A. M. Jarosz

Virulence and Molecular Diversity in Colletotrichum lindemuthianum from South, Central, and North America

ABSTRACT Isolates of Colletotrichum lindemuthianum (138 total) from Argentina, Brazil, the Dominican Republic, Honduras, Mexico, and the United States were characterized into 41 races based on virulence to 12 differential cultivars of Phaseolus vulgaris. These 41 races were categorized into two groups: those found over a wide geographic area and those restricted to a single country. Races 7, 65, and 73 were widespread. Race 73 was the most common (28%). Race 7 was found once in Argentina and Mexico but at a higher frequency in the United States. Race 65 was found repeatedly in Brazil and the United States. Although 39% of the races were detected repeatedly and three races were widespread, no race was isolated from both P. vulgaris gene pools. Phenetic analyses showed no obvious patterns correlated with virulence clusters. No geographic pattern was evident. Molecular polymorphism generated by random amplified polymorphic DNA confirmed the extensive variability in virulence of C. lindemuthianum. Virulence phenotypes were grouped into 15 clusters. The two largest clusters contained isolates from all the geographic regions sampled. Molecular polymorphism was observed among isolates from races 65 and 73 within and among countries, except among Bra-zilian isolates of race 65. The genetic diversity of C. lindemuthianum was greatest in Mexico and Honduras. Our data suggest that C. lindemuthianum may not be highly structured to specific Phaseolus gene pools.

Host-pathogen interactions in natural populations of Linum marginale and Melampsora lini. I, Patterns of resistance and racial variation in a large host population

Populations of wild flax, Linum marginale and its associated rust fungus Melampsora lini growing at Kiandra, New South Wales, Australia, were sampled during the 1986–1987 growing season. Thirteen different races of M. lini were detected in a sample of 96 isolates. The distribution of isolates was uneven: race A comprised 73% of the samples; race N, 8%; and race H, 5%; while the remaining races were represented by only one or two samples. The dominance of race A increased over the course of the growing season, comprising 67% of the early season samples and increasing to 78% for those collected late in the season. The overall diversity of the pathogen population decreased late in the growing season, but this trend was not statistically significant. The average virulence of individual isolates of the pathogen population increased during the growing season. This trend was most pronounced among the minor races, where the mean number of differential hosts infected increased from 4.58 for early season samples to 5.12 and 5.08 for mid and late season samples, respectively. In contrast to the virulence pattern in the pathogen, the L. marginale population displayed a more even distribution of resistance. In a sample of 67 plants 10 resistance phenotypes were detected from their pattern of resistance/susceptibility to seven pathogen isolates. No phenotype had a frequency that exceeded 30%. Resistance phenotypes were randomly distributed on both a population level and on a fine scale.

HOST-PATHOGEN INTERACTIONS IN NATURAL POPULATIONS OF LINUM MARGINALE AND MELAMPSORA LINI: II. LOCAL AND REGIONAL VARIATION IN PATTERNS OF RESISTANCE AND RACIAL STRUCTURE

Spatial variation in the resistance structure of Linum marginale (wild flax) populations to the rust fungus Melampsora lini, and in the racial structure of this pathogen, was investigated by sampling 10 populations distributed throughout the Kosciusko National Park, New South Wales, Australia. Considerable differences were found among populations in the structure of both host and pathogen. Host populations were divided into three broad categories: (1) populations susceptible to all testing races; (2) populations containing a strictly limited number of resistant phenotypes; and (3) populations with a considerable diversity of resistant phenotypes. The pathogen populations also showed a range of diversity. The major differences between these populations were determined by the occurrence and frequency of four common races of pathogen (races A, E, K, and N). These differences were apparent both at a regional spatial scale (over the 100 km separation of the most distant populations) and at a local scale where major differences were detected between two populations only 300 m apart. The distribution of the four common races of M. lini was consistent with the hypothesis that a fitness cost was associated with unnecessary virulence. In general, however, differences in the structure of pathogen populations from genetically very similar host populations implied that, in addition to host resistance genes, other evolutionary forces are also important in determining the genetic structure of individual pathogen populations.

