The overall symptom picture of the disorder observed during disease surveys consist mainly in poor development, rapid withering of older leaves, reduced number of functional leaves at bearing stage, poor bunch emergence, incomplete fruit filling, and premature ripening of fruits.In addition to the above symptoms, other characteristics observed are as follows:Accelerated drying of leaves, starting from the oldest leaves, and progressing towards the heart (unfurled) leaf was noted in all cases.
Leaf necrosis usually started from the apex of a leaf and progressed towards the base thus killing the entire tissue. Leaf spotting on other parts of the leaf blade was also noted, though this type of infection was less common.
Dark brown to reddish brown necrotic streaks developed along the leaf veins on young leaves which became visible to the naked eye whenthey reached about 1mm.
With progression of the disease,these streaks developed into dark spindle- shaped lesions 10-20 mm long , often surrounded by a yellow halo. Lesions coalesced under climatic conditions favorable to disease development and killed the entire leaf tissue rapidly.The centers of all lesions turned whitish in colour. In the superhumid zone affected plants often had less than four functional leaves which ultimately lead to emergence of small distorted bunches and prematurely ripening fruits.
High humidity favours development of the disease complex and can be controlled by modification ofthe canopy environment through the following:The recommended planting density of 2000 plants per hectare with spacing of 2.5 m x 2 m should notbe exceeded as this allows sun rays to reach the soil within the plantation.Maintaining sucker density at a low level (2 - 3 suckers per plant). This allows good ventilationwithin the plantations enabling the banana plant to thrive better in a well aerated environment.Ensuring a good drainage system, and effecting regular weed control. A dense weed populationincreases the humidity through transpiration.
The aim is to reduce the level of inoculum potential through:(i) Removal of plants under stress (e.g. stunted plants or plants infested by banana weevils)(ii) Removal and destruction of hanging leaves (infected or dried up)(iii) Cutting down bearing plants which are severely affected.
The vast majority of the bananas currently grown and consumed were not conventionally bred but are selections made over probably thousands of years from naturally occurring hybrids. Cultivated bananas are very nearly sterile and as a consequence are not propagated from seed but rather through vegetative propagation, primarily suckers as well as more recently micropropagated or tissue cultured bananas. These factors, very old selections, near sterility and vegetative propagation, mean that these bananas have not been genetically improved either for resistance or improved quality and are becoming increasing in affected by serious pests and diseases.
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