Abstracts of papers
Phytophthora sojae: Is it time for a new approach? A.E. DORRANCE (1) and S. St.Martin (2), (1) Department of Plant Pathology, The Ohio State University, Wooster, OH, (2) Department of Horticulture and Crop Science, The Ohio State University, Wooster, OH
Races of Phytophthora sojae with compatible interactions on all of the commonly deployed Rps genes have been identified in many US soybean production regions. In addition, more diverse pathotypes have been identified which include compatibility with Rps genes that have never been deployed. The most recently deployed resistance gene to P. sojae was Rps-1k during the early 1980's. Recently, potential new sources of resistance have been identified from southern China and the Republic of Korea. However, there is still concern whether these are novel genes or combinations of known Rps genes. Soybean cultivars with high levels of partial resistance to Phytophthora root and stem rot have not been widely planted. Cultivars with high levels of partial resistance planted in a field with high disease pressure had significantly higher yields compared to cultivars with single Rps genes and low partial resistance in 1998. Laboratory evaluations of crosses of partial-resistant by susceptible cultivars indicate that several genes may be involved.
Breeding for disease resistance to bacterial populations with a complex race structure: a case study from tomato. D.M. FRANCIS (1), E. Kabelka (1), S.A. Miller (2), J.W. Scott (3). (1) Department of Horticulture and Crop Science, The Ohio State University, Wooster, OH, (2) Department of Plant Pathology, The Ohio State University, Wooster, OH, (3) Gulf Coast Research and Education Center, University of Florida.
Bacterial diseases of tomato are common under the humid growing conditions of the Great Lakes region. The lack of resistance to bacterial pathogens in processing tomato varieties leads to frequent application of copper sprays and the failure of growers to adopt disease forecasting tools that minimize control sprays for fungal pathogens. The disease forecasting models do not predict bacterial infection and growers often default to a calendar application of copper and fungicide tank mixes. Bacterial Spot, Bacterial Canker, and Bacterial Speck all contribute to economic loss. In addition, surveys of bacterial populations have identified multiple races of the Bacterial Spot and Bacterial Speck pathogens. The population of Clavibacter michiganensis causing Bacterial Canker is genetically diverse and differences in aggressiveness have been documented. The existence of races has not been clearly established nor ruled out for Bacterial Canker. The challenge of breeding for multiple bacterial diseases superimposed upon multiple race structures is formidable. Assuming no new races are discovered, our long-term breeding will require that between five and eleven loci from seven sources be combined. Surveys of pathogen populations and the genetic characterization of resistance continue to provide valuable information used to design breeding strategies and prioritize which bacterial species and races to target. We believe a strategy that combines marker-assisted selection with populations that facilitate replicated field trials offers the best opportunity to pyramid resistance loci and select for field-resistant germplasm. In our strategy, cycles of field selection are alternated with cycles of marker assisted selection. As many as five different disease nurseries are used each year. Our goal is to combine the most important combinations of genes and to use hybrids to further pyramid resistance. This approach requires that we break up undesirable linkages, select for desirable recombination events, and pyramid unlinked genes. An interesting outcome of our work has been the suggestion that field resistance may be controlled by the same loci identified when searching for race specific resistance. Finally, because hybrids offer an opportunity to control the dose and strength of resistance, which will in turn affect the selection pressure imposed on pathogen populations, the next level of population data will require information on the fitness cost associated with avirulence.
Migrations of Phytophthora infestans awaken plant breeders. W.E. FRY, C.D. Smart and N.J. Grunwald. Department of Plant Pathology, Cornell University, Ithaca, NY.
