Effects of 3-hydroxy isoxazoles as soil-fungicides in relation to their chemical structures

Effects of a number of 3-hydroxyisoxazole derivatives substituted at 4 and/or 5 position of isoxazole ring on cucumber Fusarium wilt (Fusarium oxysporum f. cucumerinum) and cucumber seedling damping-off (Corticium rolfsii, Pythium aphanidermatum), and their antimicrobial activities on potato-sucrose-agar (PSA) medium and in soil were studied. Greenhouse evaluation (in vivo)-Compounds substituted with alkyl groups or hydrogen atom at 5-position of isoxazole ring were generally more effective than the derivatives substituted with aryl groups under greenhouse conditions. The effectiveness decreased in the following order: methyl>ethyl>propyl>butyl>hydrogen. Replacement of hydrogen atom at 4-position of isoxazole ring by nitro or methyl group or by chlorine or bromine atom reduced effectiveness against Fusarium wilt or seedling damping-off. Laboratory evaluation (in vitro)-Inhibitory effects of these derivatives on PSA medium against R. solani, C. rolfsii and P. debaryanum were generally strengthened in proportion to increase in the number of carbon atoms of alkyl groups at 5-position with the exception of activities against F. oxysporum. The growth inhibition of C. rolfsii by these derivatives in soil was not proportional to increase in the number of carbon atoms of alkyl groups at 5-position, and 3-hydroxy-5-methylisoxazole (hymexazol) showed the highest activity in soil. On the other hand, replacement of alkyl groups at 5-position by phenyl group enhanced inhibitory effect on PSA medium, but reduced it in soil. Effect of hymexazol thus selected as the most effective soil-fungicide in this series of compounds was studied extensively by large scale tests in a greenhouse.