CHARACTERISING THE PATHOGENICITY DIVERSITY OF USTILAGINOIDEA VIRENS IN HYBRID RICE IN CHINA

Fifty nine isolates of Ustilaginoidea virens, the cause of false smut of rice (Oryza sativa L.), were obtained from 46 rice hybrids in 14 counties in Sichuan, China during a survey conducted in 2006. Their pathogenicity was tested by inoculation on 3 rice hybrids, Gangyou182, Gangyou94-11 and Yixiangyou2292, 6-9 days before heading in 2007. False smut was assessed 3 weeks after inoculation using a disease index (DI) based on symptom frequency on the panicles. The sporulation capacity of these isolates was also measured. The results showed that: (i) DIs were significantly different (P<0.01) both among pathogen isolates and rice hybrids, ranging from 0 to 98.52; Q there was a significant interaction between isolates and hybrids (P<0.01); (60 significant differences in sporulation among the 59 isolates were found (P<0.01), but no relationship between sporulation and virulence on the 3 hybrids; (iv) variation in sporulation was observed among isolates originating from different counties and from the same county (P<0.05); (v) there were significant differences (P<0.01) between isolate groups from different host origin, female parents and male parents. Our results indicated a linkage between the pathogenicity of U. virens isolates and the resistance of rice hybrids. The 59 isolates could be classified into 6 groups based on their virulence to the tested rice hybrids. While variation in sporulation did not indicate host genotype-pathogen isolate interaction, pathogenicity data suggest specialisation, which is dependent on the site of origin of the isolates, the original host (rice hybrids) and the parentage of the original host.