Host-pathogen interactions in natural populations of Linum marginale and Melampsora lini. III. Influence of pathogen epidemics on host survivorship and flower production.

SummaryThe epidemiology of rust caused by the fungus Melampsora lini and the effects of infection by this pathogen on its host, the herbaceous perennial Linum marginale, were determined in the field and in garden experiments. There was considerable natural variability in disease levels over the four years (1986–1989) of the study. In two years (1986, 1989) major rust epidemics occurred. In the field, the main effect of disease was to reduce survivorship during the winter following infection. Plants which were heavily infected during the 1986 or 1989 growing seasons had reduced survivorship relative to more lightly infected plants. Melampsora lini infections did not appear to affect survivorship in either 1987 or 1988. Flowering was correlated with environmental factors and the number of stems a plant possessed. A severe drought in 1987 completely inhibited flowering. In the other three years the number of flowers produced by a plant was strongly positively correlated with the number of stems it possessed. Disease levels had no consistent effect on flowering. Controlled garden experiments were also used to examine the response of seedlings and adult plants to infection. These showed that both the timing and severity of disease appears to determine the effect of M. lini infections on L. marginale. Early, severe infection reduced growing season and overwintering survivorship as well as capsule production. However, plants in the field were most often infected only after flowering had begun, and the predominant effect of infection was a reduction in overwintering survivorship. The high variability in disease levels from year to year and the deferred nature of the effect of the rust on its host have significant implications for the design of experiments aimed at assessing the role of diseases in plant communities.

Host-pathogen dynamics in experimental populations of Silene alba and Ustilago violacea.I.Ecological and genetic determinants of disease spread.

1 To investigate the influence of within and among season factors on spread of anther-smut disease (Ustilago violacea), experimental populations of susceptible and resistant plants of (Silene alba) were established in a natural setting. 2 Frequency and density of diseased plants were both important predictors of percentage infection at the end of the growing season, but their relative importance varied across years. 3 Host families that were the progeny of relatively susceptible parents had consistently higher infection levels than families derived from more resistant parents. 4 Late flowering individuals or those producing fewer flowers were at lower risk of becoming infected. Male plants experienced a higher rate of infection because they initiated flowering earlier and produced more flowers than females. 5 There was greater over-wintering mortality of diseased plants than healthy plants in years when overall mortality was low but not in years in which mortality was high. 6 Host plants infected late in the season were more likely to recover than hosts infected early. 7 We conclude that detailed knowledge of within season environmental and genetic factors influencing disease spread does not adequately describe host-pathogen dynamics; epidemiological studies must also consider across season dynamic processes such as disease-induced mortality, host recovery, and recruitment.

Host-pathogen dynamics in experimental populations of Silene alba and Ustilago violacea. II.Experimental tests of theoretical models

1 Experimental populations of Silene alba were established with different total densities, and different frequencies and densities of plants inoculated with anthersmut (Ustilago violacea), a pollinator-transmitted floral disease. 2 Actual changes in the numbers of healthy and infected individuals in the experimental populations were compared with predicted values generated from computer simulations. Four models were tested; these assumed either linear density-dependent, linear frequency-dependent, exponential density-dependent, or exponential frequencydependent disease transmission. Model parameters were estimated from an independent subset of experimental populations and from earlier field studies. 3 Susceptible and resistant populations had qualitatively different dynamics. Results from the experimental populations were consistent with model predictions that resistant populations would lose the disease, while susceptible hosts and pathogens would coexist. 4 Comparison of the actual changes in the numbers of diseased individuals with model predictions, showed that exponential disease transmission models provided a better overall fit to the data than linear models.

