• invasion thresholds;
  • biological control;
  • percolation;
  • Rhizoctonia solani;
  • epidemiology


  • • 
    The ability to forecast invasion of harmful and beneficial organisms is becoming increasingly important in agricultural and horticultural production systems as well as in natural plant communities.
  • • 
    In this paper we examine the spread of a fungus through a population of discrete sites on a lattice, using replicable, yet stochastically variable experimental microcosms.
  • • 
    We combine epidemiological concepts to summarise fungal growth dynamics with percolation theory to derive and test the following hypotheses: first fungal invasion into a population of susceptible sites on a lattice can be stopped by a threshold proportion of randomly removed sites; second random removal of susceptible sites from a population introduces a shield which can prevent invasion of unprotected sites; and third the rate at which a susceptible population is invaded reduces with increasing number of randomly protected sites.
  • • 
    The broader consequences of thresholds for fungal invasion in natural and agricultural systems are discussed briefly.