An ideal job.

A brief personal history illustrates how fortunate I was to have stumbled into a career in plant pathology, which turned out to be the ideal job for me. Several of the people who steered me or facilitated my development in research on plant diseases are mentioned. Starting with my PhD research, I have had the good fortune to indulge a career-long fascination with epidemiology and genetics of disease resistance in plants, particularly coevolution of gene-for-gene host-pathogen systems. I hope that my example may inspire others of like minds to consider a research career in plant pathology.

Conceptual analysis of methods applied to assessment of diversity within and distance between populations with asexual or mixed mode of reproduction.

Measures of diversity within populations, and distance between populations, are compared for organisms with an asexual or mixed mode of reproduction. Examples are drawn from studies of plant pathogenic fungi based on binary traits including presence/absence of DNA bands or virulence/avirulence to differential hosts. Commonly used measures of population diversity or genetic distance consider either genotype frequencies or allele frequencies. Kosman's diversity and distance measures are the most suitable for populations with an asexual or mixed mode of reproduction, because by considering genetic patterns of all individuals they take into account not just the genotype frequencies but also the genetic similarities between genotypes in the populations. The Kosman distance and diversity measures for populations can be calculated using different measures of dissimilarity between individuals (the simple mismatch, Jaccard and Dice coefficients of dissimilarity). Kosman's distances based on the simple mismatch and Jaccard dissimilarities are metrics. Comparisons of diversity indices for hypothetical examples as well as for actual data sets are presented to demonstrate that inferences from diversity analysis of populations can be driven by techniques of diversity and distance assessments and not only data driven.

Stem rust of small grains and grasses caused by Puccinia graminis.

UNLABELLED: SUMMARY Stem rust has been a serious disease of wheat, barley, oat and rye, as well as various important grasses including timothy, tall fescue and perennial ryegrass. The stem rust fungus, Puccinia graminis, is functionally an obligate biotroph. Although the fungus can be cultured with difficulty on artificial media, cultures grow slowly and upon subculturing they develop abnormal ploidy levels and lose their ability to infect host plants [Bushnell and Bosacker (1982) Can. J. Bot. 60, 1827-1836]. P. graminis is a typical heteroecious rust fungus with the full complement of five distinct spore stages that occur during asexual reproduction on its gramineous hosts and sexual reproduction that begins in the resting spore stage and culminates on the alternate host, barberry (Berberis spp.). There appears to be little polymorphism for resistance/susceptibility in Berberis species, but complex polymorphisms of resistance/susceptibility and matching virulence/avirulence exist in gene-for-gene relationships between small grain species and the forms of P. graminis that infect them. TAXONOMY: Puccinia graminis is a rust fungus in the phylum Basidiomycota, class Urediniomycetes, order Uredinales, and family Pucciniaceae, which contains 17 genera and approximately 4121 species, of which the majority are in the genus Puccinia[Kirk et al. (2001) Ainsworth and Bisby's Dictionary of the Fungi. Wallingford, UK: CAB International]. Various subdivisions of P. graminis into subspecies, varieties and formae speciales have been proposed based on spore size and host range. Crossing studies and DNA sequence comparisons support the separation of at least two subspecies, but not the proposed separation based on spore size. HOST RANGE: The host range of P. graminis is very broad compared with that of most Puccinia spp.; it includes at least 365 species of cereals and grasses in 54 genera [Anikster (1984) The Cereal Rusts. Orlando, FL: Academic Press, pp. 115-130]. Wheat stem rust, P. graminis f. sp. tritici, was shown to infect 74 species in 34 genera in artificial inoculations of seedlings, but only 28 of those species belonging to eight genera were known to be natural hosts of the fungus. Other formae speciales of P. graminis have narrower host ranges than P. graminis f. sp. tritici. Disease symptoms: Infections in cereals or grasses occur mainly on stems and leaf sheaths, but occasionally they may be found on leaf blades and glumes as well. The first macroscopic symptom is usually a small chlorotic fleck, which appears a few days after infection. About 8-10 days after infection, a pustule several millimetres long and a few millimetres wide is formed by rupture of the host epidermis from pressure of a mass of brick-red urediniospores produced in the infection. These uredinial pustules are generally linear or diamond shaped and may enlarge up to 10 mm long. The powdery masses of urediniospores appear similar to rust spots on a weathered iron surface. With age, the infection ceases production of brick-red urediniospores and produces a layer of black teliospores in their place, causing the stems of heavily infected plants to appear blackened late in the season.

Quantitative Trait Loci Associated with Seedling Resistance to Isolates of Puccinia coronata in Oat.

ABSTRACT In our previous report, quantitative trait loci (QTL) for field adult plant resistance to crown rust were identified in an oat population of 152 F(5:6) recombinant inbred lines from the cross of 'Ogle' (susceptible)/MAM17-5 (resistant). The objectives of the present study were to identify in the same population, the number, genomic location, and effect of QTL and digenic QTL epistasis associated with greenhouse seedling resistance to isolates of Puccinia coronata to determine if the QTL detected are isolate-specific and to compare them with previously detected QTL for field resistance. Reaction type was scored on greenhouse seedlings inoculated with three isolates. Composite interval mapping was conducted to identify genomic regions associated with resistance using a framework map of 272 molecular markers. Two QTL, Pcq1 and Pcq2, were identified for resistance to each of the three isolates. Pcq1, the major QTL controlling field resistance, did not confer detectable greenhouse seedling resistance when present singly; however, Pcq1 did serve as an enhancer of seedling resistance when it was combined with Pcq2. The final model explained 76.5, 77.9, and 79.3% of total phenotypic variation for resistance to isolates MNB248, MNB249, and MNB251, respectively. Race-specificity of quantitative resistance remains to be further examined.