Farmer's Notebook
Management of cocoa diseases
By Our Agriculture Correspondent
Zinc deficiency in cocoa causes loss of cholorophyll in patches
THE GOOD relative humidity and shade needed for a good crop of cocoa also make the trees susceptible to major pests and diseases. Scientists at the Central Plantation Crops Research Institute's (CPCRI's) Regional Station at Vittal, Karnataka, have come out with suitable package of practices to effectively manage the diseases and make cocoa cultivation more remunerative to the growers.
Major diseases
"Our survey of the cocoa gardens in the country indicated diseases caused by the fungi Phytophthora palmivora and Colletotrichum gloeosporioides, Oncobasidium theobromae and zinc deficiency as the major problems affecting cocoa production.
"These diseases can be effectively managed by adopting suitable measures at the right time," say Dr. C. Kannan and Dr. R. Chandra Mohanan, Senior Scientists of CPCRI's Regional Station at Vittal.
In the diseases of young seedlings, infection by the fungus Phytophthora palmivora leads to the death of seedling either before or after emergence. This infection is commonly called seedling blight or seedling dieback.
Loss of leaves (defoliation) and drying up of the stems with brown water-soaked linear lesions from the tips are the characteristic symptoms of this disease.
Severe infection occurs in grafted and budded seedlings, which starts from the grafted or budded region and spreads to both ends, according to the scientists.
The disease can be effectively managed by providing optimum shade and improved drainage in the nursery and drenching the young plants with one per cent Bordeaux mixture or 0.1 per cent copper oxychloride just before the onset of monsoon. The same treatment should be repeated at regular intervals to get rid of the disease.
Zinc deficiency causes chlorosis (loss of chlorophyll) in patches and in advanced conditions, the green portion is found only along the sides of the veins, giving a vein-banding appearance to the leaves. Thus, the network of veins in each leaf is very distinct.
Disease symptoms
Affected leaves show symptoms such as mottling and crinkling with wavy margins and are often malformed. Younger leaves become narrow, small and sickle-shaped and twigs show signs of resetting and dieback.
To correct the zinc deficiency, the scientists recommend foliar spraying with fluid containing 3 g zinc sulphate and 1.5 g lime per litre of water.
Pod rots (also called black pod rot) are caused by the fungi Phytophthora palmivora and Colletotrichum gloeosporioides. The disease can be managed by regulating the shade by proper pruning and spacing between the trees. Other phytosanitary measures such as periodic removal of infected pods from the diseased trees will prove effective. Regular spraying of one per cent Bordeaux mixture from the onset of the monsoon is recommended to effectively manage the disease.
Control measures
Trunk and branch diseases include vascular streak dieback (VSD) caused by the fungus Oncobasidium theobromae and few other diseases caused by Phytophthora palmivora.
VSD infection mainly spreads through grafted materials and there is no evidence of transmission through seeds. Strict quarantine measures coupled with regular pruning of infected branches are recommended to keep this infection at bay.
Stem canker caused by Phytophthora palmivora can be controlled in the initial stages by the excision of diseased bark followed by swabbing with Bordeaux paste.
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