Host Resistance

*Note: the following information is for descriptive purposes only. Final, peer-reviewed results of research are not posted.

Investigating pathogen host resistance is typically associated with plant breeding, where researchers work to pass resistance traits on to crop plants. I am interested in the opposite perspective where host resistance impedes successful biological control.

Thistle infestations from former field sites were sampled to investigate if host resistance plays a part in P. punctiformis epidemiology and to quantify the potential for successful control applications. Preliminary resistance experiments performed in the greenhouse showed differential disease susceptibility between different populations of Canada thistle.

Average shoot numbers per thistle population (L) Average diseased shoot numbers per thistle population (R)

The experiment was expanded to include phylogenetic relationships between thistle samples in addition to resistance results. A strategic sampling method was employed to allow for comparisons of diversity between and within patches and sites. Three sites with naturally established Canada thistle infestations were selected and ten root accessions were collected along a a transect from each of ten patches at each site. Clones were propagated for resistance experiments and DNA was extracted to genotype samples based on microsatellite markers. The patch composition of thistle clones are being estimated, elucidating the comparative contributions of seed and vegetative reproduction to Canada thistle spread. The diversity of patches along with the possibility of susceptible and resistant genotypes within the same patch have important implications for the deployment of P. punctiformis inoculum. Biological control would not be effective overall if the patch contained a mixture of susceptible and resistant genotypes, because the resistant plants could quickly reinvade space created by the removal of susceptible genotypes.

Patch transect sampling to determine clonal composition (L) Greenhouse propagation of thistle clones (R)

Host resistance and diversity data will be paired with surveys of Canada thistle spatial distribution. The three field sites used for resistance and diversity surveys were sampled on a uniform grid at 30m intervals to generate thistle maps and calculate parameters for patchiness, variance and mean plant densities. The spatial distribution of susceptible and resistant Canada thistle will be combined with infection requirements and pathogen dispersal data to develop a comprehensive, predictive, epidemiological model. This model will combine the complex factors required for P. punctiformis epidemics. It will be useful from an applied perspective to determine the most efficient use of P. punctiformis inoculum to produce landscape-wide control of Canada thistle. An understanding of the spatial environment of susceptible host plants and pathogen potential will make this model a useful tool for biological control decision making.

Density map of Canada thistle distribution in a field site