Differential Responses of Guayule (Parthenium argentatum Gray) Genotypes to Culture Filtrate and Toxin from Macrophomina phaseolina (Tassi) Goidanich

The potential for using cell-free culture filtrate (CFCF) and toxin (phaseolinone) from Macrophomina phaseolina for rapid and effective screening procedures for charcoalrot resistance in guayule (Parthenium argentatum) germplasm was assessed. The CFCF and partially purified phaseolinone were incorporated into modified Murashige and Skoog solid medium aat the rates of 0-100% (v/v) and 0-1000 μg ml^-1 respectively. The medium pH was adjusted to 5.8 before solidifying with 0.8% agar. Fourweek-old seedlings of 10 guayule genotypes were plantanted in the medium, incubated and rated for phytotoxic symptoms and tissue damage over a 15-day period. In a green-house study, seedling growth, phytotoxicity and damage severity were compared in 12-week-old guayule seedlings root-inoculated with M. phaseolina microsclerotia. There were significant differences (P = 0.05) in genotypic responses to the fungus, the filtrate and the toxin inoculations. Time until phytotoxic symptoms developed was inversely related to the concentrations of CFCF and the toxin. Phytotoxic symptoms were produced 6 days after exposure to 50% CFCF and 48 h after exposure to 1000μg ml^-1 of partially purified phaseolinone. A comparison of photomicrographs of the control and toxintreated root tissues revealed no damage to the control roots and extensive damage to epidermal layers of the treated roots, which was evident 48 h after exposure to 100μg ml^-1 level of phaseolinone. Significant correlations were found between tolerance to the fungus and insensitivity to the culture filtrate (r = 0.89, P = 0.05) and the toxin (r = 0.95, P = 0.001) suggesting the possibility of screening for resistance to M. phaseolina using CFCF or phaseolinone. The genotypic reactions to the CFCF were also correlated with reactions to the toxin (r = 0.90, P = 0.05). Guayule breeding lines‘UC101 and‘P3-1 exhibited the greatest tolerance to the pathogen and insensitivity to the CFCF or the toxin whereas‘Ca16′,‘Cal7′,‘N576′,‘N9-5′, 11605’and‘N6-5’were very susceptible to the pathogen and sensitive to the CFCF or the toxin.