Mango seedlings as a rule start to bear fruit within 4–7 years, while grafted trees (if allowed) may bear a few fruits in their second year in the field. Mango production in Kenya has to be differentiated according to the production system. There is traditional mango growing, and commercial and market-orientated mango cultivation. Out of an average annual mango production in Kenya of about 140,000 tonnes (t) during 1999/2000, approximately 3300 t (2.3%) were exported (source: Annual Report, Horticultural Crops Development Authority (HCDA), Ministry of Agriculture, Nairobi). Some distinct differences between the location of production and the performance of the orchard can be identified, such as the harvest period, the fruit quality and the yield level. Due to the varying ecological conditions in Kenya, mangos are available almost all year round (see Appendix 3).
In the main production area, the Coast Province, two supply seasons can be differentiated. The first and main season runs from November to February and the second from June to August. In areas of higher altitude such as Murang’a and Mwea (Central Province), the harvest season is 4–6 weeks later than at the coast, with a peak in February and March. The mango picking season in Kenya competes with that of other mango producing countries (Mexico, Brazil, India, Pakistan, Israel, South Africa) and extends over a period of between 5 and 6 months (Appendix 4). Interestingly though, Kenya exports only about 3000 t out of the worldwide export tonnage of 580,000 t/year (FAO, 2001).
Productivity depends on a number of factors, including quantity of previous crop, weather and soil conditions, altitude, control of pests and diseases, fertilization and cultivar. Even in the case of the same cultivar, yields vary greatly because mango is grown under widely varying agroclimatic conditions and cultural practices.
Biennial or irregular bearing occurs often with the mango and it is common for some cultivars to bear heavily in one year and sparsely the next. One of the reasons for this phenomenon is that trees over-bear in one year, thus inhibiting adequate flower bud formation the following year. Under these circumstances, it is difficult to get accurate local long-term yield records. However, it is well known that yields of 25 t/ha and more for Kent, Sabine, Tommy Atkins and Keitt are not uncommon.
Cultivar trials carried out under rainfed conditions at government prison farms in Kenya indicate that even higher yields could be achieved. Tables 1 and 2 show the performance of some imported mango cultivars planted in the Central Province of Kenya. Additional performance figures are also shown in these tables which are taken from Griesbach (1992, page 87).
Depending on cultivars and environmental conditons it takes 90 to 160 days after flowering for Kenya mangos to reach maturity. Not all fruits on one tree will ripen at the same time. A great problem is to determine precisely the stage at which the fruit is ripe for picking. Fruits harvested too early will be of inferior quality after storage; however, fruits picked when too ripe cannot be stored for any length of time and may give rise to problems such as jelly seed. The fruit will have its best flavour if allowed to ripen on the tree. None of the tests (acid, sugar content or specific gravity) used to determine ripeness, however, are fully reliable.
The fruits are generally picked when they begin to change colour. This may occur first in a small area or the change will cover most of the fruit’s surface. However, one destructive maturity test that can be applied even before the external colour break starts is to examine the colour of the flesh around the seed. When this begins to change from green-white to yellow or orange, it indicates that the fruit is beginning to ripen and may therefore be picked. Also the greater the swelling of the shoulders above the stalk attachment, the riper the fruit is likely to be (see diagram of a mature mango fruit).
During and after harvesting the highly perishable fruit must be handled with the greatest care. The fruit is removed from the tree by cutting the fruit stalk about 2 cm from fruit. This will prevent the latex (exuded from the cut stalk) adhering to the skin of the fruit, staining it and rendering it unattractive. Ladders or long picking poles with a cutter blade and an attached canvas bag, held open by a ring, are also in use. To avoid physical damage, the picked mangos should be carefully placed into clean wooden or plastic containers and never into gunny bags. If there is a delay in the transfer of the fruits to a store or packing shed they should be kept in a sheltered place to minimize sunburn, loss of moisture and accumulation of dust.
After any sorting, grading, washing, fungicidal treatment and perhaps waxing, the fruits are ready for packing, preferably into shallow single-layered trays of 4–5 kg each. Because mangos are harvested during the summer months, the fruit temperature may be as high as 35°C and more. This has a detrimental effect on the shelf life of the fruit. It is therefore advisable to move the packed fruits into cold storage as quickly as possible to help them lose this inherent heat. The recommended storage temperature must, however, not drop below 7°C (range: 7–10°C) as otherwise cold injury may occur.
According to the Horticultural Crops Development Authority (HCDA), mangos in Kenya are available from November to April (and sometimes to July). Because of less competition better prices are fetched in Europe and the Middle East between November and December (see Appendix 5). Many techniques have been used in other countries to improve productivity and to alter the cropping season. Smudging (moist organic material—grass, leaves, etc.—is slowly burnt under the tree canopies and the resulting smoke induces flowering) is an old technique reported from the Philippines for enforcing off-season flowering, but this has largely given way to chemical induction. The application of potassium nitrate has been commercially accepted. The reasons are obvious: to have an altered earlier harvest, to take advantage of the good market price, to fill the gap of under-supply and to have flowering during a dry spell with little or no fungal diseases.
The readiness of a tree to flower is an important factor for a successful operation. For best results, choose trees with leaves that are dull green or greenish-brown and brittle when crushed by hand. The trees should have an appearance of suspended growth or be dormant. It is easier to induce mango trees to flower towards the dry season, and older trees respond better than young ones.
It is recommended that a 1% potassium nitrate solution mixed with a sticker agent (adhesive) be sprayed on to the tree, totally drenching its terminals and leaves. Make sure a knapsack sprayer has no residual herbicide in it before beginning to spray. If the timing is right, flowers will emerge 10–14 days after application. Tentative trials have been successfully implemented in Kenya.