COSTS AND BENEFITS OF PLANT RESPONSES TO DISEASE: RESISTANCE AND TOLERANCE.

A major assumption of models of the evolution of plant resistance to disease is that plant resistance involves fitness costs. To test this assumption, a field experiment was performed so that a quantitative-genetic analysis could be used to detect fitness costs to Ipomoea purpurea of resistance to different fungal isolates of Colletotrichum dematium, a pathogenic fungus causing the disease anthracnose. This experiment yielded no evidence that resistance to anthracnose involves direct fitness costs. Nevertheless, trade-offs in plant fitness that were unrelated to resistance were detected between different disease environments. Tolerance, defined as the ability to compensate in part for fitness decrements caused by disease, was found to involve fitness costs. Halfsib families that were more tolerant of disease had lower fitness in the absence of disease. The possibility that the cost of tolerance could obscure fitness costs of resistance is explored.