Abstract
The inheritance of resistance to three different pathotypes, groups I, II, III, of Xanthomonas oryzae (Uyeda et Ishiyama) Dowson in three Japanese native varieties, Mimigura Mochi, Kurokara and Kuromochi, was studied. These three varieties were found to belong to Wase Aikoku group. To check up the testing conditions adopted in this experiment, we examined the relationship between disease index number of an individual plant at two weeks and that at three weeks after inoculation and also examined the relationship between disease index number of the two leaves of a plant inoculated with a bacterial isolate. Two fully developed leaves of an adult plant grown in the experiment paddy field were used for inoculation of each isolate. The isolates of X. oryzae used were T7174 for group I, T7147 for group II and T7133 for group III. Two weeks after the inoculation scoring was made according to the standard proposed by Ezuka and Horino and the scoring was made again at three weeks after the inoculation. The reaction was considered as S (susceptible) when the disease index number was over 2.0, while considered as R (resistant) when it was 2.0 and below. The resistance degree of tested plants was accurately enabled to evaluate on the basis of the mean value of the disease index number of two leaves per plant at two weeks after inoculation. The study of F1 and F2 populations of crosses between resistant varieties and the susceptible variety Toyonishiki revealed that resistance to those three pathotypes in Mimigura Mochi, Kurokara and Kuromochi was under completely dominant monogenic control. Evidence from allele tests with the already-known gene for resistance indicated that the resistance of Kurokara and Kuromochi was also controled by Xa-w or, if not so, another gene linked very closely with Xa-w.
