EFFECT OF ARTIFICIAL EXCLUSION AND AUGMENTATION OF FUNGAL PLANT-PATHOGENS ON A REGENERATING GRASSLAND

Plots of rough grassland dominated by Holcus lanatus L. were cleared o f vegetation in three successive years and allowed to regenerate witho ut grazing or harvesting. Vegetation cover and foliar disease severity were measured using pin sampling in each plot, two to four times per year. One third of plots were sprayed regularly with propiconazole to reduce fungal disease; one third of plots were sprayed with rust (Pucc inia coronata f. sp. holci Corda.) urediospores in the second and thir d years of the study and with conidia of the leaf-spotting fungus Asco chyta leptospora (Trail) Hara. in the third year. The fungicide had di rect effects on the growth of perennial herbs and grasses, and indirec t effects through disease control on perennial herbs. Both fungicide t reatment and disease augmentation tended to raise diversity in plots w here grasses were only partly dominant. Fungicides increased diversity because both direct and indirect effects were greater in perennial he rbs than in grasses. Disease augmentation raised diversity because the diseases introduced were more specific to the most common component o f the vegetation, H. lanatus.