MYCELIAL GROWTH AND ENDOPECTINASE SECRETION OF VENTURIA-INAEQUALIS IN-VITRO AT SUBLETHAL CONCENTRATIONS OF KRESOXIM-METHYL

Defined and complex liquid media supplemented with kresoxim-methyl inh ibited growth of Venturia inaequalis. The level of inhibition was depe ndent on fungicide concentration, medium, and most significantly, the culture volume. Culture of the fungus for 20 days in 1, 25 and 250 mi potato dextrose broth containing the fungicide at 0.01 mg/l led to a g rowth inhibition of 0, 59 and 90 %, respectively. At 0.05 mg/l growth inhibition was 4 % in l-ml cultures and at about 95 % in the 25 and 25 0-ml cultures. Whereas no growth was detectable at 1 mg/l in 250-ml cu ltures and trace amounts of growth in 25-ml, in the l-ml cultures grow th reduction was only 61 and 74 % at 1 and 5 mg/l, respectively. The m aximum percentage of inhibition of colony growth on fungicide-suppleme nted agar after 28 days was 45 % and the growth inhibition was lower a fter prolonged incubation. Time course culture experiments in 25-ml li quid culture indicated that growth inhibition also depended on culture time. The progress of mycelial development led to a reduction in fung icide efficacy. The points of inflection towards a normal mycelial dev elopment were delayed according to the fungicide concentrations and th is effect was more pronounced in the defined medium. Endopectinase sec retion was not reduced with regard to mycelial weight. A trend towards higher pectinase production was detectable with increasing fungicide concentration.