cherry and apricot. There are over 2,000 of varieties of plums all differing in size, shape, and flavour. The colour of the fruit can vary from blue-black to purple-red, green, and yellow. Plums originated from three main types: European, Japanese, and Western Asian. Of the several plum spe-cies, the oriental Japanese plum, Prunus salicinia has the least chilling requirement and will produce after mild winter. Other species from mild climates are Prunus cerasifera, P. occidentalis and P. texana, the latter is a small strong flavoured fruit.
Plate 16.1 Plums tree and fruits
Nearly all plums that are grown in Kenya are of Japanese plum group (Prunus salicina; Prunus triflora). Majority of P. salicina varieties are easier to grow compared to apples and pears.
Table 16.1: Varieties
|Shiro||The trees have a semi-upright(syn.: Ogden)growth habit (low-growing and spreading rather than upright). They are very hardy and prolific so they require fruit thinning. The tree bears round to heart- shaped yellow fruits. The fruits are juicy, pleasantly sweet but without a marked flavour. The skin and the flesh around are acidic and are suitable for eating fresh, cooking, canning and des- sert.|
|Songold||This is a semi-upright and vigorous with low chilling requirements cul-tivar. The cultivar is susceptible to bacteria spot. The fruits are golden yellow and develop an amber colour when ripe.|
|Harry Pickstone||The tree is moderately spreading, very vigorous and has a very low chilling requirement. It bears a heavy crop just two years after planting. The fruit skin is green with red shade but changes to red when ripe. The yellow flesh has excellent dessert quality.|
|Reubennel||The tree has a very low chilling require- ment, is very vigorous and bears heavy crop by its second year. The fruit skin is green with dark red on its suture when picked, but changes to overall red when fully ripe or during cold storage. The flesh is orange to yellow and the texture firm and non-melting.|
|Santa Rosa||The tree is hardy, upright, vigorous and highly productive. The conical fruit is medium to large and crimson to pur plish red with light freckles and often with a whitish bloom. It is firm, sweet, juicy and aromatic, except near the pit where it is often quite acid. The flesh is dark yellow to orange and has a melting texture. It is good for dessert|
|Burbank(syn. Wright’s Early)||The tree is relatively small, spreading,flat-topped medium-sized and hardy. The fruits are round to conical in shape and develop a bright red, blushed yell low skin. The yellow to orange flesh is firm, sweet, aromatic, juicy and of a very good flavour.|
|Satsuma(syn. RedCardinal;Blood Plum)||The tree is a vigorous and upright grower, partially self-fertile; It pro- duces medium to large, almost round and dark red fruits with a small pit. The flesh is purple to red, firm, juicy and has a particularly good flavour.|
|Methly||The tree produces small to medium-sized, round, reddish-purple fruits with soft, juicy, red flesh of good flavour.|
|Cherry Plum||Many cherry plums are low growingshrubs, but some are vigorous, spread- in trees with twiggy branches. The fruits are small, oval and scarlet red. Their yellow flesh is sweet and juicy and pleasant to eat. The fruits are excellent for canning making jam, jelly and juice.|
Japanese plums grow at altitudes of about 1,700 m – 2,600 m, where their chilling requirements of 100 – 800 hours below 7 °C are met.
Up to 1000 mm of rainfall per year is required. If it is inadequate, irrigation should be used to meet these requirements.
Low chilling temperatures are necessary to help the plants break their dormancy. Temperatures of 6 0C to 8 0C are essential for this purpose. However, in the tropics where such temperatures may not prevail for long to break dormancy, chemicals such as Dinitro orthocresol (DNOC) Thiourea, MULTI-K (Potassium Nitrate) and Dormex (Hydrogen cyanamide) can be used as an alternative or a supplement.
These should be deep and well drained. Sandy loams are also suitable provided the pH range is optimum i.e. 5.5 – 6.5.
- Agronomic practices
Plums are propagated by using hardwood cuttings obtained from branches during the dormant months of the tree. A rooting powder (Seradix) together with sandy soil or other growth media that promote easy root production are used to encourage the rooting of the cuttings.
