Population dynamics of a non-cultivated biennial plant Tragopogon pratensis L. infected by the autoecious demicyclic rust fungus Puccinia hysterium (Str.) Röhl

1 Population dynamics of the biennial plant Tragopogon pratensis have been monitored 2 in the Park Grass Experiment at Rothamsted Research, Harpenden, UK, over many 3 years. Observations of diseased T. pratensis, systemically infected by the autoecious 4 demicyclic rust Puccinia hysterium, were made over the period 1995-2008, and 5 confirmed an outbreak pattern of dynamics, characterised by an increase to a 6 relatively high incidence to low almost indiscernible levels. An epidemiological 7 model was developed taking into account the biennial habit of the host plant, and the 8 systemic nature of infection during the winter period, and the partial sterilisation of 9 infected second year plants. Seedling emergence rate and natural mortality between 10 season and within season were key parameters affecting host performance. The 11 transmission rate between infected second year plants and susceptible first year 12 seedlings, and the probability that the fungus would survive the winter systemically as 13 mycelium producing aecia and telia on emerging second year plants, were key 14 parameters associated with pathogenicity. Furthermore the possibility of pathogen15 induced additional mortality was modelled. The model predicted that outbreak 16 dynamics of T. pratensis would occur with high pathogenicity and medium or high 17 host performance. In the former case the population dynamics would be cyclical, with 18 in some cases infected plants going to extinction. In the latter case both host and 19 pathogen would go to extinction. The model predicted that the two pathogenicity 20 parameters were critical in determining whether the pathogen would invade a healthy 21 population; whereas pathogen induced mortality had little influence, a result also 22 obtained in some limited potted plant experiments. Fitting the model to the field data 23 indicated that there was little or no density-dependence in seedling emergence rate, 24 and again that pathogen-induced mortality played little role in the observed population 25 dynamics. 26