Biological management of phytophthora pod rot of cocoa
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Date
2004
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Department of Plant Pathology, College of Horticulture, Vellanikkara
Abstract
Phytophthora pod rot (PPR) is one of the serious diseases of cocoa in Kerala. Fifteen isolates of the pathogen were isolated from different locations of Thrissur district and their pathogenicity established.
The cultural and morphological characters of 15 isolates of the pathogen like growth on Carrot Agar, mycelial width, length of sporangiophore, sporangial ontogeny, length and breadth of sporangia, L/B ratio, and pedicel length were studied. Based on these characters, the different isolates were identified as Phytophthora palmivora (Butler) Butler.
To find out the variations among the isolates of P. palmivora the morphological, virulence and protein profile characteristics of 15 isolates were studied in addition to their sensitivity towards fungicides and antibiotics. Analysis of data on protein profiles and sensitivity towards fungicides did not show much variation among the isolates. But the isolates showed some variations with regard to their morphology, virulence and sensitivity to antibiotics.
Among 225 cocoa types screened for host resistance, eight cocoa types viz., G VI 14, G VI 73, G VI 77, G VI 124, G VI 138, G VI 154, G VI 279, G VI 284 showed moderately resistant reaction. Sixty three types were moderately susceptible and rest highly susceptible.
Quantitative estimation of epiphytic microflora from cocoa pods yielded more bacteria than fungi and actinomycetes. The epiphytic fungi and bacteria showed antagonistic activity against P. palmivora. Actinomycetes were not antagonistic to the pathogen. Eight fungal and seven bacterial antagonists including standard cultures of T. harzianum and P. fluorescens [Pf (K) and Pf (T)] exhibited maximum inhibition on the pathogen. Studies on the detached pods revealed the efficacy of epiphytic fungal isolate 20F and two bacterial isolates 23B and 24B against the pathogen. The efficient fungal isolate 20F was identified as T. viride Pers. ex. S.F. Gray and the bacterial isolates 23B and 24B as P. fluorescens Pelleroni.
The mechanism of antagonism of epiphytic T. viride and standard T. harzianum was studied. The antagonists overgrew the pathogen and showed interactions like coiling, penetration, hyphal malformations and disintegration of host hyphae. Three P. fluorescens isolates including Pf (K) produced HCN and siderophores.
T. viride (20F) and T. harzianum were compatible with Akomin 40 and Indofil M45. They were incompatible with Bordeaux mixture, Bavistin and Kocide. Fytolan was partially compatible. The two fungal antagonists were compatible with insecticide phorate, while Sevin was partially compatible. Ekalux, Nuvacron and Endosulfan were incompatible. Ammonium chloride, Rajphos and MOP were compatible with two Trichoderma spp. while ammonium sulphate was incompatible.
Two native P. fluorescens (23B and 24B) and standard Pf (K) were compatible with Akomin 40, Indofil M45 and Bavistin and incompatible with Bordeaux mixture, Kocide and Fytolan. With regard to insecticides, the lower two concentrations of Sevin, Ekalux, Endosulfan and Nuvacron were compatible with the bacterial antagonists, while higher concentration was incompatible. Phorate was partially compatible. Fertilizers like Rajphos and MOP were compatible with antagonists. Urea was incompatible. Ammonium sulphate and ammonium chloride were partially compatible.
Among different fungicides screened, Bordeaux mixture, Fytolan and Kocide completely inhibited P. palmivora. The lower two concentrations of Akomin 40 and Indofil M45 at all concentrations permitted slight growth. Bavistin was not inhibitory to the fungus. Nuvacron at all concentrations, the highest concentration of Ekalux and two higher concentrations of Endosulfan were inhibitory to the pathogen. Sevin and phorate exerted varying levels of inhibition. Urea and MOP were not much inhibitory to the pathogen. Ammonium chloride, ammonium sulphate and Rajphos showed partial inhibition.
Field evaluation of isolates of P. fluorescens [23B, 24B, Pf (K)] and two Trichoderma spp. revealed the efficacy of fluorescent pseudomonads in reducing the incidence of PPR of cocoa. The efficacy was comparable with the recommended fungicides.
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172267