T. W. Mew

Focus on bacterial blight of rice.

Bacterial blight of rice, caused by Xanthomonas orpw. aryzap (37), was first reported in Japan over a century ago. Since hen, considerable research on the disease and its causal agent has been reported and reviewed ( 1 8,19,24,29). In the past 10 years, advances in the area of molecular genetics, monoclonal antibody technology, and microbial taxonomy have been made. Now these technologies are being used to characterize the pathogen, reevaluate the ecology and epidemiology of the disease, and study host-parasite interactions. In this article, we will focus on the use and the potentials of these technologies to clarify the taxonomy of the pathogen, describe the pathogen population structure in single plants or fields, and investigate the molecular events occurring in the hostparasite interactions.

Changes in race frequency of Xanthomonas oryzae pv. oryzae in response to rice cultivars planted in the Philippines

In an extensive survey of rice-growing areas in the Philippines from 1972 to 1988, 960 strains of Xanthomonas oryzae pv. oryzae (the bacterial blight pathogen) were collected. These strains were separated into six races based on their interactions with a distinct set of five differential cultivars carrying different resistance genes. Race 1, which was predominant before improved modern rice cultivars highly responsive to nitrogen fertilizer and management become widely cultivated, was virulent to rice with Xa-10, Xa-11, and Xa-14 genes (.)

A benefit of high temperature: increased effectiveness of a rice bacterial blight disease resistance gene.

*Continuous planting of crops containing single disease resistance (R) genes imposes a strong selection for virulence in pathogen populations, often rendering the R gene ineffective. Increasing environmental temperatures may complicate R-gene-mediated disease control because high temperatures often promote disease development and reduce R gene effectiveness. Here, performance of one rice bacterial blight disease R gene was assessed in field and growth chamber studies to determine the influence of temperature on R gene effectiveness and durability. *Disease severity and virulence of Xanthomonas oryzae pv. oryzae (Xoo) populations were monitored in field plots planted to rice with and without the bacterial blight R gene Xa7 over 11 yr. The performance of Xa7 was determined in high- and low-temperature regimes in growth chambers. *Rice with Xa7 exhibited less disease than lines without Xa7 over 11 yr, even though virulence of Xoo field populations increased. Xa7 restricted disease more effectively at high than at low temperatures. Other R genes were less effective at high temperatures. *We propose that Xa7 restricts disease and Xoo population size more efficiently in high temperature cropping seasons compared with cool seasons creating fluctuating selection, thereby positively impacting durability of Xa7.

Genotypic and Pathotypic Diversity in Xanthomonas oryzae pv. oryzae in Nepal

ABSTRACT Among the 171 strains of Xanthomonas oryzae pv. oryzae (the bacterial blight pathogen of rice) collected from eight rice-producing zones in Nepal, 31 molecular haplotypes were distinguished using two polymerase chain reaction-based assays. Six common haplotypes represented nearly 63% of the strains, and some haplotypes were geographically dispersed. Multiple correspondence analysis divided the collection into five putative genetic lineages. Lineages 1, 2, and 4 were the most frequently detected and occurred in diverse geographic populations. Twenty-six pathotypes (virulence phenotypes) of X. oryzae pv. oryzae were identified using 11 near-iso-genic rice lines, each containing a single gene for resistance. The 26 pathotypes grouped into five clusters, and cluster 1 contained wide virulence spectrum strains from all geographic populations. Although molecular variation was greatest between strains of different virulence phenotypes, some variation was observed among strains with identical virulence. There was a weak correlation (r = 0.52) between molecular haplotypes and virulence phenotypes. There are two major groups of X. oryzae pv. oryzae in Nepal. One group consists of strains with high molecular polymorphism and many pathotypes that are either virulent to the 11 major resistance genes or avirulent only to Xa21. Strains in the second group have low molecular polymorphism and are avirulent to Xa4, xa5, Xa7, and Xa21.

Virulence of Xanthomonas oryzae pv. oryzae on Rice Lines Containing Single Resistance Genes and Gene Combinations

Fifty isolates of Xanthomonas oryzae pv. oryzae were collected from different rice-producing districts of Nepal and evaluated for their virulence on these 11 rice lines having from one to four resistance genes: IRBB4 (Xa4), IRBB5 (xa5), IRBB7 (Xa7), IRBB8 (xa8), IRBB10 (Xa10), IRBB14 (Xa14), and IRBB21 (Xa21), two-gene combination AY4+5 (Xa4 and xa5), three-gene combinations NH21-37-1-1 (Xa4, xa5, and xa13) and NH24-10-1-3 (Xa4, xa5, and Xa21), and four-gene combination NH56-1-44-4 (Xa4, xa5, xa13, and Xa21). The ability of an isolate to cause lesions with different lengths across the lines was interpreted as virulence. Isolates that were consistently associated with high or low virulence were statistically differentiated. Most isolates produced large lesions on near-isogenic lines with single genes and small lesions on lines with different gene combinations. Based on infection responses on the two-, three-, and four-gene combinations, five virulence groups were identified. Isolates in virulence group I were widely distributed. The line × isolate interactions were generally not significant with gene combinations, indicating a low possibility of specificity. A line with a four-gene combination, NH56, showed wider spectrum and higher level of resistance to X. oryzae pv. oryzae than the other lines. The results of this study will facilitate the deployment of effective resistance to X. oryzae pv. oryzae in Nepal.