Isozyme Analysis of Plant Mating Systems

The last decade has seen a marked increase in the number of genetical studies of plant mating systems. Earlier detailed work had largely been limited to economic plants in which morphological markers were available. Fryxell (1957) comprehensively reviewed this period and compiled an extensive list of all plant species studied. Each species was classified into one of seven classes according to its predominant mode of reproduction. Classification was based either on genetical evidence (from the segregation of morphological marker genes) or on studies of reproductive morphology and development. Table 3.1 gives the total number of taxa in each class as listed by Fryxell, and shows the diversity of plant mating systems, even at this simplified level.

The Acquisition of Hypovirulence in Host-Pathogen Systems with Three Trophic Levels

A major focus of research on the dynamics of host‐pathogen interactions has been the evolution of pathogen virulence, which is defined as the loss in host fitness due to infection. It is usually assumed that changes in pathogen virulence are the result of selection to increase pathogen fitness. However, in some cases, pathogens have acquired hypovirulence by themselves becoming infected with hyperparasites. For example, the chestnut blight fungus Cryphonectria parasitica has become hypovirulent in some areas by acquiring a double‐stranded RNA hyperparasite that debilitates the pathogen, thereby reducing its virulence to the host. In this article, we develop and analyze a mathematical model of the dynamics of host‐pathogen interactions with three trophic levels. The system may be dominated by either uninfected (virulent) or hyperparasitized (hypovirulent) pathogens, or by a mixture of the two. Hypovirulence may allow some recovery of the host population, but it can also harm the host population if the hyperparasite moves the transmission rate of the pathogen closer to its evolutionarily stable strategy. In the latter case, the hyperparasite is effectively a mutualist of the pathogen. Selection among hyperparasites will often minimize the deleterious effects, or maximize the beneficial effects, of the hyperparasite on the pathogen. Increasing the frequency of multiple infections of the same host individual promotes the acquisition of hypovirulence by increasing the opportunity for horizontal transmission of the hyperparasite. This effect opposes the usual theoretical expectation that multiple infections promote the evolution of more virulent pathogens via selection for rapid growth within hosts.

The effect of small-scale environmental changes on disease incidence and severity in a natural plant-pathogen interaction

SummaryThe incidence and severity of Rhynchosporium secalis infections were assessed in a large population of Hordeum leporinum. Transects were set out in four directions from five trees to determine the effect of shading. Under the tree canopy 60.3% of H. leporinum plants were infected while only 11.2% were infected away from the canopy. Disease severity, on those plants which were infected, was higher under the canopy (mean 12.4% and 13.0% leaf area diseased for the flag and first leaves, respectively) than away from the canopy (means of 7.8% and 5.0% for the flag and first leaves respectively). Plants under the tree canopy contained on average 23% more nitrogen, raising the possibility that the susceptibility of the host changed in response to nitrogen levels. However, the observed pattern is also consistent with the hypothesis that shade-associated changes in the environment enhanced the ability of the pathogen to infect and develop on the host. The data clearly demonstrate the importance of small-scale environmental factors on natural host-pathogen interactions. These environmental factors may cause differential selection for disease resistance within a host population, which may ultimately lead to the formation of sub-populations with differing levels of resistance.

Predominance of a single major gene for resistance to Phakopsora pachyrhizi in a population of Glycine argyrea

Four phenotypic patterns of resistance and susceptibility to nine races of Phakopsora pachyrhizi were found among 104 lines derived from seeds collected from 49 plants of a Glycine argyrea population. Genetic analyses suggested that the three most common phenotypic patterns were determined by two alleles at a single locus. The fourth phenotypic pattern was represented by only a single line which was not analyzed genetically. These results contrast with those of a related species, G. canescens, where ten or more resistance genes were detected within each of two populations, and individuals often possessed two or more resistance genes. It is postulated that G. argyreas relatively high rate of outcrossing and the apparent prevalence of forms of resistance which delay the development of the pathogen may have contributed to the small number of major resistance genes in this population.