Selection of diploid and screening of polyploid guayule lines for Verticillium tolerance

Guayule (Parthenium argentatum Gray) produces a high quality hypoallergenic latex, that is of interest for use in the medical industry. In order to meet the anticipated demand, guayule will need to be planted in mild climatic zones over a wide geographic area, and often following cotton (Gossypium sp.) or other crops susceptible to the root-invading fungus Verticillium dahliae Kleb. Since guayule has been shown previously to be susceptible to V. dahliae, the goals of this work were: (1) to develop a diploid guayule line that is tolerant to V. dahliae through mass selection, and (2) to evaluate improved guayule lines for Verticillium tolerance. To achieve the first goal, a diploid guayule population was subjected to three cycles of selection, with only the plants with the greatest tolerance and the highest rubber yields used in the subsequent cycles of selection. Second, fifteen improved polyploid and three diploid lines were evaluated for tolerance to V. dahliae under greenhouse conditions. Cuttings were found to be more tolerant than seedlings, and tolerance increased with age of the plant at the time of inoculation. The Verticillium screened diploids expressed greater tolerance than an unscreened diploid population in both greenhouse and field trials. Among the 18 improved lines, 13 of which had not been previously evaluated, a Verticillium-screened diploid line (418-6) had the highest tolerance ranking, but was not significantly better than four unscreened polyploid lines. Continued evaluation of improved lines will help growers in making informed decisions about which lines to plant for their specific locations. The Verticillium-tolerant diploid germplasm material developed in this study is available to plant breeding programs.