Virulence Variability and Genetic Diversity in Blumeria graminis f. sp. hordei in Southeastern and Southwestern China.

Powdery mildew is a key airborne foliar disease of barley in southeastern and southwestern China. Barley varieties usually partially or wholly lose resistance to the pathogen Blumeria graminis (DC.) f. sp. hordei 3 to 5 years after release due to the frequent acquirements of new virulences in the pathogen population. However, no B. graminis f. sp. hordei virulence detection has been carried out in the recent decade and, thus, no information is available on the present virulence components and major pathotypes in epidemic regions. Twenty-one near-isogenic lines of Pallas were selected to detect B. graminis f. sp. hordei virulence variation, with 97 pathotypes identified from the isolates collected from 2015 to 2019. The virulence complexities ranged from 1 to 12, with 1.5 isolates on average assigned per pathotype, suggesting a natural trait of high pathotype diversity and low virulence complexity in the Chinese B. graminis f. sp. hordei populations. Eleven high-virulence pathotypes were detected in the traditional barley-growing regions in Yunnan and Zhejiang. Six virulent pathotypes to resistance gene mlo-5 were detected only in the two traditional epidemic regions, with a virulence frequency (VF) of 4.8% (7 of 147). Compared with the results from a decade ago, VFs for resistance alleles Mla3, mlo-5, Mla6 + Mla14, Mla7 + Mlk, Mlg + MlCP, and Mla13 + MlRu3 + MlaRu4 increased from 0 to 0.7 to 25.8%. Isolates from Yunnan and Zhejiang had similar virulence profiles, which differed from those identified in Tibet. In addition, genetic diversities differed in the isolate groups collected from Tibet, Yunnan, and Zhejiang.

Identification and molecular mapping of YrBm for adult plan resistance to stripe rust in Chinese wheat landrace Baimangmai.

KEY MESSAGE: A new adult plan resistance gene YrBm for potentially durable resistance to stripe rust was mapped on wheat chromosome arm 4BL in landrace Baimangmai. SSR markers closely flanking YrBm were developed and validated for use in marker-assisted selection. The wheat stripe rust pathogen Puccinia striiformis f. sp. tritici (Pst) frequently acquires new virulences and rapidly adapts to environmental stress. New virulences in Pst populations can cause previously resistant varieties to become susceptible. If those varieties were widely grown, consequent epidemics can lead to yield losses. Identification and deployment of genes for durable resistance are preferred method for disease control. The Chinese winter wheat landrace Baimangmai showed a high level of adult plant resistance (APR) to stripe rust in a germplasm evaluation trial at Langfang in Hebei province in 2006 and has continued to confer high resistance over the following 15 years in field nurseries in Hebei, Sichuan and Gansu. A recombinant inbred line population of 200 F10 lines developed from a cross of Baimangmai and a susceptible genotype segregated for APR at a single locus on chromosome 4BL; the resistance allele was designated YrBm. Allelism tests of known Yr genes on chromosome 4B and unique closely flanking marker alleles Xgpw7272189 and Xwmc652164 among a panel of Chinese wheat varieties indicated that YrBm was located at a new locus. Moreover, those markers can be used for marker-assisted selection in breeding for stripe rust resistance.

Pathotype Identification and Virulence Variation in Cochliobolus sativus in China.

Spot blotch caused by Cochliobolus sativus has become an important disease in the wheat-growing regions in China that has resulted from changes in the regional climate, agricultural cultivation patterns, and the susceptible wheat varieties that are widely grown. Little information is available about virulence variability and pathogenic specialization of the C. sativus isolates from major wheat-growing regions in China. Here, 12 representative wheat varieties and foundation breeding stocks were selected to characterize the pathotypes of C. sativus isolates from infected wheat plants. Based on the infection phenotypes in the 12 differential genotypes at the seedling stage, 70 Chinese pathotypes were identified from 110 isolates and clustered into three virulence groups. The high virulence isolates were collected from wheat leaves, crowns, and roots, with most (10 of 14) from the Henan province in the Huang-Huai plain. No relationship was evident between virulence variability of C. sativus isolates and their geographic origins or types of diseased wheat tissues. C. sativus showed a significant pathogenic specialization in hosts of wheat and barley. Most of the wheat isolates (50 of 65) were avirulent to all the differential barley genotypes, and a few were virulent only to highly susceptible barley genotypes. These results indicated that C. sativus isolates from the wheat-growing regions in China varied considerably for their virulence in wheat varieties, and showed significant pathogenic specialization to the wheat and barley hosts.

Virulence and Molecular Diversity in the Cochliobolus sativus Population Causing Barley Spot Blotch in China.

Spot blotch, caused by the fungal pathogen Cochliobolus sativus, is a limiting factor for barley (Hordeum vulgare) production in northeast China, which causes significant grain yield losses and kernel quality degradation. It is critical to determine the virulence diversity of C. sativus populations for barley resistance breeding and the judicious grouping of available resistance varieties according to the predominant pathotypes in disease epidemic regions. With little information on the barley pathogen in China, this study selected 12 typical barley genotypes to differentiate the pathotypes of C. sativus isolates collected in China. Seventy-one isolates were grouped into 19 Chinese pathotypes based on infection responses. Seventeen isolates were classified as pathotype 3, which has only been identified in China, whereas most (52 of 71) were classified as pathotype 1. All of the tested isolates had low virulence on the North Dakota (ND) durable, resistant line ND B112. Using 22 selected amplified fragment-length polymorphism (AFLP) primer combinations, genetic polymorphism was used to analyze 68 isolates, which clustered into three distinct groups using the unweighted pair group method average with the genetic distance coefficient. No relationship was found between the virulence of isolates and their origins. Isolates of the same pathotype or those collected from the same location did not group into clusters based on the AFLP analysis.