日本レース判別体系を用いて， 雲南省梗稲栽培地帯のレース分布を調査した。広範な地域， 1筆圃場から多数のレースが分離され， レース分布は日本よりも複雑であると考えられた。同地帯から採取したいもち病菌の病原性の変動を分離直後と一定期間後のレース検定結果を比較して検討した。検定結果が一致しない菌があった。これは判別品種関東51号， ツユアケおよびヤシロモチの反応が不安定なためであり， いもち病菌の病原性そのものが変動したためでないと考えられた。Blast disease is the most destructive disease for cultivated rice in Yunnan Province, China. In order to develop an effective breeding program for stable resistance to blast disease, it is important to analyse in more detail the pathogenic specialization of the blast fungus. This report deals with studies on the pathogenic specialization and changes in the virulence of the blast fungus in Yunnan Province. Rice samples, including leaves and panicles, infected with rice blast were collected in various districts of Yunnan. To differentiate blast races, Japanese differential varieties were used.
(1) Race distribution in the area cultivated with the Japonica rice varieties.
One hundred and sixty-four isolates were collected in various districts in 1988. The isolates tested were divided into 21 races. Among the 21 races, race 001 predominated, the relative frequency of which was 25.6%, followed by race 117 with a frequency of 11.6%, and races 003 (9.8%) ,007 (9.1%) ,013 (7.3%) ,005 (6.5%) ,017 (6.1%) ,011 (4.3%) ,031 (3.7%) and 037 (3.7%). With a view to identifying the composition of pathogenic races in a paddy field, a number of diseased panicles were collected from 8 fields in the area in 1988. Two races were isolated from 2 fields, 4 races from 2 fields, 5 races from 2 fields, 6 races and 7 races were isolated from one field, respectively. One race predominated in each field, although it was likely that multiple races coexisted in a field. Race composition in a field of Yunnan Academy of Agricultural Science varied from year to year.
(2) Change in virulence of some isolates in Yunnan Province
A survey was carried out to analyze the variation in virulence of the blast fungus. Thirty-five isolates collected from different locations in Yunnan Province were identified for races in 1984. In 1986, the race of each isolate was reidentified. Among the 35 isolates tested, 13 isolates showed the same reaction pattern to the diffentials in those two years, while the other 22 isolates showed different reactions. Out of the 22 isolates, 10 isolates were avirulent and three were virulent to Kanto 51, respectively. Four isolates were avirulent to Tsuyuake. Seven isolates were avirulent and one was virulent to Yashiromochi. It is worth noting that the changes in the virulence of the isolates under testing affected exclusively the following 3 rice varieties; Kanto 51, Tsuyuake and Yashiromochi. The isolates classified into such races as 013,017,031,033 or 037, which were virulent to Kanto 51 or Tsuyuake, were divided into 2 groups in relation to the formation of the susceptible type of lesions. One group formed a number of lesions on each of Shin 2, Kanto 51 or Tsuyuake, while the other group formedfewer lesions on Kanto 51 or Tsuyuake as compared with those on Shin 2. When a large quantity of spores avirulent to Yashiromochi was inoculated to spindly growing plants of Yashiromochi, susceptible-like type of lesions was formed on the rice plants. It is therefore assumed that the results of race identification may vary due to the differences in the number of spores inoculated and physiological status of rice plants, as well as in the environmental conditions after inoculation. These results indicates that different races may be associated with the unstable nature of the reaction of rice varieties such as Kanto 51, Tsuyuake and Yashiromochi.