Nitrite as a factor in the decline of Fusarium oxysporum f. sp. dianthi in soil supplemented with urea or ammonium chloride

Addition of 1 g urea or NH₄Cl per kg dry soil (0.1%) reduced the population ofFusarium oxysporum f. sp.dianthi in one of two soils tested. Ammonia does not seem to be the responsible factor since it accumulated similarly in both soils upon addition of NH₄Cl or urea. Addition of Nitrapyrine in combination with 0.1% urea or NH₄Cl increased ammonia concentrations in soil but decreased the population-declining effect. Addition of nitrate in amounts corresponding to those measured after decomposition of urea in soil had no effect on population development. Addition of nitrite in amounts corresponding to those measured during decomposition of urea in soil decreased the population ofF. oxysporum f. sp.dianthi. In vitro, nitrite inhibited chlamydospore formation. Upon addition of 0.1% urea, nitrite accumulated 10 to 100 times more in the susceptible soil than in the not-susceptible soil. It is concluded that nitrite rather then ammonia is responsible for the decline effect of ammonia-generating compounds on populations ofF. oxysporum f. sp.dianthi in soil.