| Barley powdery mildew caused by Blumeria graminis. f.sp. hordei (Bgh), which is a windborne and obligate biotrophic fungal pathogen, is one of the most destructive foliar diseases of barley. It is particularly prevalent in cooler and more humid regions of temperate climates, and it is one of more serious barley diseases in the winter barley regions of China. Barley powdery mildew can reduce grain yield by20%to30%. In recent years, barley powdery mildew in China is getting increasingly serious because of densely seeding, using larger amount of nitrogenous fertilizer and planting the same barley cultivars. Breeding for genetic resistance is an economically and environmentally sustainable way to control powdery mildew. Growing resistant cultivars against powdery mildew not only diminishes crop losses and fungicide spraying costs, but also reduces environmental pollution. Knowledge of the genetic structure and evolution of Bgh populations and identification of resistance genes to powdery mildew is prerequisite for an efficient use of available resistance genes. In this study, the frequency, distribution and dynamics of virulence and pathotypes in Bgh populations in the winter barley regions of China were investigated on’Pallas near-isogenic lines’ and other five barley varieties as differential hosts. The genetic diversity of79Bgh isolates, selected from different pathotypes in respective region, was detected by AFLP to demonstrate further genetic diversity level in Bgh populations of China. Resistance and resistance genes to powdery mildew in some leading barley cultivars and backbone parents from major barley growing areas in China were identified by analysis of resistance reaction spectra;112widely representative Bgh isolates were selected to inoculate some core barley varieties (lines) for screening differential barley hosts for determining Bgh pathotypes in China。575isolates were collected from seven populations of Bgh on different cultivated barley in seven geographically distant locations in2006. Their virulence was determined by inoculation onto26differential host lines. All of the isolates belonged to58pathotypes and13of which included81%of these isolates. The most abundant pathotype was pathotype 0047(18.3%), the second most abundant was pathotype0045(11.8%) and the third most abundant was pathotype0057(7.8%). Most of virulent genes investigated in this study showed similar frequencies in the seven different areas. These indicate that the seven locations may be in a uniform epidemiological region and barley cultivars in these areas may have similar genetic background. Diversities within these populations and distances between these populations measured by KOIND package were different. Correlations were not found between the genetic distances and the geographical distances between different locations. This suggested that long distance spread and local epidemics existed in the major winter barley growing regions in China.Molecular markers, which can detect high level of allelic polymorphism, have become a important tool for studying genetic diversity in pathogen populations. In this study, genetic diversity of79Bgh isolates collected from7different locations in the winter barley regions of China, which belong to12different pathotypes, was assessed using AFLP (amplified fragment polymorphism, AFLP) markers. Cluster analysis of UPGMA (unweighted pair group method average, UPGMA) based on a total of337polymorphic AFLP bands detected among the79isolates showed seven different genetic lineages. There was some correlation between pathotypes and AFLP groups, and a close correlation between the genetic lineages and origins of the genotypes. The diversity parameter obtained with POPGENE program showed the Bgh population in the winter barley regions of China possessed relatively high levels of genetic diversity, and the level of genetic diversity detected in some Bgh population was different. These results were related with some factors causing evolution of Bgh such as the size of barley area planted every year, the environmental conditions which impaired life cycle of Bgh on barley host. The higher genetic similarity between different Bgh populations was found, and the positive correlations between the geographical distances and the genetic distances amongst these locations were found. These results indicated that the main allelic variation existed within Bgh populations, and there is an extensive gene flow and migration of pathogen among the different regions. Some results of genetic variation of Bgh populations obtained with AFLP analysis were not in agreement with some of results detected by virulence assessments, but the causes for evolution in natural Bgh populations concluded from AFLP analysis was more reasonable. ’Pallas’ isogenic lines possessing known Bgh resistant genes were used in virulence analysis for the population consisting of729isolates collected from the winter barley region of China in2005and2006, and resistance and resistance genes to powdery mildew in328barley varieties (lines) were identified. All isolates were avirulent to Mlal Mla (A12), Mla3, Mla6Mlal4, Mla7Mla (No3), Mla7Ml (Lg2), Mla9Mlk, Mla9, Mlal3MlaRu3, Mlpl, Mlg (Cp) and mlo5, and the frequencies of isolates overcoming the genes Mlal2MlaEm2(0.1%), Mla7Mlk (0.4%), Mlat Mla8(0.9%), Mla10MlaDu2(2.8%) and Mlkl (4.2%) were very low. These results indicated that these resistance genes can be using in barley breeding program. Majority of328varieties (lines) tested were susceptible to powdery mildew, but37of these were resistant to all dominant pathotypes. Resistance genes can be postulated in less of the lines investigated, and the resistance genes in these lines were Ml (Bw), Mla8, Mlg, Mlra Mla8, Mla9Mlk, Mlal Mla (A12) or mlo5.112representative isolates of Bgh selected from different locations and different pathotypes, were inoculated on112barley varieties including leading barley cultivars and backbone parents in China. A set of differential hosts which is suitable to test pathotypes of Bgh in China was established by investigating on infection types and virulence frequency of112isolates on112barley varieties. The variation of virulence frequency from0to60%was continuous, and virulence frequency within60%to70%was not found, one was within70%to80%, one was within80%to90%, and one was100%. Infection symptom of most inoculated hosts was obvious and stable. Nine barley varieties were chosen differential hosts to identify pathotypes of Bgh in China, and they were Daner, Dulihuang, Yun2, E6, Supi4, Beiqing8, Pu7, Zhenongda7, and Zhepi3respectively, and more, Xiaoshanglixiahuang was used as a susceptible control variety. These barley varieties were used to test the112isolates,50pathotypes, of which a few were predominent pathotypes, were found. These results showed that these differential barley varieties were suitable to identify pathotypes of Bgh population in China. |
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