1College of Agriculture, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan; 2Kosugi Country Club, Kosugi, Toyama 939-0321, Japan; 3College of Technology, Toyama Prefectural University, Kosugi, Toyama 939-0398, Japan

Background and objectives
Pythium spp. are major causal agents in the crown and root rot of bentgrass green in golf courses. Although the pathogenicity of Pythium spp. on bentgrass has been studied extensively, no information is available concerning its behavior during nonsymptomatic periods. This study reports on the seasonal population changes of Pythium spp. in bentgrass greens, primarily to clarify the factors affecting development of disease caused by Pythiumspp. in bentgrass greens.

Material and methods
Soil and root samples were obtained from the bentgrass greens of two golf courses, the Kosugi Country Club and the Taikoyama Country Club (KCC and TCC, respectively), which neighbour each other and are located in Toyama, Japan. Fungicides which are effective at eliminating Pythiumspp. are little used in both golf courses. Isolations were made at 1-month intervals from April to December 1997 in each of two greens of the golf courses. Densities of Pythium spp. in the top 3 cm of the soil was investigated by the soil dilution plate method using a Pythium-selective medium at 25C. Pythium spp. were also isolated from the crowns and roots of the bentgrass in the same sample on the medium. All isolates obtained were incubated by the grass-blade culture method modified as described previously [1] and identified according to the keys of van der Plaats-Niterink [2]. The pathogenicity of Pythiumspp. was analysed by in vitro inoculation tests. The tests were conducted on seedlings of creeping bentgrass grown on a plant culture medium in a test tube and incubated with continuous light at 28C for 3  weeks. Disease rate and plant dry weight were assessed after 3  weeks of incubation.

Results and conclusions
The predominant Pythium spp. recovered from the soil and roots of the bentgrass greens of the two golf courses were P.  torulosum, P. rostratum, P. vanterpoolii; P. torulosum was the species found most frequently throughout the experimental period. P. vanterpoolii dominated among the Pythium spp. isolated from the crowns and roots in the late summer. In a pathogenicity test, P. vanterpoolii caused pre-emergence damping-off of the bentgrass. Pythium spp. reduced the seedling growth. No or low pathogenicity was observed in P. torulosum and P. rostratum. Fluctuation patterns of the total population of Pythium spp. in bentgrass green differed between the two golf courses. The population in KCC was stable at ranges from 83 to 285 propagules/g soil throughout the period. While the population in TCC fluctuated at ranges from 102 to 1964 propagules/g soil and increased especially in May and August. Yellow spot, which is one symptom of the summer decline complex of bentgrass green involving Pythium spp. appeared in TCC in late August to September.

These results indicate that Pythium spp. which have low to high pathogenicity to bentgrass fluctuated in the nonsymptomatic bentgrass greens. The different phenology patterns of Pythiumspp. between the two neighbouring golf courses may relate to the differences in soil construction and cultural management practices.

1. Tojo M, Nakayama K, Sasaki T, Ohki ST, Osaki T, 1997. In: Ogoshi A, Kobayashi K, Homma Y, Kodama F, Kondo N, Akino S, eds, Plant Growth-Promoting Rhizobacteria - Present Status and Future Prospects. Sapporo, Japan: The 4th PGPR International Workshop Organizing Committee, pp. 203-204.
2. van der Plaats-Niterink AJ, 1981. Studies in Mycology 21, 1-242.