SUMMARY
The relationships between the orchid Spiranthes sinensis (Persoon) Ames. var. amoena (M. Bieberstein) Hara and Rhizoctonia spp. were investigated in situ at germination and in adult plants, Seeds of the orchid placed in cotton gauze were buried at 210 sampling points in turf grassland, the orchid habitat (in situ germination). Eight weeks later, protocorm development of the orchid was confirmed at 67 of the 210 sampling points. Isolation of fungi from protocorms showed that in situ germination was induced mainly by Rhizoctonia rapens Bernard. Similarly, R. repens was the dominant mycorrhizal fungus isolated from roots of adult plants. The number of adult plants within a radius of either 30 or 50 cm of burial points did not influence seed germination.
The distribution of Rhizoctonia spp. other than R. repens in the sample site was examined with a baiting method using buckwheat stems. Thirty‐two isolates consisting of binucleate Rhizoctonia anastomosis group (AG)‐A, AG‐B, AG‐G, and AG‐1, R. solani Kuhn AG‐4, Waitea circinata Warcup & Talbot, which anastomozed with WAG‐O and WAG‐Z, and a multinucteate Rhizoctonia sp. were isolated. Three AG‐G isolates were obtained from the points at which protocorm development was induced by R. repens. Seeds of S. sinensis var. amoena were inoculated in vitro with these isolates to test for symbiotic germination. Most Rhizoctonia spp, not associated with the germination in situ induced seed germination in vitro. Seedlings which developed with these isolates in vitro were transferred to ex vitro conditions. New leaves developed and elongated as seedlings continued to grow for 3 months, The seed burial method enabled the clarification of the differences in orchid‐fungal specificity in situ and in vitro. We concluded that the specificity between S. sinensis var. amoena and fungi in situ conditions was different to that in vitro.
Variations in the prevalence of races 56 and 15B-1 (Can.) of wheat stem rust (Puccinia graminis Pers. f. sp. tritici Erikss. and Henn.) have occurred that cannot be explained by changes in the wheat varieties grown in Western Canada. The reasons for the changes in prevalence were investigated by comparing in growth cabinets the reproductive potentials or aggressiveness of the two races and the factors that might influence them. When a mixture of urediospores of the two races was used to inoculate three susceptible varieties for successive uredial generations, race 56 predominated at 25 and 20 °C, and race 15B-1 (Can.) predominated at 15 °C. Experimentation showed that the urediospores of race 56 were more infective than those of 15B-1 (Can.) and that the incubation period of race 56 was shorter than that of race 15B-1 (Can.). Pustules of race 56 grew faster than those of race 15B-1 (Can.) during early stages of development but those of 15B-1 (Can.) ultimately became larger than those of race 56. Race 56 produced more urediospores per pustule than race 15B-1 (Can.) on the susceptible varieties Little Club, Red Bobs, and Marquis at 15 and 20 °C. Infection density markedly affected competitive abilities. Race 56 tended to predominate in light infections but race 15B-1 (Can.) tended to predominate in heavy infections. Pustule size decreased as infection density increased. The possible significance of these findings under field conditions is discussed. It seems likely that the rise of race 15B-1 (Can.) to predominance in 1950 resulted not only from the selection pressure of the widely grown varieties that were resistant to race 56 and susceptible to 15B-1 (Can.) but from the lower temperatures that prevailed and favored 15B-1 (Can.) over 56. The rise to predominance of race 56 from 1956 to 1961 may have resulted from the greater aggressiveness of that race under the moderate or warm conditions that prevailed.
A total of 46 single uredinial isolates of Phakopsora pachyrhizi were collected from rust-infected soybean and wild host plants (Pueraria lobata and G. soja) at different localities in central and southwestern Japan. Eighteen pathogenic races were identified using a set of differential varieties composed of nine cultivars of soybean and two accession lines of G. soja. Nine and 11 races were found on soybean and wild host plants, respectively. Two races were common to soybean and wild host plants.
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