SPREAD OF ANTHER-SMUT DISEASE (USTILAGO-VIOLACEA) AND CHARACTER CORRELATIONS IN A GENETICALLY VARIABLE EXPERIMENTAL POPULATION OF SILENE ALBA

1 The anther-smut fungus Ustilago violacea produces spores in diseased flowers of the dioecious plant Silene alba; spores are transferred to healthy plants by insect pollinators. 2 To study the ecological and g enetic determinants of disease spread and its demographic consequences , an experimental population of S. alba was created that contained bot h anther-smut inoculated plants (as an inoculum source) and uninoculat ed plants which were the progeny of crosses between genotypes with hig h or low disease resistance. 3 Genetic analyses confirmed that variati on in resistance is heritable. 4 Total disease incidence increased ove r the 3-year study, but rates of establishment of new infections decre ased and estimates of the latent period increased each year. By the en d of the study, the remaining healthy plants were more likely to be fr om resistant parents, suggesting that changes in genetic structure of the healthy plants may contribute to the decline in new infections ove r time. 5 Diseased plants were significantly more likely than healthy plants to die during the first winter of the study but there was no di fference in the second year. A compilation of several data sets sugges ts that diseased plants have higher mortality than healthy plants only in winters with overall high plant survivorship. 6 Phenotypic correla tions revealed that plants with early flowering dates and high rates o f flower production are more likely to become diseased. 7 There was ev idence for a genetic correlation between disease resistance and flower ing date in males, such that early flowering families were more prone to infection. Such a relationship suggests that resistance could have a fitness cost, since earlier flowering plants are likely to have high fitness in the absence of disease.