World Journal of Microbiology and Biotechnology 7. 494-501 Effects of temperature, pH and water po- tential on biomass production or hyphal ex- tenslon of Gllocladlum vlrens (620) and three Trichoderma Isolates were deter- mlned in v&o. Optlmum blomass productlon occurred between 20 and 30°C and at pH ranges between 4.6 and 6.6. Two isolates of 7. v/ride grew at 5°C and 0. v&ens grew at 35°C but no isolates grew at 40°C. Hyphal extension rates and conldlal germination of all fungi declined with decreasing water potential over the range -0.7 to -14.0 MPa. In general, growth rates for each Isolate were lower on potato/dextrose agar with water potential adjusted with poly- ethylene glycol than when adjusted with NaCl or glycerol. No mycelial growth or spore germination occurred on agar at -14.0 MPa. The authors are with the Microbiology and Crop Protection Department, Horticulture Research international, Littlehampton, West Sussex BN17 6LP, UK. J.M. Whipps is the corresponding author @ 7991 Rapid Communications of Oxford Ltd. All antagonists were maintained on potato/dextrose agar (PDA; Oxoid CM Effects of temperature, pH and water potential on growth of four fungi with disease biocontrol potential A.M. Jackson, J.M. Whipps and J.M. Lynch A&m white rot (AWR) disease of onion and related crop species caused by Sclerotium cepivowm Berk. results in severe crop losses in many parts of the world (Utkhede & Rahe 1980). The problem is increasing owing to the lack of reliable methods of cultural control and the loss of dicarboximide fungicide effectiveness (Entwistle 1986; Walker et al. 1986). There is thus considerable potential for biological control of this disease. Very few biological control agents are commercially available for the control of plant pathogens (Ayers & Adams 1981; Chet et al. 1984; Kerr & Tate 1984; Whipps 1986; Lynch 1988). In part this is due to erratic or incomplete levels of control. For example, a number of Buci& isolates reduced AWR in initial field trials but failed to reduce disease in subsequent years (Utkhede & Rahe 1983; Rahe 1986). Consequently, biological control has often been considered to be less effective and a greater risk than chemical control. This could perhaps be improved if physiological and ecological attributes of potential biocontrol organisms were determined before field use. This would enable predictions of their ability to grow and survive in soil to be made and assist in improving inoculum production. A series of in vitro screens have detected four fungi with potential biological control activity against AWR (Jackson et al. 1991a). Methods of inoculum production and some physiological growth parameters have been examined already (Jackson et al. 1991 b,c,d) and this paper extends these studies to the effects of temperature, pH and water potential on growth and spore germination. Materials and Methods Fmgal Isolates The antagonists used in this study were: Gliocladiwn virens Miller, Giddens & Foster (GZO) which was provided by R.D. Lumsden, Beltsville, USA; Trichderma viride Pers.ex Fr. (IMI 322659) which was isolated from a sclerotium of Sclerotiniu sclerotiorwn Lib. de Bary; T. viride Pers. ex Fr. (IMI 322663) and T. pseudokoningii Rifai (IMI 322662) which were isolated from Allirrm white-rot-diseased onion bulbs. 494 World Journaf of Microbiology and Biotechnology, Vol 7, 1991