High disease incidence and apparent disease tolerance in a North American Great Basin plant community

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