The potential for the biological control of cavity‐spot disease of carrots, caused byPythium coloratum, by streptomycete and non‐streptomycete actinomycetes

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Abstract
Summary: Actinomycetes isolated from carrot rhizosphere were screened for theirin vitroandin vivoantagonism toPythium colorationVaartaja, a causal agent of cavity‐spot disease of carrots (Daucus carataL.). Forty‐five streptomycete and non‐streptomycete actinomycete isolates were screened forin vitroantagonism in a carrot bioassay. Of these, seven which reduced or prevented lesion formation were identified using cultural, morphological, physiological, biochemical and cell wall characteristics asStreptomyces janthimts, S. cinerochromogenes, Streptoverticilium netropsis, Actinomadura rubra, Actinoplanes philippinensis, Muromonospora carbonaceae, andStreptosporangium albidum.All seven isolates tested produced non‐volatile antifungal metabolites, but failed to produce inhibitory volatile compounds.Actinoplanef philippincnsisand M.carbonaceagrew epiphytically on the hyphae and oospores ofP. eoloratum.The external surface of the oospores of the pathogen was heavily colonized by both hyperparasites, their hyphae were found to coil tightly around the oospore wall, and frequently caused cytoplasmic collapse of oospores. Sporangia ofA. philippinensiswere often seen to emerge from the colonized hyphae and oospores ofP. eoloratum.None of the other actinomycete isolates showed hyperparasitism.All seven isolates significantly reduced the incidence of cavity spot in soil artificially infested with the pathogen in the glasshouse.Streptomyces janlhinusandStfepto. albidumwere the most effective in reducing the disease in inoculated plants. In addition, all the actinomycetes species exceptAc. rubraandM. carbonacea, in the presence or absence of the pathogen, significantly (P< 0.05) increased mean fresh root weight compared to the treatment which includedP. thoriumonly. This study shows that these actinomycetes have considerable potential for future use as biocontrol agents of cavity spot under natural field conditions. This is the first report of cavity‐spot disease of carrots being controlled by microbial antagonists, and is the first report of non‐streptomycete actinomycetes to control aPythiumdisease.