Study of pathogenicity and genetic diversity of Magnaporthe oryzae isolated from rice hybrid Wuyou 308 and detection of resistance genes

To understand the cause of loss of rice blast resistance, we studied the pathogenicity of Magnaporthe oryzae strains isolated from rice hybrid Wuyou 308 and evaluated its resistance genes. A total of 62 M. oryzae strains were isolated and tested in 7 Chinese rice varieties with varying degrees of resistance to rice blast and 30 blast-resistant monogenic lines. Fourteen physiological races of M. oryzae were identified: 8.55% belonging to the ZA group, 86.67% to the ZB group, and 5.00% to the ZC group. ZB15 was the most abundant race (45.00%). Five resistance genes, Pi-3(1), Pi-z5, Pi-k, Pi-kp(C), and Pi-k(C), conferred good resistance to the 62 strains, with resistance frequencies of 95.56, 91.11, 88.89, 82.22, and 82.22%, respectively. In contrast, Pi-a(2) had a resistance frequency of 0%. The hybrid combination Wuyou 308 was found to carry Pi-ta and Pi-b genes. Because Pi-ta and Pi-b both showed low resistance frequencies to M. oryzae isolated from Jiangxi, the hybrid rice variety Wuyou 308 could be infected by most of the 62 M. oryzae strains. The emergence and spread of rice blast disease in Wuyou 308 may thus be difficult to avoid when climatic conditions are favourable.