Inheritance of Biochemical and Morphological Characters Associated with Two-spotted Spider Mite Resistance in Pelargonium ×hortorum

in Journal of the American Society for Horticultural Science
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  • 1 Intercollege Graduate Program in Genetics, The Pennsylvania State University, University Park, PA 16802
  • | 2 DEKALB Plant Genetics, 62 Maritime Drive, Mystic, CT, 06355
  • | 3 Department of Entomology, The Pennsylvania State University, University Park, PA 16802
  • | 4 Schering Plough, 2015 Galloping Hill Road, Kenilworth, NJ 07233
  • | 5 Department of Entomology, The Pennsylvania State University, University Park, PA 16802
  • | 6 Department of Biology, The Colorado State University, Fort Collins, CO 80523
  • | 7 Department of Horticulture, The Pennsylvania State University, University Park, PA 16802
  • | 8 Department of Entomology, The Pennsylvania State University, University Park, PA 16802

Diploid zonal geraniums (Pelargonium ×hortorum) are able to resist attack by small arthropod pests such as the two-spotted spider mite (Tetranychus urticae Koch) when exudate produced by tall glandular trichomes contains a high percentage of ω5-unsaturated anacardic acids. Trichomes of susceptible plants exude primarily saturated anacardic acids. Inbred mite-resistant and -susceptible geraniums were reciprocally crossed and the F1, F2, and backcross generations were examined for anacardic acid composition and trichome density. Selected F2 plants were bioassayed for resistance to two-spotted spider mites. High concentrations of ω5-unsaturated anacardic acids in resistant plants are conditioned by a single dominant allele. We propose that inheritance of tall glandular trichome density can be controlled by a small number of loci (possibly as few as one) exhibiting codominance. F2, with low densities of tall glandular trichomes and producing ω5-unsaturated anacardic acids, displayed effective resistance to two-spotted spider mites as measured by mite mortality and fecundity. A genetic model for the biosynthesis of anacardic acids is proposed.

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