Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2006

Pathotype Diversity of Phytophthora sojae in Eleven States in the United States

Pathotype diversity of Phytophthora sojae was assessed in 11 states in the United States during 2012 and 2013. Isolates of P. sojae were recovered from 202 fields, either from soil samples using a soybean seedling bioassay or by isolation from symptomatic plants. Each isolate was inoculated directly onto 12 soybean differentials; no Rps gene or Rps 1a, 1b, 1c, 1k, 3a, 3b, 3c, 4, 6, 7, or 8. There were 213 unique virulence pathotypes identified among the 873 isolates collected. None of the Rps genes were effective against all the isolates collected but Rps6 and Rps8 were effective against the majority of isolates collected in the northern regions of the sampled area. Virulence toward Rps1a, 1b, 1c, and 1k ranged from 36 to 100% of isolates collected in each state, while virulence to Rps6 and Rps8 was less than 36 and 10%, respectively. Depending on the state, the effectiveness of Rps3a ranged from totally effective to susceptible to more than 40% of the isolates. Pathotype complexity has increased in populations of P. sojae in the United States, emphasizing the increasing importance of stacked Rps genes in combination with high partial resistance as a means of limiting losses to P. sojae.

Virulence Characterization and Identification of Maize Lines Resistant to Puccinia sorghi Schwein. Present in the Argentine Corn Belt Region

Puccinia sorghi Schwein., the causal agent of maize common rust, is an endemic disease in the Argentine Corn Belt region. Virulence surveys of the pathogen population within the region have not been performed; thus, the understanding of the pathogen population is low and it is difficult to deploy resistance genes that could be effective at controlling the disease. In total, 58 single-uredinial isolates derived from infected maize leaves collected in different locations throughout the Argentine Corn Belt region during 2010 to 2012 were tested on a set of 25 maize lines carrying different Rp genes. Maize lines Rp3-A and PIO19802 showed the lowest virulence frequencies (3.4 and 1.7%, respectively) for all tested isolates. Moreover, the combination in a single genotype of the resistance genes carried by lines Rp3-A and PIO19802 or either of these lines combined with the resistance genes from PIO12345 would confer resistance to all isolates tested. Virulent isolates on maize lines Rp-G, Rp1-K, and Rp-GI were most frequent in 2012. Twenty-four virulence phenotypes were identified, with phenotypes TCCG (17.2%), TTBB (15.5%), and TCFG (10.3%) being the most common throughout the region. Adult plant resistance associated with hypersensitive response was identified at vegetative stage 6 in maize lines PIO68752, PIO28427, and PIO36420.

Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2012

Collections of Puccinia triticina were obtained from rust-infected leaves provided by cooperators throughout the United States and from wheat fields and breeding plots by United States Department of Agriculture-Agricultural Research Service personnel and cooperators in the Great Plains, Ohio River Valley, southeastern states, and Washington State and Idaho in order to determine the virulence of the wheat leaf rust population in 2012. Single uredinial isolates (501 in total) were derived from the collections and tested for virulence phenotype on 20 lines of 'Thatcher' wheat that are near-isogenic for leaf rust resistance genes. In 2012, 74 virulence phenotypes were described in the United States. Virulence phenotypes TNBGJ, TCRKG, and MBTNB were the three most common phenotypes. Phenotype TNBGJ is virulent to Lr39/41 and was widely distributed throughout the hard red winter wheat region of the Great Plains. Phenotype TCRKG is virulent to Lr11, Lr18, and Lr26 and was found mostly in the soft red winter wheat region in the eastern United States. Phenotype MBTNB is virulent to Lr11 and was also found mostly in the soft red winter wheat region. The frequency of isolates with virulence to Lr39/41, which is present in many hard red winter wheat cultivars in the Great Plains region, continued to increase. Isolates with virulence to Lr21, which is present in many hard red spring wheat cultivars, also continued to increase in frequency in the northern Great Plains region.

Genetic Analysis of Adult Plant Resistance to Yellow Rust and Leaf Rust in Common Spring Wheat Quaiu 3

Identifying and utilizing rust resistance genes in wheat has been hampered by the continuous and rapid emergence of new pathogen races. A major focus of many wheat breeding programs is achieving durable adult plant resistance (APR) to yellow (stripe) rust (YR) and leaf (brown) rust (LR), caused by Puccinia striiformis and P. triticina, respectively. This study aimed to determine the genetic basis of resistance to YR and LR in the common spring wheat 'Quaiu 3'. To that end, we evaluated 198 F₅ recombinant inbred lines (RILs), derived from a cross of susceptible 'Avocet-YrA' with Quaiu 3, for APR to LR and YR in artificially inoculated field trials conducted in Mexico during the 2009 and 2010 growing seasons. High narrow-sense heritability (h²) estimates, ranging between 0.91 and 0.95, were obtained for both LR and YR disease severities for both years. The quantitative and qualitative approaches used to estimate gene numbers showed that, in addition to known resistance genes, there are at least two to three APR genes associated with LR and YR resistance in the RIL population. The moderately effective race-specific resistance gene Lr42 and the pleiotropic slow-rusting APR gene Lr46/Yr29 were found to interact with additional unidentified APR genes. The unidentified APR genes should be of particular interest for further characterization through molecular mapping, and for utilization by wheat breeding programs.

