An experimental genetic system for Puccinia striiformis was recently developed using the alternate (sexual) host, Berberis vulgaris. Selfing of an aggressive Pst isolate resulted in an S1 generation of 16 progeny, which were confirmed by segregating SSR markers. We analyzed the inheritance of avirulence/virulence in the S1 generation using wheat genotypes representing 21 Pst resistance genes. All S1 progeny were virulent for 14 of 15 Yr genes where the parental isolate was virulent. No segregation was observed for 5 of 6 host genes for which the parental isolate was avirulent. Segregation was observed with respect to Yr8 where the parental isolate gave infection type (IT) 0, and to Yr17 where the parental isolate gave IT 5-6 (0-9 scale). Avirulence/virulence to Yr8 (Compair and AvS+Yr8) was represented by two phenotypes, and avirulence/virulence to Yr17 (VPM1, AvS+Yr17, and Baltimore) was represented by three host phenotypes. On both Yr8 host genotypes, some progenies produced IT 0 and others produced IT 1-2, suggesting that the parental isolate was heterozygous for two different Avr8 alleles resulting in different, but clearly avirulent phenotypes. On the Yr17 genotypes, two distinct phenotypes, IT 2-4 and 5-6 (occasionally 7), were observed, the latter being similar to the parental isolate. None of the progenies was considered virulent (IT 7 to 9), as observed for Yr17-virulent reference isolates. This unusual segregation pattern could be explained by the presence in the parental isolate of a heterozygous modifier gene influencing the phenotypic expression of avirulence. In order to resolve the genetics in detail, additional progeny are being produced from the parental isolate, and selfings of additional Pst isolates with divergent levels of aggressiveness are in progress.
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Berberis holstii, native to the highlands of East Africa, is susceptible to Puccinia graminis and P. striiformis in artificial inoculations. However, it is not known whether these pathogens complete their sexual cycles in the region. In an attempt to understand the role of B. holstii in pathogen variation and epidemiology of wheat stem rust and stripe rust, we investigated the functionality of B. holstii as an alternate host. Natural aecial infections on B. holstii were observed and sampled in August at Mt. Kenya and Narok (Kenya), and June to December at North Shewa (Ethiopia) from 2008. Aeciospores from the collections were inoculated to a panel of cereal species, including Line E and ‘Morocco’ wheat, 'Hiproly’ barley, 'Prolific' rye, and ‘Marvelous’ oat. For the majority of aecial samples, aeciospore viability was lost during shipment and storage; thus inoculations were not successful. Inoculations using relatively fresh samples collected at North Shewa in 2012 and 2014, resulted in stem rust infections on Line E, Prolific, Hiproly, and Marvelous. DNA assays using real-time PCR confirmed the presence of P. graminis in these samples. While it is likely that the pathogen infecting Line E, Prolific and Hiproly is P. graminis f. sp. secalis (Pgs), the inoculation and DNA assays did not provide sufficient resolution to distinguish Pgs from Pgt. Stem rust infections on Marvelous were assumed to involve Pg f. sp. avenae. Experiments are in progress to characterize isolates derived from these samples, and to determine if other rust fungi are present in these samples. Based on these preliminary data, we conclude that P. graminis completes its sexual cycle in Ethiopia. The contribution of the sexual cycle to the observed variation within the Pgt population in the region remains unclear.
Identifying and tracking new races of wheat rust pathogens in a timely manner is important for early warning of disease potential in wheat-growing regions. Ecuador, located in northwestern South America, serves as a strategic monitoring location for rust fungi between the wheat production areas of North and South America. New races are likely to occur more often when the fungus is in the proximity of the alternate Berberis species host. More than 30 Berberis species have been reported in Ecuador, most of them endemic. However, most herbarium collections correspond to types, have only been found once, and/or date back 20 years or more. Therefore, the current status of diversity in Berberis spp. in Ecuador is largely unknown. Our goal is to collect Berberis species in Ecuador, document their distribution, prepare herbarium specimens, and identify the species morphologically and genetically. We will use this information to establish the relationships of neotropical Berberis species with Berberis in other parts of the world, and determine the pathogenicities of various rust fungi associated with them. Preliminary results show that the Ecuadorian Berberis spp. are phylogenetically distinct from those of Argentina and Brazil. To date, the rust fungi on Ecuadorean Berberis do not infect wheat. We have identified three potentially new rust fungal species based on DNA analysis.