Bull. Environm. Contam. Toxicol. 25,554-561 (1980) Interactions of the Solvent Acetone with the Fungicides Benomyl and Captan in Fungal Assays Robert E. Burrell and Charles T. Corke Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada NIG 2W1 The poisoned-agar technique for assessing toxicity involves the addition of volumes of stock solutions of a compound to specific volumes of an agar medium. The differences in radial growth, generated from inoculated mycelial discs on control and experimental plates, are used to assess the toxicity of the chemical. Water- insoluble compounds are usually added to the medium in organic solvents, such as acetone or ethanol. The solvents used, and the volumes added, are sometimes defined (MANTEN et al. 1950; EDGINGTON et al. 1971), or such information may not be specified (ALI et al. 1979; BUCHENAUER and ERWIN 1976). In spite of the frequent use of solvents in bioassays, little attention has been given to their possible interactions with the candidate pesticide and the organism being tested. MANTEN et al. (1950) reported that 2% acetone in a medium did not affect the growth of the fungi used in their studies. However, evidence has been presented to show that acetone markedly affects both the replication of vaccinia and rabbit-pox viruses (GHENDON and SAMOILOVA 1968; CHERNOS et al. 1972) and the ultrastructure of the alga Chlorella (PARASHER et al. 1978). DALELA et al. (1979) observed that solvents influenced the degree of inhibition of ATPase activity induced by insecticides in tissues from a fresh- water teleost. They concluded that it was obligatory when report- ing experimental data to clearly define details of the solvents used. In this paper, the effects of acetone on the growth of fungi are reported. A theoretical discussion of possible interactions between a solvent system, a toxic chemical, and a test organism, which result in additive, antagonistic and synergistic responses, is presented. Experimental data to illustrate the interactions of acetone and two fungicides, benomyl and captan, on the growth of four fungi are detailed. MATERIALS AND METHODS The phytopathogenic fungi, Sclerotinia homeocarpa, Fusarium oxysporum f. lycopersici, Polyporus hirsutus and Pestalotia sp. were used in this study. The effects of acetone on growth were determined by adding the solvent to i00 mL of melted Difco potato- dextrose agar medium to give final levels of 0 (control), 0.i, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.5 and 2% (v/v). Bottles of medium were shaken for 2 minutes on a rotary shaker and I0 mL volumes of control and solvent-treated medium were dispensed into petri dishes. 0007-4861/80/0025-0554 $01.60 O 1980 Springer-Verlag New York Inc.