Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv oryzae (Xoo) is one of the major constraints to productivity in South-East Asia. The strategy of using major genes, singly or in combination, continues to be the most effective approach for BB management. Currently, more than two dozen genes have been designated but not all the known genes are effective against all the prevalent pathotypes. The challenge, therefore, is to continue to expand the gene pool of effective and potentially durable resistance genes. Wild species constitute an important reservoir of the resistance genes including BB. An accession of Oryza nivara (IRGC 81825) was found to be resistant to all the seven Xoo pathotypes prevalent in northern states of India. Inheritance and mapping of resistance in O. nivara was studied by using F2, BC2F2, BC3F1 and BC3F2 progenies of the cross involving Oryza sativa cv PR114 and the O. nivara acc. 81825 using the most virulent Xoo pathotype. Genetic analysis of the segregating progenies revealed that the BB resistance in O. nivara was conditioned by a single dominant gene. Bulked segregant analysis (BSA) of F2 population using 191 polymorphic SSR markers identified a approximately 35 centiMorgans (cM) chromosomal region on 4L, bracketed by RM317 and RM562, to be associated with BB resistance. Screening of BC3F1 and BC2F2 progenies and their genotyping with more than 30 polymorphic SSR markers in the region, covering Bacterial artificial chromosome (BAC) clone OSJNBb0085C12, led to mapping of the resistance gene between the STS markers based on annotated genes LOC_Os04g53060 and LOC_Os04g53120, which is approximately 38.4 kb. Since none of the known Xa genes, which are mapped on chromosome 4L, are effective against the Xoo pathotypes tested, the BB resistance gene identified and transferred from O. nivara is novel and is tentatively designated as Xa30(t). Homozygous resistant BC3F3 progenies with smallest introgression region have been identified.

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Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo) is a major biotic constraint in the intensive irrigated rice belt comprising Punjab and adjoining north-western states of India. Development and deployment of host resistance is the only effective means of BB management. The pathogen is highly variable, and the current Xoo population from the state could be classified into seven distinct pathotypes (PbXo-1 to PbXo-7) by inducing differential reactions on a set of near-isoganic lines in the background of IR24 and some international, national and regional cultivars. Known BB resistance genes (Xa1, Xa3, Xa10, Xa11, Xa14, Xa18) were ineffective, whereas xa13, Xa4 + xa13, xa5 + xa13, xa13 + Xa21, Xa4 + xa5 + xa13, Xa4 + xa5 + Xa21, Xa4 + xa13 + Xa21, xa5 + xa13 + Xa21 and Xa4 + xa5 + xa13 + Xa21 and rice line IET8585 ⁄ Ajaya were effective against all the seven pathotypes analysed. Xa21 was effective against all the pathotypes except PbXo-3 and PbXo-4. PbXo-7, the most dominant pathotype, was found to be virulent and induced susceptible ⁄ moderately susceptible reaction on 22 of the 40 test genotypes followed by PbXo-1, PbXo-5 and PbXo-6; PbXo-2 was the least virulent pathotype. Molecular profiling of these pathotypes using random amplified polymorphic DNA (RAPD) and IS1112-based polymerase chain reaction (PCR) generated specific and reproducible fingerprint patterns. Primers S1117, S112, S109, S1106 and JEL are more informative in distinguishing pathotypes. At a similarity of 0.50, pathotypes PbXo-1 and PbXo-2 were grouped together, whereas other five pathotypes showed separate lineage. The data using RAPD-PCR and IS1112-based PCR approaches revealed their potential in generating unique DNA fragments specific for different pathotypes that may lead to the rapid assessment of genetic variation in the pathogen population. Pyramiding of two ⁄ more partially effective known Xa genes and ⁄ or search for new disease resistance genes effective against the wider Xoo population appears to be the most appropriate approach for BB management in the near future.

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Resistance of rice (Oryza sativa) to sheath blight, caused by Rhizoctonia solani, is quantitative and involves two mechanisms: physiological resistance and disease escape. The epidemiological concept of components of resistance was applied using a detached tiller method under controlled conditions, to specifically address physiological resistance to sheath blight in rice. A sclerotium was inserted below the leaf collar of individual rice tillers maintained in tubes filled with water. Different variables were measured after incubation: number of lesions, lesion length, vertical sheath colonization, presence or absence of dark margin at the edge of lesions, and survival duration of the leaf blade. Several rice varieties reported to have different levels of susceptibility to sheath blight were assessed, together with varieties that are cultivated over large areas. Although numerical differences between rice varieties were observed for all disease variables, only the number of lesions significantly differed among varieties tested in this study. The varieties Pecos and IR64 had the consistently lowest and highest disease intensities, respectively. This methodology may allow the detection of sources of resistance that specifically involve defense mechanisms. When combined with field assessment, this methodology should also enable to quantitatively assess the relative role of both mechanisms of resistance to sheath blight.