INHERITANCE OF BIOCHEMICAL AND MORPHOLOGICAL CHARACTERS ASSOCIATED WITH 2-SPOTTED SPIDER-MITE RESISTANCE IN PELARGONIUM X HORTORUM

Diploid zonal geraniums (Pelargonium xhortorum) are able to resist att ack by small arthropod pests such as the two-spotted spider mite (Tetr anychus urticae Koch) when exudate produced by tall glandular trichome s contains a high percentage of omega(5)-unsaturated anacardic acids, Trichomes of susceptible plants exude primarily saturated anacardic ac ids. Inbred mite-resistant and -susceptible geraniums were reciprocall y crossed and the F-1, F-2, and backcross generations were examined fo r anacardic acid composition and trichome density. Selected F-2 plants were bioassayed for resistance to two-spotted spider mites. High conc entrations of omega(5)-unsaturated anacardic acids in resistant plants are conditioned by a single dominant allele. We propose that inherita nce of tall glandular trichome density can be controlled by a small nu mber of loci (possibly as few as one) exhibiting codominance. F-2, wit h low densities of tall glandular trichomes and producing omega(5)-uns aturated anacardic acids, displayed effective resistance to two-spotte d spider mites as measured by mite mortality and fecundity. A genetic model for the biosynthesis of anacardic acids is proposed.