Population biology of Xanthomonas oryzae pv. oryzae and approaches to its control

Abstract New insights into the population structure of Xanthomonas oryzae pv. oryzae, the pathogen causing bacterial blight of rice, have been achieved through the use of molecular markers in combination with virulence typing. A hierarchical analysis to evaluate diversity and structure of X. oryzae pv. oryzae populations between countries, between different agro-ecosystems, and within and between farmers fields has revealed differentiation of subpopulations on various spatial scales. Such information on the pathogen populations might be used to refine disease management strategies and to investigate pathogen evolution.

Analysis of DNA Polymorphism and Virulence in Philippine Strains of Xanthomonas oryzae pv. oryzicola

Molecular tools were used to analyze the genetic diversity and population structure of Xanthomonas oryzae pv. oryzicola, the bacterial leaf streak pathogen of rice in the Philippines. Representative pathogen strains were selected and used to assess resistance in rice germplasm. A partial genomic library of X. oryzae pv. oryzicola was constructed, and a 459-bp clone containing the repetitive DNA element R41 was selected as a probe for restriction fragment length polymorphism (RFLP) analysis and sequenced. R41 shared 44% sequence homology with the putative transposase gene of IS1112, an insertion element cloned from X. oryzae pv. oryzae. Using R41 as a probe for RFLP analysis, 26 band profiles were discerned in a collection of 123 strains of X. oryzae pv. oryzicola. Analysis of PstI digestion patterns of DNA from the same collection resolved 36 haplotypes. Several clusters of strains were detected after grouping of data based on either pR41 as a probe or Pst1 digestion patterns. However, based on bootstrap analysis, the clusters were not robust. Genetic diversity was high for the entire collection as well as within spatially and temporally defined subsets of strains. Even a set of strains collected from a single site at a single time was highly diverse. Strains representing the different DNA types were inoculated to a set of diverserice cultivars. Consistent rice varietal groupings were obtained from disease reaction data, but there was no correlation between pathogen isolate cluster and host reaction across inoculation trials. Isozyme group I of rice, representing tropical japonica and javanica germplasm, is a promising source of resistance to bacterial leaf streak.

Problems in detection of Xanthomonas oryzae pv. oryzae in rice seed and potential for improvement using monoclonal antibodies

Four semiselective media that varied with respect to carbon source, amino acids, and antibiotics were evaluated for the growth of various strains of the rice bacterial blight pathogen, Xanthomonas oryzae pv. oryzae in pure and mixed culture. Plating efficiencies in pure culture ranged from 40 to 90% on these media, and colonies developed in 3-7 days, depending on the strain of X. o. oryzae. In mixtures with Erwinia herbicola and Pseudomonas putida(1:1:1), X. o. oryzae was recovered only on XOS medium. Recovery of a faster growing X. o. oryzae strain (X1-5) was improved by adding 50-100 mg/L of FeEDTA to XOS medium, but recovery of other strains was reduced or unaffected

Use of partial host resistance in the management of bacterial blight of rice

The progress of bacterial blight epidemics, caused by Xanthomonas oryzae pv. oryzae, varies with environment. The irrigated lowland rice production environment in central Terai (plain) is less conducive to the disease than the irrigated lowland rice production environment in eastern Terai in Nepal. The effect of partial resistance on bacterial blight was studied in central Terai during the wet seasons of 1994, 1995, and 1996. Three partially resistant rice cultivars, Sabitri, Laxmi, and IR54 (possessing the Xa4 gene), and susceptible check IR24 were included in this study. Analysis of pooled data from the 3 years of experiments indicated that rice cultivars differed in resistance based on three epidemiological parameters: disease severity (DS), area under the disease progress curve (AUDPC), and rate of disease increase (r). Estimates of DS, AUDPC, and r were reduced and yield loss was negligible in the partially resistant cultivar Laxmi compared with the susceptible check IR24. IR54, which has partial resistance to X. oryzae pv. oryzae, also showed low DS, AUDPC, and r, which prevented yield loss due to bacterial blight. The locally adapted cultivar Sabitri showed intermediate estimates of DS, AUDPC, r, and yield loss. Earlier and more severe disease developed in the susceptible check IR24, resulting in a yield loss of 22%. Yield losses were mainly due to reductions in number of tillers, number of grains per panicle, and 1,000-grain weight. These data indicate that the use of rice cultivars with high levels of partial resistance will be an effective tool for the management of bacterial blight in central Terai and similar environments in Nepal.

Resistance of rice to Xanthomonas oryzae pv. oryzae in Nepal

Adhikari, T. B., and Mew, T. W. 1994. Resistance of rice to Xanthomonas oryzae pv. oryzae in Nepal. Plant Dis. 78:64-67. Resistance to bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae in rice cultivars, was evaluated in the field and under greenhouse experiments from 1987 to 1991. In the field studies, plants were assessed by measuring lesion length (LL) and disease severity (percentage of leaf diseased). The area under the disease progress curves (AUDPC) and LL were used to compare rice cultivars. Rice cultivars BR-34-13, PAU-50-B-25, Laxmi, Sabitri, BW293-21, IR7167-33, Rodina, and Amonghaud had significantly (P< 0.05) shorter LL and smaller AUDPC than the susceptible check IR24. In the greenhouse studies, highly significant (P < 0.01) cultivar, strain, dose, cultivar X strain, and cultivar X dose effects were observed, indicating a differential host-pathogen interaction. Differences in virulence among bacterial strains and resistance among rice cultivars were observed. Inoculum that contained 109 cfu/ ml induced larger differences in LL between resistant and susceptible cultivars. Laxmi consistently exhibited the highest level of resistance as indicated by reduced LL and AUDPC, and could be a source of resistance to BB in Nepal. Additional keywords: dose-response curve, Oryza sativa