Genetics of Leaf Rust Resistance in the Soft Red Winter Wheat Cultivars Coker 9663 and Pioneer 26R61

Leaf rust, caused by the fungus Puccinia triticina, is an important disease of soft red winter wheat cultivars that are grown in the southern and eastern United States. The objectives of this study were to identify the leaf rust resistance genes in two soft red winter wheat cultivars, Coker 9663 and Pioneer 26R61, that have been widely grown and were initially highly resistant to leaf rust. Both cultivars were crossed with the leaf-rust-susceptible spring wheat cv. Thatcher and the F₁ plants were crossed to Thatcher to obtain backcross (BC₁) F₂ families. In seedlings, the Thatcher/Coker 9663 BC₁F₂ families segregated for three independent seedling resistance genes when tested with different leaf rust isolates. The leaf rust infection types of selected BC₁F₃ lines, when tested with different leaf rust isolates, indicated that seedling resistance genes Lr9, Lr10, and Lr14a were present. In field plot tests, BC₁F₄ lines that were seedling susceptible had some adult plant resistance to leaf rust. Seedlings of the Thatcher/Pioneer 26R61 BC₁F₂ families segregated for two independent resistance genes. Infection types of selected BC₁F₃ lines indicated the presence of Lr14b and Lr26. The adult plant gene Lr13 was determined to be present in selected BC₁F₄ lines that were tested with different leaf rust isolates in greenhouse tests.

Identification of novel QTLs for seedling and adult plant leaf rust resistance in a wheat doubled haploid population

Pyramiding of genes that confer partial resistance is a method for developing wheat (Triticum aestivum L.) cultivars with durable resistance to leaf rust caused by Puccinia triticina. In this research, a doubled haploid population derived from the cross between the synthetic hexaploid wheat (SHW) (xAegilotriticum spp.) line TA4152-60 and the North Dakota breeding line ND495 was used for identifying genes conferring partial resistance to leaf rust in both the adult plant and seedling stages. Five QTLs located on chromosome arms 3AL, 3BL, 4DL, 5BL and 6BL were associated with adult plant resistance with the latter four representing novel leaf rust resistance QTLs. Resistance effects of the 4DL QTL were contributed by ND495 and the effects of the other QTLs were contributed by the SHW line. The QTL on chromosome arm 3AL had large effects and also conferred seedling resistance to leaf rust races MJBJ, TDBG and MFPS. The other major QTL, which was on chromosome arm 3BL, conferred seedling resistance to race MFPS and was involved in a significant interaction with a locus on chromosome arm 5DS. The QTLs and the associated molecular markers identified in this research can be used to develop wheat cultivars with potentially durable leaf rust resistance.

Differentiation of molecular genotypes and virulence phenotypes of Puccinia triticina from common wheat in North America

Wheat leaf rust caused by Puccinia triticina is widely distributed in the wheat growing regions of the United States and Canada, and is subject to selection for virulence phenotype by leaf rust resistance genes in wheat cultivars. The objective of this study was to determine the number of genetically differentiated groups of P. triticina that are currently present in North America. In total, 148 isolates of P. triticina from the 1980s to 2005 were collected from wheat-growing regions of the United States and Canada and tested for virulence on 20 lines of wheat with single genes for leaf rust resistance and for molecular genotype with 23 simple sequence repeat (SSR) markers. In total, 91 virulence phenotypes and 65 SSR genotypes were found. After removal of isolates with identical virulence and SSR genotypes, 125 isolates were included for further analysis. Bayesian cluster analysis indicated five different groups of isolates based on SSR genotypes that also differed for virulence to leaf rust resistance genes Lr2a, Lr2c, Lr3bg, Lr17, and Lr28. Isolates avirulent to Lr14a and Lr20 that have increased since 2003 had SSR genotypes identical or similar to older isolates in one of the five groups, indicating that these isolates were derived by mutation from the previously existing population of P. triticina. The representative collection of P. triticina isolates had characteristics consistent with an asexual dikaryotic population of genetically differentiated groups of SSR genotypes with high levels of heterozygosity and disequilibrium within which stepwise mutation at avirulence or virulence loci regularly occurs.

Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2007

In 2007, leaf rust of wheat was severe throughout the Great Plains region of North America. Yield losses in wheat due to leaf rust were estimated to be 14% in Kansas. Collections of Puccinia triticina were obtained from rust-infected leaves provided by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio River Valley, southeast, California, and Washington State in order to determine the virulence of the wheat leaf rust population in 2007. Single uredinial isolates (868 in total) were derived from the collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3a, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17a, Lr30, LrB, Lr10, Lr14a, Lr18, Lr21, and Lr28, and on winter wheat lines with genes Lr41 and Lr42. Fifty-two virulence phenotypes were found. Virulence phenotypes TDBJG, MFPSC, and TDBJH were among the four most common phenotypes and were all virulent to resistance gene Lr24. These phenotypes were found throughout the Great Plains region. Phenotype MLDSD, with virulence to Lr9, Lr17, and Lr41, was also widely distributed in the Great Plains. In the soft red winter wheat region of the southeastern states, phenotypes TCRKG, with virulence to genes Lr11, Lr26, and Lr18, and MFGJH, with virulence to Lr24, Lr26, and Lr11, were among the common phenotypes. Virulence phenotypes with virulence to Lr16 were most frequent in the spring wheat region of the northern Great Plains. Virulence phenotypes with virulence to Lr11, Lr18, and Lr26 were most common in the soft red winter areas of the southeastern states and Ohio Valley. Virulence to Lr21 was not found in any of the tested isolates.