LACK OF ASSOCIATION BETWEEN TOLERANCE TO THE ELM PHYTOALEXIN MANSONONE-E AND VIRULENCE IN OPHIOSTOMA-NOVO-ULMI

The effect of the elm phytoalexin, mansonone E, on linear growth of 17 fungal species was examined to determine whether the Dutch elm diseas e fungus Ophiostoma novo-ulmi is more tolerant of mansonone E than oth er fungi. Linear growth of O. novo-ulmi was less inhibited by mansonon e E than that of most, but not all, other fungi examined, suggesting t hat O. novo-ulmi is relatively tolerant of mansonone E. To determine w hether this tolerance is required for pathogenicity, we generated muta nts of O. novo-ulmi with reduced tolerance to mansonone E by N-methyl- N'-nitro-N-nitrosoguanidine mutagenesis followed by a screen for reduc ed tolerance to 1,2-naphthoquinone, a compound similar in structure to mansonones. Reduced tolerance to 1,2-naphthoquinone and mansonone E w as accompanied by reduced tolerance to another elm phytoalexin, manson one F. Two mutants exhibited wild-type levels of virulence on three el ms, while three other mutants exhibited markedly reduced virulence com pared with the wild-type field isolate from which they were derived. W hen the weakly virulent mutants were crossed with wild-type isolates, progeny that had wild-type levels of virulence but low tolerance to ma nsonone E were recovered. These results suggest that although O. novo- ulmi is relatively tolerant of mansonone E, this tolerance may not be required for high levels of virulence on elm.