Distribution and ecology of Pythium species in Japan.
The distribution of P. spp. is discussed on the basis of data obtained by assaying 80 plant root samples and 263 soil samples collected in various districts during 1968-84. P. cultures were obtained from plant roots by placing washed tissue pieces on plain water agar plates and single hyphal tips from the fungal growth were removed and subcultured. Either a modification of Waksman's direct inoculation method or trapping with cucumber seed and other substrates was used to isolate P. from soil. At least 2 temp. levels were used (<10° and >20°C) and 1-7 d incubation were allowed. The dominant spp. associated with plant roots were P. sylvaticum, P. spinosum, P. aphanidermatum and P. ultimum, in descending order. These spp. were each isolated from >10 samples. P. spp. were ubiquitous in Japanese soils, being detected in 247 out of 263 samples. The 4 most dominant spp. were the same as those from plant roots. A 5th group, which formed only hypha-like sporangia and zoospores and had no sexual organs, was tentatively named H-Zs (hyphae-zoospores). This was followed by P. irregulare, P. vexans, P. splendens and P. torulosum, in frequency of isolation. The dominant sp. varied with districts; P. aphanidermatum was more frequent in those with a high annual av. air temp. Based on total soil samples the av. yield was 2.6 spp./sample while the av. was 2-3.4 spp./sample, depending on district. In assaying P. population by a modification of Waksman's direct inoculation method 12, 17 and 19 out of 48 soil samples from Tohoku and the Ryukyu Islands yielded a total number of propagules >201, in the range 51-200 and <50/g soil, respectively.