Antimicrobial nectar inhibits a florally transmitted pathogen of a wild Cucurbita pepo (Cucurbitaceae).

PREMISE OF THE STUDY: Floral nectars of many species contain antimicrobial chemicals, but their function in nectar is subject to debate. Previously, we have shown that Erwinia tracheiphila, the causative agent of bacterial wilt disease in cucurbits, can be transmitted via the floral nectaries. • METHODS: We used a disk diffusion assay (DDA) to determine the antimicrobial effects of nectar from a wild gourd on lawns of Escherichia coli and Erwinia tracheiphila. We also used E. tracheiphila to inoculate flowers of wild gourd plants, with and without nectar. • KEY RESULTS: The DDA showed that paper disks saturated with 10 µL of nectar inhibited the growth of E. coli on a larger area of the lawn than 40% glucose but a smaller area than 5% ampicillin for 12 h. On lawns of E. tracheiphila, nectar inhibited growth on a larger area than glucose for 24 h and there were no significant differences between ampicillin and nectar for12 h. A significantly larger proportion of the plants inoculated via flowers without nectar contracted wilt disease than plants with nectar. • CONCLUSIONS: These findings indicate that nectar reduces transmission of E. tracheiphila via the nectaries and reveal the potential for florally transmitted pathogens to influence the evolution of floral traits.

Indirect costs of a nontarget pathogen mitigate the direct benefits of a virus-resistant transgene in wild Cucurbita.

Virus-resistant transgenic squash are grown throughout the United States and much of Mexico and it is likely that the virus-resistant transgene (VRT) has been introduced to wild populations repeatedly. The evolutionary fate of any resistance gene in wild populations and its environmental impacts depend upon trade-offs between the costs and benefits of the resistance gene. In a 3-year field study using a wild gourd and transgenic and nontransgenic introgressives, we measured the effects of the transgene on fitness, on herbivory by cucumber beetles, on the incidence of mosaic viruses, and on the incidence of bacterial wilt disease (a fatal disease vectored by cucumber beetles). In each year, the first incidence of zucchini yellow mosaic virus occurred in mid-July and spread rapidly through the susceptible plants. We found that the transgenic plants had greater reproduction through both male and female function than the susceptible plants, indicating that the VRT has a direct fitness benefit for wild gourds under the conditions of our study. Moreover, the VRT had no effect on resistance to cucumber beetles or the incidence of wilt disease before the spread of the virus. However, as the virus spread through the fields, the cucumber beetles became increasingly concentrated upon the healthy (mostly transgenic) plants, which increased exposure to and the incidence of wilt disease on the transgenic plants. This indirect cost of the VRT (mediated by a nontarget herbivore and pathogen) mitigated the overall beneficial effect of the VRT on fitness.