Plums are usually budded on peach seedlings (such as Nemaguard and Lovell) where they tend to produce earlier and set more consistently. Peach seedlings are compatible with most commercial plum cultivars, although a large scion overgrowth may sometimes develop as the tree grows older. Mariana and Myrobalan (Prunus cerasifera) are recommended plum rootstocks. Myrobalan seedlings have been used as the principal rootstock for Japanese plums. Their roots are adapted to a wide range of soil and climatic conditions. They are resistant to drought and crown rot, but susceptible to root knot nematodes and bacterial canker. Mariana plum stock has good nematode resistance.
The budding method used is T-budding
The land should be cleared of all vegetative material and if possible ploughing should be done. Planting holes dug 60 cm x 60 cm x 60 cm should be prepared well in advance.
The spacing is 8 m x 8 m for larger varieties like Methly, 5 m x 6 m for medium sized varieties and 4 m x 5 m for trees that are regularly pruned.
Planting is done bare rooted in the dormant season. The soils should be firmed around the plants and watered generously. Watering frequency depends on rainfall; however in dry periods watering 2 – 3 times per week is adequate.
Most plum varieties are self-fertile and will not require cross pollination. However, it is a good idea to grow more than one tree to ensure effective pollination.
Cover crops can be used during the early growing stages but should not be allowed to compete in the later years.
The preferred tree shape is the ‘open centre’ with good light distribution even for larger tree sizes. Most of the young plum trees obtained from the nursery will be in the shape of a straight whip without lateral branches.
- Pruning at planting usually consists of cutting this whip back to 50 cm – 60 cm from the ground. This stimulates side branching below the point of cutting.
- Pruning during the tree’s formative years is light: interior branches and water sprouts are controlled by pruning or bending, and growing laterals (scaffolds) are headed to induce branching.
- At maturity, vigorous upright shoots are removed since fruiting occurs mostly on spurs of older wood as trees age. To maintain fruit size, renewal of fruiting wood is necessary since spurs live for only about 5 – 8 years.
- It is necessary to remove the weak, diseased or over crowded branches.
- Pruning should be carried out before the rainy season.
- To start a new crop cycle trees should be defoliated.
- In most locations the leaves do not detach easily and therefore chemicals like sodium chlorate (0.5 % – 1%), copper or zinc sulphate and cyanamide (all at about 2% concentration) is used to defoliate
- Plums will fruit on both the old and new wood and a balance should be kept between the two kinds.
- Suckers growing on the rootstocks should be removed, if possible by pulling rather than cutting.
Manure and fertilizer
The need for nutrients varies from tree to tree depending on, among other factors, the cultivar, age, and soil condition. In this context, leaf analysis will help determine deficiencies and/or excesses of the nutrients needed for good growth and production.
- Fertilizers are applied at the beginning of the growing season when the soil is moist and well soaked. It should be spread around the plant and worked in shallowly.
- The recommended amount of nitrogen should be split into two top dressings, spaced about 2 – 3 months apart.
- Apply 0.5 Kg of TSP per mature tree. CAN fertilizer can be top dressed at the same rate. Apply also 1 to 2 ‘debes’ (20 Kg – 40 Kg) of manure per tree.
- Fertilizer application before dormancy period enhances good vegetative growth after bud break.
Cultivate lightly or remove all weeds for the first 3 years. An orchard may be kept under grass and mowed down or controlled with herbicides. Mulch checks the weeds and is a source of organic matter. A cover crop may also be grown.
To avoid problems associated with heavy crops, it is necessary to thin the fruits. This should be done after natural drop and before seed hardens (about 6 weeks to 2 months after flowering), since thinning at a later stage will have little or no effect on fruit size. Remove all but the best plum per cluster or spur. Depending on cultivar, the remaining fruit clusters should be spaced 7 cm – 15 cm apart. This is to facilitate the tree to bear the following year. Prop up the heavily laden branches.
