Genetic analysis of selected PSB rice varieties and evaluation of elite breeding lines for resistance to bacterial blight (Xanthomonas oryzae pv oryzae ex Ishiyama)

Bacterial Blight (BB) caused by Xanthomonas oryzae pv oryzae ex Ishiyama is a serious disease and one of the major constraints in rice production in many Asian countries. Effective chemical-based control measures are not available against BB and the use of resistant varieties remains the most important ecological control method. More than 25 genes are available for resistance (R) to BB in rice including the Xa-21, Xa-7, Xa-4, xa-8, xa-5, Xa-13, Xa-10 and Om gene. It was in this light that this study aimed to analyze BB resistance observed in some PSB rice varieties possibly caused by new resistance genes. it also aimed to evaluate backcross progenies for any presence of the Xa-21 and xa-5 R genes by using DNA markers and by analyzing their reaction to Xoo race 6, the most virulent strain. Seven BB isolates were used in the inoculation using a clipping method for introducing the bacteria into the plant leaves, Six modern varieties were subjected to genetic analysis for resistance to BB. These included the PSB Rc2, PSB Rc34, PSB Rc56, PSB Rc64, PSB Rc80 and PSB Rc82 varieties. The PSB Rc2 variety was found to contain the Xa-4 gene; PSB Rc34 had 4 genes effective against races 1, 6 and 9; PSB Rc56 had Xa-4, Xa-14 and an unknown gene found to be very effective for race 8, PSB Rc64 had Xa-4, Xa-14, Xa-7 and a putative new gene that was effective against race 6; PSB Rc80 contained Xa-4 gene found effective against races 1 and 7; and PSB Rc82 had at least nine different genes that made it resistant to Xoo races 1, 6, 7, 8 and 9 based on the analysis of two populations (PSB Rc82 X IR24 and PSB Rc82 X IRBB5). Out of the 56 backcross-derived lines from General Yield Trial and Preliminary Yield Trial tested, only 23 showed consistent resistant reaction to Xoo race 6 at 21 days after inoculation (DAI). A DNA marker analysis using the Xa-21 and RG556a primers, revealed that 13 lines had both the Xa-21 and xa-5 genes, respectively. Twelve lines had Xa-21 gene while five lines had xa-5 alone. Knowledge on the properties of bB resistance genes, their effectiveness against specific races and the R genes present in certain commercial varieties are useful in the deployment of suitable varieties for locations where specific Xoo races are prevalent.