Inheritance of biochemical and morphological characters associated with two-spotted spider mite resistance in Pelargonium xhortorum

Diploid zonal geraniums (Pelargonium x 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.