VARIATION IN DISEASE INCIDENCE IN GRAZED AND UNGRAZED SITES FOR THE SYSTEM PULSATILLA-PRATENSIS PUCCINIA-PULSATILLAE

We have studied the interaction between the perennial host Pulsatilla pratensis and the systemic rust fungus Puccinia pulsatillae. The disease sterilizes plants by inhibiting the production of flowers. Diseased plants were also vegetatively more vigorous, producing more leaves than healthy ones. This enhanced growth of diseased plants is interpreted as being due to a trade-off between loss of reproductive capacity and vegetative growth. A 3-yr demographic study detected a tendency for greater survivorship among diseased P. pratensis plants. This may be due to the increased vigour of infected plants. No herbivore (leaf eating insects or cattle) was observed to consume P. pratensis, but disease levels varied greatly between localities that had been grazed and ungrazed by cattle. On grazed localities low disease levels prevailed while at ungrazed localities disease levels were high. This pattern appears to be due to greater survivorship among diseased plants and the failure of seedlings to establish in ungrazed localities.