Digitonin as a model biologically active saponin forming specific quality of Digitalis sp in bioassays in vitro

Polygenic system of heredity of resistance responds to production of broad complex of biological active substances. These compounds have different eff ect against pathogens. Basic importance of digitonin is of course in its pa rticipation in active defensive mechanisms of plants against pest organisms . Fungitoxicity of digitonin is well-known when fungi are cultivated in vit ro. Therefore digitonin was used as a model substance for investigation of its effect on phytopathogenic fungi cultivated in vitro. At the same time w e presented routine bioassay for testing of potential fungicidal biological ly active natural substances. The microscopic filamentous fungi Fusarium cu lmorum (FC), Fusarium solani (FS), Cladosporium herbarum (CH), Alternaria a lternata (AA) and Botrytis cinerea (BC) were used to bioassay of toxicity. One mi of 2 degrees liquid malt, 1 ml of suspension of the fungi and water solution of the tested substance (digitonin and different pesticides) was s uccessively added into test tubes. The fungi were cultivated in the dark in thermostat at 25 degrees C. The evaluations of a mycelial growth of the in vestigated clone fungal cultures by four level of scale were performed in f ive days and then in 10 to 14 days for each treatment of the testing substa nces. Results are described in Tabs. II to VI. We used bioassay to test fun gicidal attributes of natural substance in conditions in vitro in compariso n with pesticides. We used determined toxicity of substances to verify this investigated method, too. We selected among insecticides Actellic 50 EC wi th an active ingredient (a, i.) pirimiphos-methyl (without important fungic ide effect), of fungicides Impact 125 EC with a. i. flutriafol, Tilt 250 EC with a. i. propiconazole, Tilt CB FW with a. i. carbendazim and propiconaz ole and of biological active natural substances digitonin. We showed advant age to compare toxicity of pesticides to determined toxicity of natural sub stances. We see advantage of comparison there, that commercial producer alr eady determined toxicity of used pesticides against fungi. These compounds, at which important fungicidal effect was found, were used as fungicides. C oncentration of 25 mu g of digitonin on 1 mi of media manifested significan t toxic effect on tested fungi. The effective concentration of digitonin wa s lower than the usual concentration of digitonin in seeds of Digitalis sp. (Bylka ct al., 1976; Czabajska et al., 1980). This method demonstrated a h igher sensitivity of the toxic effect of digitonin on fungi comparing with the results described in papers by Wolters (1968). Toxicity of digitonin wa s significantly higher in comparison with the toxicity of insecticide Actel lic. Also the fungicide Impact showed a lower toxicity than digitonin. The toxicity of digitonin was in a close relation with toxicity of Tilt 250 and Tilt CB. Triazole fungicides, that inhibit the biosynthesis of sterols, we re also compared with digitonin known to form a specific complex with stero ls. The negative interaction of these substances is thus expressed in the m etabolism of sterols.