Fungal pathogens can alter host fitness by affecting a plant's ability to survive, reproduce, compete, grow or defend itself against herbivores and other parasites. However, infections need not have uniformly negative effects, and the outcome of a fungus-plant interaction can range from strongly parasitic, through commensalistic, to mutualistic. The outcome of the interaction is determined by characteristics of the fungus and plant, as well as ecological conditions.
Damping off and root rot diseases severely reduce plant survivorship. These pathogens appear to be highly aggressive, a trait that may be evolutionarily constrained because of the necrotrophic infection habit. Incidence is locally high with patch size being variable. Canker and wilt pathogens reduce plant survivorship and growth, and tend to be highly aggressive. Two diseases within this group, chestnut blight and Dutch elm disease, have caused sustained highly destructive pandemics on their hosts. Neither pathogen appears to be evolving towards genetically based reduced aggressiveness despite the fact that disease incidence is near 100 % over a wide area. Less aggressive strains may not be selectively favoured because plants can be multiply infected, which would allow more aggressive strains to invade trees previously infected by less aggressive strains. 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 between plant and pathogen range from strongly parasitic to nearly commensalistic. Disease incidence is highly variable across space and time, and much of the variability appears to be influenced by climatic and environmental variables. Non-systemic foliar diseases reduce plant reproduction, 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. Systemically infecting pathogens can alter plant survivorship, reproduction, growth, competitive ability and susceptibility to herbivores. The effects can be positive as well as negative, and the net outcome of the interaction can range from strongly parasitic to unconditionally mutualistic. Trends within this group support Clay's New Function Hypothesis that pathogens can reduce their aggressiveness by acquiring new functions which increase plant fitness, and not by the amelioration of the original disease symptoms.