Studies on the vicissitude of Pellicularia filamentosa in soil. Part II

Abstract

From the results of the previous experiment on the crown rot of sugar beet in Hokkaido, it was pointed out that Rhizoctonia solani in the soil of sugar beet field becomes active in two different seasons of a year, i.e., usually May to June and July to August. The reason of the fluctuation of the fungus activity in soil was considered by the senior author to be due to alternation of different strains of R. solani in the soil depending on the advance of season.In the present experiment, the range of Rhizoctonia strains in a particular soil of flax field, and their comparative characteristics were studied.Isolations of the causal Rhizoctonia were made from May to August in the flax growing plots of the University Farm in Sapporo. More than 150 isolates were obtained from affected flax tissues and also about 100 cultures were isolated from the soil by means of the flax trap technique. All of them could be divided according to their cultural characters into two apparent groups: one called “spring strain” was isolated from the soil and damped-off seedlings in May and June; the other, “summer strain”, was obtained in July and August from the soil and lesions on the stems of flax at the post-flowering stage.Comparative studies were carried out with several isolates each of the two strains. The effect of temperature on the growth and pathogenicity of both strains is quite different: the temperature which induces the fastest mycelial growth of the spring strain on PDA (Table 3) or through soil-tubes (Fig. 1) is generally lower than that in the summer strain. The spring strain causes severe damping- off at low temperature such as 13°and 18°C (Figs. 2, 3).The results of pathogenicity trials on the flax of different ages (Tables 4, 5, 6) proved that the pathogenicity of strains is specialized by the age of the host plant: isolates of the spring strain are pathogenic only to young seedlings and can not attack mature plants, while those of the summer strain are slightly pathogenic on seedlings, but readily invade mature stems of flax in the post-flowering stage.The results of isolation experiments from the soil which was artificially inoculated with both strains showed that all of the isolates caught by the flax trap from the soil incubated at low temperature (5°to 18°C: average 13°C) and of those isolated from infected host plants which were grown under the same condition, were found to belong to the spring strain; when the soil was kept at higher temperature (12°to 28°C: average 20°C) both strains were obtained from the soil and host plants (Tables 7, 8).The cultural characters of all isolates of each strain are quite uniform, but there are remarkable differences between the two strains. The spring strain is identified as Pellicularia filamentosa from its morphological character as well as from the perfect stage formed on the flax stem and the surface of soil particles in the experimental plots during the isolation experiments in May and June. Because of the lack of perfect stage and of a few differences in its vegetative characters from the original description, the summer strain cannot be clearly identified as Pellicularia praticola. On this account the authors propose the name Rhizoctonia solani, the classic complex species, to cover the both strains.