Am. Jarosz et Al. Davelos, EFFECTS OF DISEASE IN WILD PLANT-POPULATIONS AND THE EVOLUTION OF PATHOGEN AGGRESSIVENESS, New phytologist, 129(3), 1995, pp. 371-387
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.