Migrations of Phytophthora infestans during the last quarter of the twentieth century changed the structure and composition of P. infestans populations throughout much of the world (except apparently for much of Africa, Australia, and Mexico). These changes have led to more serious problems with potato and tomato late blight worldwide. The introduced strains have been more aggressive than the previously dominant strains; they have been largely insensitive to metalaxyl in the USA and Canada, and we are just learning that the introduced strains have a different mix of pathogenicity characteristics. The dramatically more serious late blight problems necessitated use of large amounts of fungicide to suppress the disease adequately. Whereas before the mid 1990s, breeding for late blight resistance was a very low priority in most of the United States and Canada, it has now again become important to potato breeders. In addition, plant molecular biologists have become interested and novel approaches to creating resistant potatoes are under investigation. The migrations have illustrated that our previous understanding of the host range of P. infestans was conditioned by a genetically depauperate pathogen population, and the current more diverse population has pathogenic characteristics not previously expected.
Genetic variation in cereal rust populations and breeding for resistance to the cereal rusts. J.A. KOLMER, USDA-ARS, North Carolina State University, Raleigh NC.
Cereal rust populations can be highly diverse for virulence phenotypes. Many virulence phenotypes of the wheat leaf rust fungus, Puccinia triticina, and the oat crown rust fungus P. coronata are found on an annual basis in North America. Distinct regional populations of both P. triticina and P. coronata are found in North America. P. triticina reproduces asexually which maintains associations between virulences and molecular markers. P. coronata reproduces sexually, which results in random associations between virulences. Leaf and crown rust populations respond rapidly to selection pressures imposed by the release of cultivars with specific rust resistance genes. P. trticina phenotypes with virulence to resistance genes Lr11, Lr3ka, Lr16, and Lr17 have increased since 1987. Isolates of P. coronata with virulence to Pc38 and Pc39 have increased due to the release of cultivars with these genes. Effective leaf and crown rust resistance in wheat and oats can be maintained by introgression of genes into adapted germplasm that provide effective resistance before rust phenotypes with virulence to the current resistance genes become prevalent.
Applying Magnaporthe grisea population analyses for durable rice blast resistance: 10 years after. R. S. ZEIGLER (1), F. Correa Victoria (2), M. Jones (3). (1) Department of Plant Pathology, Kansas State University, Manhattan KS 66506, (2) CIAT, Cali, Colombia, (3) WARDA, Bouake, Cote d’Ivoire.
Blast disease, caused by Magnaporthe grisea (anamorph Pyricularia grisea), can cause devastating losses to rice in virtually all rice-growing environments world-wide. The preferred means of disease management includes disease resistance, which typically is overcome within only a few years. Founded in the principle that resistance cultivars are deployed against populations of pathogens, rather than particular races, population biology concepts entered the field of plant pathology and resistance breeding in the 1980s. Resistance screening techniques were devised to expose breeding lines to populations believed to be representative of the target environments for varietal release. It was postulated that analyzing the full virulence spectrum of target populations would suggest potentially stable and unstable resistance gene combinations and, therefore, increase the efficiency of resistance breeding. Populations were characterized using two types of tools: cultivars with known resistance genes and neutral molecular markers, with transposable element MGR586 being the most commonly used. Both approaches grouped population samples and, while the simple 1:1 relationship between group and pathotype that was first observed in the US was not found elsewhere, MGR586-defined pathogen groups (or "lineages") in studies from Colombia and the Philippines were observed to have unique virulence spectra. Virulence spectra of lineages in a population were such that in most cases relatively simple combinations of would confer resistance against all detected lineages ("lineage exclusion"). MGR586 characterization of populations was useful in the New World, Europe and West Africa (accounting for ~5 % of the global rice area), with resistance gene combination predictions still holding up and the efficiency of breeding increased in Colombia and Ivory Coast. The potential for the durability of a 2 gene combination (Pi-1 and Pi-2), first suggested by population analyses world-wide, is discussed. In Asia MGR586 is useful in areas where modern rice varieties are exclusively grown (up to 70 million hectares) and a putative bottleneck-effect produces simple populations; but, in traditional rice growing areas where the same cultivars have been grown for centuries the tool fails. It seems that in ancient populations an active transposable element like MGR586 quickly obscures, rather than clarifies, the relationships among strains. The test of the potential for lineage exclusion breeding in most of Asia awaits a more suitable neutral marker for population analysis.
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