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 (p haseolinone) from Macrophomina phaseolina for rapid and effective scre ening procedures for charcoal rot resistance in guayule (Parthenium ar gentatum) germ-plasm was assessed. The CFCF and partially purified pha seolinone were incorporated into modified Murashige and Skoog solid me dium at the rates of 0-100% (v/v) and 0-1000 mu g ml(-1), respectively . The medium pH was adjusted to 5.8 before solidifying with 0.8% agar. Four-week-old seedlings of 10 guayule genotypes were planted in the m edium, incubated and rated for phytotoxic symptoms and tissue damage o ver a 15-day period. In a greenhouse study, seedling growth, phytotoxi city and damage severity were compared in 12-week-old guayule seedling s root-inoculated with M. phaseolina microsclerotia. There were signif icant differences (P=0.05) in genotypic responses to the fungus, the f iltrate and the toxin inoculations. Time until phytotoxic symptoms dev eloped was inversely related to the concentrations of CFCF and the tox in. Phytotoxic symptoms were produced 6 days after exposure to 50% CFC F and 48 h after exposure to 1000 mu g ml(-1) of partially purified ph aseolinone. A comparison of photomicrographs of the control and toxin- treated root tissues revealed no damage to the control roots and exten sive damage to epidermal layers of the treated roots, which was eviden t 48 h after exposure to 100 mu g ml(-1) level of phaseolinone. Signif icant correlations were found between tolerance to the fungus and inse nsitivity 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 resis tance to M. phaseolina using CFCF or phaseolinone. The genotypic react ions 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 o r the toxin whereas 'Ca16', 'Ca17', 'N576', 'N9-5', '11605' and 'N6-5' were very susceptible to the pathogen and sensitive to the CFCF or th e toxin.