Lack of trade-off between indirect defense mediated through defense mutualism and direct defense against grape powdery mildew in riverbank grape.
English-Loeb, Greg*,1, Norton, Andrew2, 1 Cornell University, Geneva, NY2 Colorado State University, Fort Collins
ABSTRACT- In previous work we found evidence of a defense mutualism in the riverbank grapevine Vitis riparia involving tuft-form domatia, non-glandual trichomes present in vein axils on the underside of leaves, and a mycophagous mite Orthotydus lambi, (Tydeidae). Domatia provide shelter for tydeid mites and the mites suppress grape powdery mildew (GPM), an important foliar and fruit disease of grapevines caused by the fungus Uncinula necator. In this study we tested the hypothesis that V. riparia genotypes that invest more in indirect resistance via larger domatia will invest less in direct mechanisms of resistance, a pattern that has been shown to occur in ant-plant defense mutualisms. Multiple cane cuttings from 24 accessions of V. ripara maintained at the USDA grape repository in Geneva, NY were rooted, placed in 4-liter pots, and grown outdoors. Half of the replicate vines were kept free of tydeid mites using an acaracide and were inoculated with GPM conidia. The other half of the vines were kept free of GPM using a fungicide and were infested with tydeid mites. Genotypes varied significantly in size of domatia on leaves (P = 0.025) and density of tydeid mites (P = 0.001). There was a significant positive correlation between size of domatia and density of tydeid mites (P = 0.005). Genotypes also varied significantly in direct resistance to GPM (absence of tydeid mites) (P = 0.02). We did not detect, however, a relationship (positive or negative) between level of direct resistance to GPM and size of domatia among the 24 genotypes (r = 0.03, P = 0.79). Hence, we found no evidence of a trade-off between investment in a defense mutualism involving domatia and mites and investment in direct defense mechanisms in this system unlike more tightly co-evolved ant-plant defense mutualisms.
Key words: defense mutualism, host plant resistance, plant pathogen, plant defense
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