Figure 1. Remove all leaves with leaf spot. Here at least two leaves have leaf spot. Do not remove only the bottom two leaves. All areas with leaf spot should be removed regularly.
Deleafing is the main activity for controlling the spread of yellow and black sigatoka. Deleafing stops necrotic patches on leaves from developing, reducing the number of ascospores that are emitted by the diseases.
Chemical sprays and oil only work on the early stages of the disease. Once a spot turns dark there is no chemical or oil that will stop it developing. It will go on to form large dead patches on the leaf that will emit many spores and continue the cycle of infection. They only way to break up the cycle is by deleafing.
Leaf area is essential for photosynthesis in the plant. By deleafing, the area available for photosynthesis is reduced, slowing the development of the bunch and towards harvest the bunch may ripen prematurely. This can lead to mixed-ripe at ripening centres. This is where some of the bananas in a carton ripen faster than others.
It is important to deleaf regularly to keep the number of spores in the air to a minimum, but in order to avoid mixed-ripe, it is also important to only remove the areas of the leaf with sigatoka infections. Growers should educate workers about the right leaf portions to remove so that the leaf area left is adequate for photosynthesis.
(Left) A mild case of leaf spot. Only the end of this leaf has been removed.The remainder is free of leaf spot and should be left on the plant.
(Right) The severe leaf spot located around the base of this leaf means that the whole leaf should be removed.
Remember to also examine suckers. It is easy to confuse the disease cordana with leaf spot, but if in doubt, the spotted leaves should be removed. Cordana infects through sigatoka infections and often masks a sigatoka leaf spot that can cause re-infection.
Deleafing cannot be done in the evening or at night because visibility is poor and it is impossible to remove leaf spot properly. If a leaf only has a few spots on it at one end, then remove the leaf area only at that end (see figure 2).
(Top) A blade with both the upper and lower surfaces sharpened. This gives more flexibility when deleafing and makes completing the task more efficient.
(Bottom) A traditional deleafing blade with only the lower surface sharpened. This limits cutting motions and slows down work.
By choosing the correct deleafing tools, the task can be made easier and more efficient. Knives and blades should be sharp enough and long enough to move leaves aside to look for leaf spot and to cut pieces of leaf from the highest leaves.
The older style of curved blades tends to only allow a downward chopping action (see figure 3). It is more efficient to have a knife with both sides sharpened to allow upwards and downwards cutting motions (see figure 3). Vertical blades also have good flexibility. The pole must be long enough and light enough to allow fast and efficient cutting.