EFFECTS OF DISEASE IN WILD PLANT-POPULATIONS AND THE EVOLUTION OF PATHOGEN AGGRESSIVENESS

Fungal pathogens can alter host fitness by affecting a plant's ability to survive, reproduce, compete, grow or defend itself against herbivo res and other parasites. However, infections need not have uniformly n egative effects, and the outcome of a fungus-plant interaction can ran ge from strongly parasitic, through commensalistic, to mutualistic. Th e outcome of the interaction is determined by characteristics of the f ungus and plant, as well as ecological conditions. Damping off and roo t rot diseases severely reduce plant survivorship. These pathogens app ear to be highly aggressive, a trait that may be evolutionarily constr ained because of the necrotrophic infection habit. Incidence is locall y high with patch size being variable. Canker and wilt pathogens reduc e plant survivorship and growth, and tend to be highly aggressive. Two diseases within this group, chestnut blight and Dutch elm disease, ha ve caused sustained highly destructive pandemics on their hosts. Neith er pathogen appears to be evolving towards genetically based reduced a ggressiveness despite the fact that disease incidence is near 100% ove r a wide area. Less aggressive strains may not be selectively favoured because plants can be multiply infected, which would allow more aggre ssive strains to invade trees previously infected by less aggressive s trains. Obligate foliar diseases can affect plant fitness by reducing survivorship, reproduction, growth or competitive ability of the host. However, overall effects are often slight and the relationship betwee n plant and pathogen range from strongly parasitic to nearly commensal istic. Disease incidence is highly variable across space and time and much of the variability appears to be influenced by climatic and envir onmental variables. Non-systemic foliar disease reduce plant reproduct ion, but some interactions may be nearly commensalistic because of low seed set in healthy individuals. Thus, disease effects are minimized in environments where reproduction by seed is not important. Systemati cally infecting pathogens can alter plant survivorship, reproduction, growth, competitive ability and susceptibility to herbivores. The effe cts can be positive as well as negative, and the net outcome of the in teraction can range from strongly parasitic to unconditionally mutuali stic. Trends within this group support Clay's New Function Hypothesis that pathogens can reduce their aggressiveness by acquiring new functi ons which increase plant fitness, and not by the amelioration of the o riginal disease symptoms.