Actinomycetes isolated from carrot rhizosphere were screened for their in vitro and in vivo antagonism to Pythium coloratum Vaartaja, a causal agent of cavity-spot disease of carrots (Daucus carota L.). Forty-five streptomycete and non-streptomycete actinomycete isolates were screened for in vitro antagonism in a carrot bioassay. Of these, seven which reduced or prevented lesion formation were identified using cultural, morphological, physiological, biochemical and cell wall characteristics as Streptomyces janthinus, S. cinerochromogenes, Streptoverticillium netropsis, Actinomadura rubra, Actinoplanes philippinensis, Micromonospora carbonaceae, and Streptosporangium albidum. All seven isolates tested produced non-volatile antifungal metabolites, but failed to produce inhibitory volatile compounds. Actinoplanes philippinensis and M. carbonacea grew epiphytically on the hyphae and oospores of P. coloratum. 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 of A. philippinensis were often seen to emerge from the colonized hyphae and oospores of P. coloratum. 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 janthinus and Strepto. albidum were the most effective in reducing the disease in inoculated plants. In addition, all the actinomycetes species except Ac. rubra and M. carbonacea, in the presence or absence of the pathogen, significantly (P<0·05) increased mean fresh root weight compared to the treatment which included P. coloratum only. 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 a Pythium disease.