Plant population heterogeneity and pathogen and herbivore levels: a field experiment.

The identity of neighbouring plants in a population is thought to affect the disease levels of an individual plant: disease levels are expected to be higher for susceptible plants surrounded by other susceptible plants as compared to susceptible plants that have resistant plants as neighbours. To explore this idea, plants in a natural population of the annual sunflower, Helianthus annuus, were scored in 1988 for levels of the rust pathogen Puccinia helianthi. Progeny of plants with high rust levels (families predicted to be susceptible to the disease) were grown in 1989 in a series of field plots at either high frequency (all plants in a plot of one family) or in low frequency (a plot with plants of many families). There was a positive correlation between rust levels of the parent plants in 1988 and their progeny in 1989, but the frequency of a family in the experimental plots did not affect rust levels. Generally low rust levels at the site, as well as other factors, probably contributed to the absence of a frequency effect. Herbivore levels and plant survival and reproduction varied among plots, but were not affected by the plot composition.

Anther-smut infection of Silene alba caused by Ustilago violacea: factors determining fungal reproduction.

The anther-smut fungus Ustilago violacea sporulates in flowers of the dioecious host plant Silene alba. Growth chamber comparisons of healthy and diseased plants, with the genetic background of host and pathogen controlled, revealed that fungal infection increases the number of flowers produced per plant and decreases the size of individual flowers. There were few consistent effects of plant genotype or fungal isolate on diseased flower traits, but differences between the plant sexes were apparent. Stimulation of flower production is proportionally greater in females than males: thus, although healthy male plants produce many more flowers than healthy females, sexual differences in diseased flower number are reduced. Sexual differences in diseased flower size also exist, with male flowers smaller than females. A field inoculation study confirmed dimorphism in diseased flower size and demonstrated that spore production per flower was greater for males than females for all flower size classes.

AN EXPERIMENTAL FIELD STUDY OF ANTHER-SMUT DISEASE OF SILENE ALBA CAUSED BY USTILAGO VIOLACEA: GENOTYPIC VARIATION AND DISEASE INCIDENCE.

Twenty cloned genotypes of Silene alba differed greatly (0-100%) in the percentage of flowering plants that became diseased by the anther-smut fungus Ustilago violacea following natural spore dispersal in a two-year field experiment. Male genotypes with the highest percentage of disease had high rates of flower production; this trait may increase the probability of spore deposition on flowers, a common site of infection. Because of this relationship, male genotypes with the highest percentage of disease also produced the most healthy flowers in the two-year period. Flowering early in the season was also a predictor of high disease levels for male genotypes in the first year. Variation among female genotypes in disease levels was not correlated with either flower production or phenology, suggesting that the sexes differ in their interaction with the pathogen. Plants of both sexes that remained nonreproductive the first year but flowered the second year could become diseased due to infection of vegetative tissue. Disease levels of the genotypes following natural spore dispersal were not correlated with disease levels of the genotypes following inoculation of vegetative tissue. This discrepancy points out that the methodology used to investigate genetic variation in disease resistance may affect the results obtained.

Intraspecific variation in the response to CO2 enrichment in seeds and seedlings of Plantago lanceolata L.

Four genotypes of P. lanceolata were grown to maturity at combinations of two levels of atmospheric CO2 concentrations and two temperature conditions. Seed weight was determined, and seed germination and seedling growth were measured for the progeny of each genotype under the same environmental conditions. Overall, high CO2 levels decreased seed weight, increased germination percentage and rate, and increased seedling size. Families differed in their response to CO2 enrichment, and to combinations of CO2 and temperature levels for several characters. These results suggest the existence of genetic variability in P. lanceolata in response to CO2 enrichment.

Spatial patterns of disease induced by Fusarium moniliforme var. subglutinans in a population of Plantago lanceolata.

At low densities, Plantago lanceolata was less likely to have the inflorescence disease caused by the fungus Fusarium moniliforme var. subglutinans. Where the disease was found, as host density increased, the proportion diseased remained constant or decreased. The pathogen's inability to exploit high host densities may be due to both the long disease cycle and the reduced efficiency of disease spread in heterogeneous habitats. Diseased plants were randomly spaced early in the summer but were later found in clusters, probably as a result of localized spore dispersal. Spatial patterns of disease levels remained constant from year to year.

The effect of disease induced by Albugo candida (white rust) and Peronospora parasitica (downy mildew) on the survival and reproduction of Capsella burna-pastoris (shepherd's purse).

Disese induced by Albugo candida or Peronospora parasitica was found to have significant effects on the survival and reproductive output of affected individuals of Capsella bursa-pastoris. The time of infection by either pathogen was of considerable importance in determining the ultimate effect of disease. Systemic (primary) infection of seedling plants led to a high degree of mortality (88%) prior to reproductive maturity, however, localized or systemic secondary infection did not affect survival. Fruit production of surviving infected plants was negatively correlated with disease severity.

Relationship of phenotypic and genetic variation in Plantago lanceolata to disease caused by Fusarium moniliforme var. subglutinans.

Naturally established individuals of Plantago lanceolata with the inflorescence disease caused by Fusarium moniliforme var. subglutinans had more inflorescences and were more likely to be male-sterile than healthy plants. Half-sib families planted in the field varied in the percentage of diseased plants, the number of inflorescences per plant, the incidence of male-sterility, and the pattern of inflorescence phenology. The rankings of families with respect to disease incidence was, however, not simply related to their reproductive phenotypes. Plants derived from field genotypes with a history of disease were slightly more likely to become diseased than plants derived from healthy genotypes. Inflorescence infection was more severe on plants derived from genotypes with a known history of disease. Since the fungus reduces seed production in the plants it infects, differential incidence of disease based on plant phenotype and genotype may have ecological and evolutionary consequences for the host population.