Differential production of extracellular laccase in the Dutch elm disease pathogens Ophiostoma ulmi and O-novo-ulmi

Extracellular laccase production of the Dutch elm disease pathogens Ophiostoma ulmi and O. novo-ulmi was monitored during growth in vitro. Under conditions conducive to mycelial growth, O. novo-ulmi secreted appreciable amounts of laccase (0 . 26+/-0 . 06 U ml(-1)) in parallel with growth whereas laccase activity in O. ulmi was barely detectable (less than or equal to 0 . 02 U ml(-1)). In contrast yeast phase cells of both Dutch elm disease fungi produced little or no detectable laccase. These findings were not significantly influenced by the temperature. For O. novo-ulmi laccase production was significantly affected by pH. Levels of laccase were 2 . 5 fold lower at pH 4 . 0 and 7 . 0 compared with pH 5 . 5 (P < 0 . 01). No significant difference in enzyme secretion could be observed between Eurasian (EAN) and North American (NAN) race isolates of O. novo-ulmi (0 . 27+/-0 . 06 and 0 . 24 +/- 0 . 07 U ml(-1) respectively). This differential production of laccase provides a further example of the extensive physiological differences between O. ulmi and O. novo-ulmi.