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Wheat yellow (stripe) rust, caused by Puccinia striiformis f. sp. tritici ( Pst ), is one of the most devastating diseases of wheat worldwide. Pst populations are composed of multiple genetic groups, each carrying one or more races characterized by different avirulence/virulence combinations. Since the severe epidemics in 2017, yellow rust has become the most economically important wheat foliar disease in Uruguay. A set of 124 Pst isolates collected from wheat fields in Uruguay between 2017 and 2021 were characterized phenotypically, and 27 of those isolates were subsequently investigated in-depth by additional molecular genotyping and race phenotyping analyses. Three genetic groups were identified, PstS7 , PstS10 , and PstS13 , with the latter being the most prevalent. Two races previously reported in Europe, Warrior ( PstS7 ) and Benchmark ( Pst S10), were detected in four and two isolates, respectively. A third race, known as Triticale2015 ( PstS13 ), that was first detected in Europe in 2015 and in Argentina in 2017 was detected at several locations. Additional virulence to Yr3 , Yr17 , Yr25 , Yr27 , or Yr32 was detected in three new race variants within PstS13 . The identification of these new races, which have not been reported outside South America, provides strong evidence of the local evolution of virulence in Pst during the recent epidemic years., Competing Interests: The author(s) declare no conflict of interest.

Stripe rust, caused by the fungal pathogen Puccinia striiformis f. sp. tritici , is a major threat to wheat ( Triticum spp.) production worldwide. The objective of this study was to determine the virulence of P. striiformis f. sp. tritici races prevalent in the main wheat growing regions of Kenya, which includes Mt. Kenya, Eastern Kenya, and the Rift Valley (Central, Southern, and Northern Rift). Fifty P. striiformis f. sp. tritici isolates collected from 1970 to 1992 and from 2009 to 2014 were virulence phenotyped with stripe rust differential sets, and 45 isolates were genotyped with sequence characterized amplified region (SCAR) markers to differentiate the isolates and identify aggressive strains PstS1 and PstS2 . Virulence corresponding to stripe rust resistance genes Yr1 , Yr2 , Yr3 , Yr6 , Yr7 , Yr8 , Yr9 , Yr17 , Yr25 , and Yr27 and the seedling resistance in genotype Avocet S were detected. Ten races were detected in the P. striiformis f. sp. tritici samples obtained from 1970 to 1992, and three additional races were detected from 2009 to 2014, with a single race being detected in both periods. The SCAR markers detected both Pst1 and Pst2 strains in the collection. Increasing P. striiformis f. sp. tritici virulence was found in the Kenyan P. striiformis f. sp. tritici population, and different P. striiformis f. sp. tritici race groups were found to dominate different wheat growing regions. Moreover, recent P. striiformis f. sp. tritici races in East Africa indicated possible migration of some race groups into Kenya from other regions. This study is important in elucidating P. striiformis f. sp. tritici evolution and virulence diversity and useful in breeding wheat cultivars with effective resistance to stripe rust.

An isolate of the fungus Puccinia striiformis, causing yellow (stripe) rust on cereals and grasses, was selfed on the alternate (sexual) host, Berberis vulgaris. This enabled us to investigate genetic variability of progeny isolates within and among aecia. Nine aecial clusters each consisting of an aecium (single aecial cup) and nine clusters containing multiple aecial cups were selected from 18 B. vulgaris leaves. Aeciospores from each cluster were inoculated on susceptible wheat seedlings and 64 progeny isolates were recovered. Molecular genotyping using 37 simple sequence repeat markers confirmed the parental origin of all progeny isolates. Thirteen molecular markers, which were heterozygous in the parental isolate, were used to analyse genetic diversity within and among aecial cups. The 64 progeny isolates resulted in 22 unique recombinant multilocus genotypes and none of them were resampled in different aecial clusters. Isolates derived from a single cup were always of the same genotype whereas isolates originating from clusters containing up to nine aecial cups revealed one to three genotypes per cluster. These results implied that each aecium was the result of a successful fertilization in a corresponding pycnium and that an aecium consisted of genetically identical aeciospores probably multiplied via repetitive mitotic divisions. Furthermore, the results suggested that aecia within a cluster were the result of independent fertilization events often involving genetically different pycniospores. The application of molecular markers represented a major advance in comparison to previous studies depending on phenotypic responses on host plants. The study allowed significant conclusions about fundamental aspects of the biology and genetics of an important cereal rust fungus., (Copyright © 2017 British Mycological Society. Published by Elsevier Ltd. All rights reserved.)

A procedure for virulence phenotyping of isolates of yellow (stripe) rust using spray inoculation of wheat seedlings by spores suspended in an engineered fluid, Novec™ 7100, is presented. Differential sets consisting of near-isogenic Avocet lines, selected lines from the "World" and "European" sets, and additional varieties showing race-specificity facilitate a robust assessment of race, irrespectively of geographical and evolutionary origin of isolates. A simple procedure for purification of samples consisting of multiple races is also presented.