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
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
report of non-streptomycete
actinomycetes to control a Pythium disease.