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 disease fungus Ophiostoma novo-ulmi is more tolerant of mansonone E than other fungi. Linear growth of O. novo-ulmi was less inhibited by mansonone E than that of most, but not all, other fungi examined, suggesting that O. novo-ulmi is relatively tolerant of mansonone E. To determine whether this tolerance is required for pathogenicity, we generated mutants of O. novo-ulmi with reduced tolerance to mansonone E by N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis followed by a screen for reduced tolerance to 1,2-naphthoquinone, a compound similar in structure to mansonones. Reduced tolerance to 1,2-naphthoquinone and mansonone E was accompanied by reduced tolerance to another elm phytoalexin, mansonone F. Two mutants exhibited wild-type levels of virulence on three elms, while three other mutants exhibited markedly reduced virulence compared with the wild-type field isolate from which they were derived. When the weakly virulent mutants were crossed with wild-type isolates, progeny that had wild-type levels of virulence but low tolerance to mansonone 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.