Evaluation of resistance in rice plants to Myanmar isolates of Xanthomonas oryzae pv. oryzae

To obtain genetic resources of resistance to bacterial leaf blight of rice caused by Xanthomonas oryzae pv. oryzae in Myanmar, 40 Myanmar rice varieties, 10 international differentials of near isogenic lines and 17 varieties of pyramided lines were tested for the resistance to three different pathotypes isolated in Myanmar. The bacterial isolate used were MKM13 (belonging to predominant race in Myanmar and showing wide range of pathogenicity), M 3-1 (showing intermediate range of pathogenicity) and MKM39 (belonging to rare race in Myanmar and showing narrow range of pathogenicity). The fully developed leaves at maximum tillering stage were used for inoculation of each isolate. Three weeks after the inoculation scoring was made according to the lesion length. The reaction was considered as S (susceptible) when the lesion length was more than 5 cm, while considered as R (resistant) when it was 5 cm below. In the inoculation test using international differentials of near isogenic lines, the resistance genes, Xa21 and Xa3, would be effective resistant resources to the bacterial blight diseases in Myanmar. Although bacterial isolate MKM39 was avirulent to IR24, some Myanmar varieties were susceptible to this isolate. Rice varieties cultivated in Myanmar were classified into four groups based on their reactions to three Myanmar bacterial isolates. Group I contained one variety was resistant to all the three isolates of pv. oryzae and group II contained 14 varieties was susceptible. Group III contained 23 varieties resistant to bacterial isolate MKM39 but susceptible to MKM13 and M3-1. Group IV contained two varieties was susceptible to bacterial isolate MKM13 but resistant to M3-1 and MKM39. Furthermore, gene combinations Xa3+Xa7, Xa3+ Xa10, Xa4+xa5 and Xa4+xa5+Xa13+Xa21 conferred a broad spectrum of resistance to all three Myanmar isolates evaluated, supporting the strategy of pyramiding appropriate resistance genes.