Study On The Mating Type, Chinese Pathotypes And Population Genetic Diversity Of The Rice Blast Fungus (Magnaporthe Grisea) From Blast Nurseries In Hunan Province, China

The rice blast disease caused by Magnaporthe grisea Barr is one of the most destructive rice diseases worldwide. Most study on genetic diversity of the blast was always based on large-scale collection but few on micro-geographic area where the sexual reproduction can take place. In Hunan province the blast nurseries where diversified rice cultivars are grown every year under favorable environmental condition to the blast pathogen are not explored for genetic diversity of blast fungus. This kind of research will be helpful to understand rice blast diversity in typical small area like blast nursery and provide basically information for improving resistance breedingThis study was aimed at investigating mating type, pathotype and population genetic diversity of the blast fungus M. grisea collected from 3 blast nurseries in Hunan province in September 2001.Sixty-two blast isolates were harvested in Anhua, Taojiang and Songbai, and monoconidial isolation was performed. The distribution of the mating type and female fertility of isolates was investigated. Among 55 isolates collected from Anhua, 22% was Mat 1.1 (11 male and one female hermaphrodite) and 11% Mat 1.2 (4 male and 2 hermaphrodites). In Taojiang, where 3 isolates were sampled, 2 isolates were Mat 1.1 with one female hermaphrodite while one was unknown. Four fertile isolates were sampled in Songbai among which three were Mat 1.1 and one was a female hermaphrodite Mat 1.2. These results showed the coexistence of both isolates from Mat 1.1 and Mat 1.2 in these areas but all the isolates were inter-sterile since the crossing of isolates from opposite mating type failed to produce any perithecium in laboratory. The occurrence of the perfect stage of M. grisea in the field was discussed.The pathotype diversity of the isolates was examined based on their pathogenic reaction to the seven Chinese differential rice cultivars. Results showed that the isolates were distributed into twenty Chinese physiological races (pathotypes). These pathotypes belonged to ZB (46.15%), ZA (30.76%), ZC (13.46%), ZG (7.69%) and ZH (1.92%) groups. Isolates from the same site showed different pathogenic reactions to the seven Chinese differentials cultivars. The races of the groups ZA and ZB were dominant and the race ZB 15 was predominant among the 20 races obtained.The population genetic diversity of 44 isolates was studied with 13 random amplified polymorphic DNA (RAPD) primers. Forty-four isolates were distributed into 6 genetic lineages including four main lineages of 8 to 11 isolates and two lineages of one and 5 isolates. The 20 Chinese pathotypeswere also diversified among lineages, as no genetic lineage clustered only a specified group of Chinese pathotype except the lineage 3 that comprised only one isolate. The isolates with Mat 1.1 and Mat 1.2 belonging to different Chinese pathotypes were grouped in the same lineage. This research showed that the population genetic diversity of the blast in blast nurseries in Hunan province was complicated.To study the genetic diversity of the fertile isolates of Magnaporthe grisea 10 pairs of primers were used for Sequence Characterized Amplified Regions (SCARs) analysis. The 24 isolates (including 2 control isolates) were clustered into 6 genetic lineages among which lineage II and HE clustered isolates from the same mating type while other lineages clustered isolates from both Mat 1.1 and Mat 1.2, on the other hand, lineage II and VI clustered isolates from the same collection site. These results further confirmed the population genetic diversity of blast fungus by RAPD. In conclusion, it is suggested that in the restricted areas like the blast nurseries in Hunan province, China the great genetic diversity of blast fungus could be presented as reported in the macro-geographic areas. However, more investigation needs to be done in the rice field to prove the presence of perithecia under natural condition.