The effect of small-scale environmental changes on disease incidence and severity in a natural plant-pathogen interaction.

The incidence and severity of Rhynchosporium secalis infections were assessed in a large population of Hordeum leporinum. Transects were set out in four directions from five trees to determine the effect of shading. Under the tree canopy 60.3% of H. leporinum plants were infected while only 11.2% were infected away from the canopy. Disease severity, on those plants which were infected, was higher under the canopy (mean 12.4% and 13.0% leaf area diseased for the flag and first leaves, respectively) than away from the canopy (means of 7.8% and 5.0% for the flag and first leaves respectively). Plants under the tree canopy contained on average 23% more nitrogen, raising the possibility that the susceptibility of the host changed in response to nitrogen levels. However, the observed pattern is also consistent with the hypothesis that shade-associated changes in the environment enhanced the ability of the pathogen to infect and develop on the host. The data clearly demonstrate the importance of small-scale environmental factors on natural host-pathogen interactions. These environmental factors may cause differential selection for disease resistance within a host population, which may ultimately lead to the formation of sub-populations with differing levels of resistance.