Comparative Toxicity of Copper – Based Fungicides against Phytophthora megakarya; a Causal Agent of Black Pod Disease of Cocoa

Abstract

Aim: The study determines the estimated toxicity and efficacy of copper-based fungicides use to control Phytophthora megakarya. Study Design: Toxicity of fungicides against Phytophthora megakarya was determined in-vitro using mycelial growth inhibition. Methodology: Isolates of P. megakarya were collected from cocoa experimental plots in Cocoa Research Institute of Nigeria, Ibadan, South Western Nigeria. Three active ingredients: Cuprous oxide, Copper hydroxide and Copper hydroxide + metalaxyl in fungicides retailed in open market were assayed in-vitro at 0.5 µg/ml, 1.0 µg/ml, 1.5 µg/ml, 2.0 µg/ml and 2.5 µg/ml of active ingredient against mycelial growth of P. megakarya; pathogen of black pod disease. The antifungal index and effective concentration at which mycelial growth was inhibited by 50% (EC50 value) was calculated for pathogen/fungicide combination and probit analysis. Results: Toxicity of fungicides against P. megakarya using the EC50 showed significant variation as estimated from the mycelial growth inhibition. Highest mycelial inhibition (85.25%) of P. megakarya was recorded at 2.5 µg/ml of Copper hydroxide + metalaxyl with estimated EC50 value of 0.18 µg/ml and highest toxicity was recorded in Copper hydroxide + metalaxyl  while Cuprous oxide was least toxic against P. megakarya. The toxicity responses of these Copper-based fungicides against P. megakarya vary with active ingredients and Copper hydroxide + metalaxyl gave the highest fungitoxic effect.