- Pests and Diseases
Table 16.2: Pests
|Red spider||Leaves fall earlier than||Use recommended|
|(tetranychus spp)||normal and are bronze in||miticides e.g. Mitac|
|colour.||20EC or Dynamec|
|.8EC. Spray white oil.|
|Leaf curling plum||These tiny green and black||Spray white oil in|
|aphid||insects feed by attaching||dormant season|
|(Prociphillus||their mouths to leaves,|
|fraxinifolis)||buds, stems or the new|
|growth of the tree. Leaves|
|curl and new shoots are|
|Scales||Cause serious damage by||Spray white mineral oil until|
|(Red,Grey)||feeding||grey scales run-off. Mineral|
|(Aonodiella auramtin||oil can be mixed with|
|such as diazinon based|
|(Diazol 60EC, Agrozinon|
|60EC, Dizon 60EC) or Decis|
Table 16.3: Diseases
|Bacterial||Shallow depressions at the||Remove diseased parts by|
|Canker||base of branches which en-||cutting the branch off and|
|(Pseudomonas||large. Amber like gum may||cutting away the diseased|
|syringae)||also appear. They will quickly||wood until healthy wood is|
|encompass the entire branch||reached.|
|and the branch will perish.|
|Rust||Appears as slightly raised||Any leaves that appear to be|
|(Tranzschellia discolor)||rust coloured (brown-||affected by rust should be|
|red) round spots on the||plucked from the tree and|
|underside of the leaves.||promptly destroyed.|
|Prevent infestation by dispos-|
|ing of all leaves around the|
|base of the tree.|
|Prune tree to allow air flow|
|through the branches.|
|Die back||Rapid wilting of leaves, dead||Spray the plum tree with|
|leaves remain attached to||recom|
|tree. Fruit often remains||mended fungicide e.g.|
|small, ripens before fruit on||Dithane M|
|healthy branches.||45, Ridomil Gold MZ 68 WG,|
|Infected areas on branches||Antracol WP 70|
|become brown and dry|
|Brown rot||The disease is caused by a||Spray the plum tree with|
|(Monilinia||fungus. Cankers develop||recom|
|fructicola)||which girdle the branch or||mended fungicide, Apply|
|Young shoots may also be||cide e.g. Antracol WP 70,|
|Milraz WP76 as soon as the|
|infected near the tips, which||blossoms show colour.|
|then wither rapidly.|
|Brown rot on green fruits ap –||Proper pruning of trees|
|pears as small, circular spots|
|that are light brown in color.|
|The disease is most serious|
|on ripe maturing fruits. In|
|wet weather pale brownish,|
|felt-like masses of fungus|
|spores cover the rotting fruit.|
|Bacterial shoot wilt||Attack shoots of plums in||Remove and burn infected|
|(Pseudomonas pru-||wet weather.||plants|
|nicola)||Black streak on one side of|
|the wilting shoot|
|Leaves also wilt|
- Harvesting and marketing of plums
Plums take 3 years after establishment to start bearing fruits. Depending on cultivar, plums are ready to be picked 80 – 120 days after flowering. Fruits may be picked before they are completely ripe since they will finish ripening off the tree. Santa Rosa and other varieties will ripen satisfactorily when harvested at the straw pink stage.
Fruit maturity first begins at the top of trees and later at the bottom, which usually necessitates more than one picking stretched over a period of 7 – 10 days. Plums are highly perishable and should be picked and handled with care.
Plums yields are variable depending on the season, orchard management and variety. During the “On” season, a yield of up to 30 – 40 Kgs/tree is expected while” Off” season yield is about 10Kgs/tree. The yields can reach 10 – 30 tons per Ha per year, depending on the variety and management.
Fruits should be wrapped in grease-proof paper and stored in cool airy place. Late varieties can be stored for 2 – 4 weeks at -10 0C to 0 0C and 90% relative humidity. Temperatures of 3 0C – 10 0C can preserve plums for 1 – 2 weeks.
Plums are a good source of vitamins A and C and potassium. They contain more antioxidants than any other fruit. Most dessert plums can be cooked, as well as eaten raw. They are sweet and very juicy. They can also be canned or frozen.
Cooking plums are drier with a tart flesh, making them ideal for pies or jam.