Etiology and control of seedling blight of cocoa

Abstract

The seedling blight is a serious nursery disease of cocoa. The fungus causing seedling blight of cocoa was isolated and Koch’s postulates were established. On carrot agar medium, the fungus produced sparse, striate growth. The mature sporangia were near spherical to ovoid with round base, papillate, caduceus with an average L/B ratio of 1.74. Sporangia were borne terminally in a sympodial fashion. It had a short and thick stalk with an average length of 4.1µm. Based on these characters, the pathogen causing seedling blight of cocoa was identified as Phytophthora palmivora (Butler) Butler. For maximum growth of the fungus, oat meal and corn agar were the best. But, for maximum sporangial production, carrot agar and oat meal agar were good. Among the liquid media, oat meal and corn meal broth supported good growth of the fungus. The disease produced various type of symptoms on leaves and stem of seedlings and budded plants like water soaking, leaf blight, defoliation, black discolouration, cotyledonary infection, wilting and die back. Stem of budded plants were more prone to infection than that of seedlings. The seedlings blight pathogen infected plants such as Piper nigrum L., Cocos nucifera L., Hevea brasiliensis L., Bougainvillea sp. And Colocasia esculenta L. but did not infect Areca catechu L., Piper betle L. and Piper longum L. on artificial inoculation. The youngest seedlings of age group less than 18 days after germination were more vulnerable to infection. Similarly, the youngest budded plants of age group 45 days after budding showed maximum infection. Among the 62 cocoa types screened for host resistance the cocoa types GIV-36.6 (local), GVI-23 (P9 x P4), GIV-4.6 (local) and S-45.5 (local) showed minimum percentage of mortality. Among the different fungicides/antibiotics screened in in vitro, Fytolan, Captaf, Bordeaux mixture, Akomin and Ridomil at all concentrations and 0.3 per cent Foltaf and, Chloramphenicol and Terramycin each at 400 and 500 ppm completely inhibited the growth of the fungus. Indofil-M.45, Aureofungin and Amoxycillin exhibited less inhibitory effect. Foltaf, Fytolan and Akomin each at 0.3 per cent concentration showed better effect in cheching the lesion development on detached cocoa leaves. Result of the different in vivo control experiment indicated that, the fungicides like Akomin, Foltaf, Fytolan and Bordeaux mixture had promising effect in checking the incidence and severity of the disease.