Effects of various fungicides in vitro on Phytophthora palmivora from cocoa

Abstract

Of eight fungicides tested in vitro against a strain of Phytophthora palmivora pathogenic to cocoa, cuprous oxide, cycloheximide and mancozeb were highly toxic at low concentrations at most stages in the development of the fungus with ED 50 < 100 p.p.m. for growth of mycelium, <200 p.p.m. for formation of sporangia, <500 p.p.m. for germination of sporangia, and <50 p.p.m. for germination of zoospores. Captafol was very toxic to mycelia and zoospores, greatly decreased formation of sporangia at low concentrations but was not very active against sporangia. Metalaxyl greatly decreased mycelial growth at < 1 p.p.m., was highly toxic to zoospores and greatly decreased formation of sporangia at an ED 50 of approx. 87 p.p.m. But it markedly suppressed germination of sporangia only at 1000 p.p.m. and higher. Milfuram decreased growth of mycelia and production of sporangia at low concentrations at which it had little effect against zoospores and sporangia. Efosite-Al was active against mycelia and sporangia only at >1000 p.p.m.; it suppressed formation of sporangia at much lower concentrations with an ED 50 approx. 42 p.p.m. Propamocarb was poorly active against all stages of the fungus. Efosite-Al, metalaxyl, propamocarb and milfuram at low concentrations caused earlier encystment and increased the germination of zoospores.