High Disease Incidence and Apparent Disease Tolerance in a North American Great Basin Plant Community

Pattern and consequences of plant disease at the community level have rarely been studied. We surveyed fungal infections in a Great Basin community of perennial shrubs over 4 years. Repeat surveys in fixed plots and along transects showed that disease incidence in the dominant perennial species was often very high, with up to 100% of all individuals infected. Despite the widespread prevalence of infection, and its severity on individual plants (which sometimes had over 1/3 of their leaves covered in pustules), its effects on survival and flowering were undetectably small. Thus, this perennial community appears to be stable, despite widespread disease. There are two potential explanations for this pattern; either the pathogens have evolved to be avirulent, or the hosts have become tolerant to being infected. Avirulence is not likely, because multiple infections are common in this system, and multiple infections have been shown in other species to favor strains that are faster reproducing and thus more virulent. Instead, it is more likely that tolerance has evolved in these host species, because infection in each year is practically inevitable and because the host plants are long-lived, giving little opportunity for new resistance genotypes to evolve.