Inbreeding depression of plant quality reduces incidence of an insect-borne pathogen in a wild gourd

The distribution of infectious diseases within populations can be characterized in terms of variation in the susceptibility and exposure of individual hosts. In general, selfing has been associated with an increase in susceptibility to pathogens, though selfing effects on plant quality may further affect pathogen exposure due to foraging insects. We observed lower incidence of a beetle-vectored bacterium, Erwinia tracheiphila, in self-fertilized wild gourds (Cucurbita pepo ssp. texana) over 2 yr of field-scale epidemics in 50 : 50 mixtures of selfed and outcrossed wild gourds. Subsequent inoculation experiments revealed no significant association between inbreeding and susceptibility, suggesting differential exposure in selfed and outcrossed plants as an explanation for the observed pattern of incidence. Selfed C. pepo tend to be smaller and produce fewer flowers and fruits, which are attractive to the specialist beetles that vector Erwinia. We experimentally manipulated plant size by transplanting first-generation selfed and outcrossed seedlings to the field on three staggered dates and found that larger plants had higher incidence of Erwinia regardless of inbreeding. We conclude that vector selection of larger plants leads to increased exposure of outcrossed plants. Thus, disease mortality risk due to vector behavior is a potential cost to outcrossed plants.