Deleafing is the sanitary removal of leaves, or parts of them, that are infected with black leaf streak (BLSD). Banana leaves being the only source of inoculum of BLSD, and the capacity to produce ascospores being higher in living leaves than in cut leaves laying on the soil, less ascospores will be released. Deleafing increases the efficiency of fungicide applications and has been shown to reduce premature ripening. However, it involves a trade-off between the removal of infected leaves and the preservation of a minimum leaf surface area to ensure an adequate development of the bunch until harvest. Studies have shown that the weight of the bunch is not appreciably affected when the number of leaves between flowering and harvest is between 5 and 7.
Sanitary deleafing consists in the surgical removal of part of a leaf. It generally follows the order of apparition of the symptoms: the tip, the left blade and the right blade. The removed leaves are generally at stages 5 or 6 of the disease (also stage 4 if the density of spots is high), these stages being the most visible. A weekly deleafing is recommended. It's also recommended to deleaf before the application of fungicides to avoid the selection of resistant strains. All the plants must be examined, even the suckers.
- For infections that cover less than 30% of the leaf area, it's preferable to remove the infected part;
- When more than 40% of the leaf area is infected, it is recommended to remove the entire leaf.
This method consists in the weekly removal of the leaf tip (about 20 cm) of one of the 5 oldest leaves before the appearance of necroses. It is complementary to sanitary deleafing and is particularly useful in areas where the conditions are favourable to the development of BLSD or during the rainy season.
Deleafing at flowering
This method consists in systematically removing the 3 oldest leaves at flowering. This strategy has a strong impact on the plant's leaf area. It anticipates, but does not replace sanitary deleafing since symptoms can appear on other leaves before harvest.
- The regular application of various products (e.g. glyphosate, chlorotalonil, bacteria, molasses, urea) in order to accelerate decomposition. Mixed results have been obtained, except for the weekly application of urea (5 or 10%) which has given interesting results.
- Composting the leaf and crop residues between the rows could reduce inoculum and bring nutrient and organic matter to the soil. The effect remains to be evaluated.
A study conducted in Guadeloupe at a time when only Mycosphaerella musicola was present, showed that although deleafing reduced the weight and diameter of fruits harvested at 900 degree-days, their green life was significantly increased. In highly infested plots, leaf removal increased the green life from an average of 7.7 days to 45.8 days. The authors also note that intense deleafing (leaving less than 3 leaves on a plant) negatively impacts fruit weght. However, leaving 5 or more leaves should not significantly affect yield. Since the presence of necrotic tissues during the last month of fruit growth seems to be responsible for the physiological changes linked to early ripening, the authors suggest that in severe infestations, yield losses could be reduced by deleafing one month before harvest.
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