Effect of osmotic potential on the germination of conidia and colony growth of Verticillium lecanii

The effects of osmotic potential on the germination of conidia and the radial extension of colonies of three isolates of Verticillium lecanii were studied. Both germination and colony extension decreased with lower (more negative) osmotic potential. Germination was most rapid at −0·2 MPa and severely restricted below −4·2 MPa. Intra-specific differences were observed in germination and only one isolate of V. lecanii (isolate 1.72) achieved a high level of germination (95% in under 12 h) at −2·8 MPa. The extension rate of colonies was also dependent on osmotic potential; the lower limit for growth was −12·3 MPa, with extension severely restricted below −7·0 MPa. When V. lecanii is used as a biological control agent, it is desirable that conidia germinate as quickly as possible after application. Thus intra-specific differences in response to water potential should be considered during the selection of isolates for application in glasshouses.