INBREEDING DEPRESSION OF PLANT QUALITY REDUCES INCIDENCE OF AN INSECT-BORNE PATHOGEN IN A WILD GOURD Matthew J. Ferrari, 1 Daolin Du, James A. Winsor, and Andrew G. Stephenson Center for Infectious Disease Dynamics, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.; Department of Biology, Hainan Normal University, Haikou 571158, China; Department of Biology, Pennsylvania State University, Altoona, Pennsylvania 16601, U.S.A.; and Department of Biology, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. 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. Keywords: cucumber beetles, disease susceptibility, herbivory, inbreeding depression, mixed mating systems, vector-borne pathogen, Cucurbita pepo, Erwinia tracheiphila. Introduction Self-pollination and inbreeding depression may generate variability in both the susceptibility and the exposure of indi- viduals to pathogens. Susceptibility, the occurrence of disease following challenge with a pathogen, may be affected by both direct and indirect means. The increases in homozygosity that accompany inbreeding may have effects on the many genes that determine pathogen resistance (Thompson and Burdon 1992). Furthermore, inbreeding is generally associated with a reduction in plant vigor (Charlesworth and Charlesworth 1987; Husband and Schemske 1996), which may indirectly affect sus- ceptibility to pathogens by reducing the resources available to fight off infection. Less often considered is the potential ef- fect that inbreeding may have on pathogen exposure through altered interactions with disease vectors (though see Ouborg et al. 2000). Many plant pathogens are vectored by foraging insects; inbreeding effects on plant quality may alter vector foraging behavior, resulting in differential rates of visitation and foraging, which may affect rates of contact with pathogens. In plant systems, individual variation, genetic or otherwise, can result in differential visitation by foraging arthropods (pol- linators and herbivores) that can act as vectors for pathogens. Individuals of Silene latifolia with many flowers receive more visits from pollinators and have higher rates of the pollinator- borne smut fungus Microbotryum violaceum (Alexander 1987; Thrall and Jarosz 1994; Shykoff and Bucheli 1995; Ferrari et al. 2005). Power (1991) found that genetic variability in a population of oats resulted in increased aphid movement and shortened foraging bouts, resulting in reduced transmission of barley yellow dwarf virus. The net effect of self-pollination on interactions with vec- tors may be either positive or negative, as it affects both host quality and resistance to herbivores (Ouborg et al. 2000; Carr and Eubanks 2002). On average, selfing results in detrimen- tal changes in phenotype that affect the preference and per- formance of herbivores (Charlesworth and Charlesworth 1987; Carr and Eubanks 2002). Intuitively, selfing may reduce the quality of host plants, resulting in relatively lower exposure of inbred hosts to the herbivore vectors (Ouborg et al. 2000; Carr and Eubanks 2002). Alternatively, selfing may reduce host plant defensive capabilities and/or lengthen the time that host plants remain in the most vulnerable stages of develop- ment and thereby may result in relatively more herbivory and pathogen exposure (Carr and Eubanks 2002). In this article, we consider the effect of self-pollination on the incidence of the beetle-transmitted pathogen Erwinia tra- cheiphila in the self-compatible, monoecious wild gourd Cu- curbita pepo ssp. texana. Field observation over 4 yr revealed higher incidence of Erwinia in outcrossed plants. We examine this pattern through retrospective analyses of Erwinia incidence in experimental plots of wild gourd, field-scale experimental 1 Author for correspondence; e-mail mferrari@psu.edu. Manuscript received August 2006; revised manuscript received November 2006. 603 Int. J. Plant Sci. 168(5):603–610. 2007. Ó 2007 by The University of Chicago. All rights reserved. 1058-5893/2007/16805-0008$15.00