Although the mango in Kenya is spread throughout all feasible agroclimatic zones it has relatively few major problems with pests and diseases. These problems can be significantly reduced through a number of management decisions, for example:
• selection of proper orchard site
• selection of cultivars
• controlled fertilizer application
• timely spray application programmes
• orchard sanitation
• timing of irrigation
However, even when implementing these decisions there is no guarantee that some of these stubborn pests/diseases will not occur. Trees should be examined frequently to check for any infestations so that control measures, particularly for export fruits, can be applied before extensive damage can occur.
Where specific insecticides/fungicides have been mentioned in the following text, these are generally given as examples and should not be regarded as exclusive of others. In addition, trade names have been avoided as much as possible as one active ingredient could have several trade names from different manufacturers. It is important to rotate pesticides so that no resistance can build up especially in the nursery. The author has previously used the pesticides mentioned during his field research trials although the reader is strongly advised to check with his/her horticultural extension officer for the latest control recommendations and the respective recommended pre-harvest intervals (see Appendix 6).
In areas where chemical control agents are not available or affordable it is possible to use phytopesticides. Tephrosia vogelii and Azadirachta indica (neem tree) are probably the most readily available.
Besides powdery mildew, anthracnose, caused by the fungus Colletotrichum gloeosporioides, is undoubtedly the most common and widespread fungus disease of mango and is a major factor limiting production in areas where conditions of high humidity prevail. The fungus invades inflorescences, fruits, leaves and twigs. Substantial losses due to this disease are recorded every year not only at premature stages of the crop but also during storage after picking.
Humidity, rains and heavy dew during critical infection periods greatly increase the disease incidence. Most infections occur from the beginning of flowering in gradually decreasing severity until the fruit is about half-grown. Infections on the flower and panicle appear first as minute brown or black spots which slowly enlarge. Infected flowers usually wither and die before fruit set.
Young fruits are readily infected. Spots may remain as pinpoint latent infections or they may enlarge in wet weather. Wet weather also causes characteristic tear-stain symptoms due to the spread of fungal spores by raindrops. The latent infections on young fruits cause much of the decay which occurs in mature fruits. Nearly mature to ripe fruits will have black spots of varied form which may be slightly sunken and show surface cracks penetrating deeply into the fruit causing extensive rotting or complete blackening of the fruit surface.
To control the disease, orchard sanitation and pruning of dead twigs and branches—which may harbour the fungus—are the principal control measures used to reduce the source of a new infection cycle. The widespread occurrence of the inoculum of the fungus makes it impossible to control the disease by pruning and the removal of dropped leaves alone. To be more successful, the above mentioned measures have to be supplemented by spray applications using Mancozeb, copper oxychloride, Maneb, Propineb, Benomyl etc.
It is recommended to start spraying at the stage of flower-bud formation. During flowering/fruit set and until the fruits have developed to half their size, spraying should take place at fortnightly intervals. After this, it is sufficient to treat the trees once a month. It is very important to apply a full cover spray for the first two applications. Since this period is also the critical stage during which powdery mildew and the mango weevil attack, counteractions should be implemented using recommended fungicide/insecticide combinations.
All cultivars are to some extent susceptible to anthracnose. The range of resistance (with Tommy Atkins being the most resistant) is: Tommy Atkins, van Dyke, Sabine, Ngowe, Gesine, Apple, Keitt, Kent, Kensington, Chino, Sensation, Batawi, Boribo, Haden, Maya.
There are several other diseases of mango fruits that have been reported occasionally. These include alternaria rot, mango scab, stem-end rot, algal leaf spot and sooty mould.
Different types of fruit flies are known to attack ripening mangos in almost all mango-producing areas. Yield losses of more than 50% have been reported. Ceratitis cosyra followed by C. rosa and C. capitata have been found to be the major pests of mango.
The females lay their eggs under the surface of the fruit skin. After hatching, the maggots penetrate the flesh and destroy the fruit from inside. The infested part becomes mushy and causes premature colouring of the already useless fruit. Fruits of some cultivars are more susceptible to attack than those of others. Successful control of fruit flies in mango orchards depends on a combination of:
• eradication of non-economic host plants (such as neglected citrus, peach, guava)
• regular orchard sanitation
• determination of population density by using traps
• regular poison-bait applications
Chemical control of adult fruit flies in orchards is based on a weekly bait spray: protein hydrozylate or molasses mixed with Malathion, Trichlorphon, Fenitrothion or Fenthion. The bait is applied in large drops at a rate of 200–1000 ml/tree, depending on tree size. It is not necessary to wet the whole tree; only part of the foliage needs to be covered.