EFFECTS OF GENETIC DIVERSITY IN EXPERIMENTAL STANDS OF SOLIDAGO-ALTISSIMA - EVIDENCE FOR THE POTENTIAL ROLE OF PATHOGENS AS SELECTIVE AGENTS IN PLANT-POPULATIONS

1 Pathogens are assumed to maintain high genetic diversity and the need for sexual reproduction in plants by continually reducing the fitness of genotypes that become dominant in a population. If this were true, a mixed stand should perform better than the average pure stand of the same plant genotypes. To test this hypothesis I deliberately created thirty-two 16-shoot stands of different genetic diversity of Solidago altissima, a plant species which in the field can form mono- or polyclonal patches, and allowed natural infestations by the mildew Erysiphe cichoracearum to occur. 2 Preliminary experiments revealed significant genetic variation in the source population of S. altissima with regard to pathogen level. However, the pattern of this variation changed between years, indicating that different strains of E. cichoracearum were dominant at different times. 3 In the main experiment pathogen levels at the beginning tended to be higher in more diverse than in less diverse plant stands. As the growing season progressed, levels of E. cichoracearum became higher in the genetically less diverse than in the genetically more diverse stands. 4 Despite the effects on pathogen level, genetic diversity did not significantly influence mean plant performance per stand (height growth and biomass at harvest). However, individual plant performance and pathogen level were positively correlated with each other at low levels of E. cichoracearum and negatively correlated at higher levels. Small plants possibly were in a phenological condition of low susceptibility to pathogen attack. The proportion of above-ground biomass allocated to leaves steadily decreased with increasing pathogen level. 5 The present study suggests that small-scale genetic diversity in the perennial plant Solidago altissima can influence pathogen levels which in turn may affect plant performance and ultimately fitness. I suggest that selection pressures exerted by plant pathogens may play a major role not only in the maintenance of sexual reproduction but also in the maintenance of lateral clonal growth that leads to a mixing of genotypes in polyclonal patches within plant populations and communities.