Effect of p-hydroxybenzoic and cinnamic acids on soil fungi (Fusarium oxysporum f.sp. cucumerinum) growth and microbial population

This study was done to determine the effects of p-hydroxybenzoic acid and cinnamic acid on growth of Fusarium oxysporum f. sp. cucumerinum and soil microbial populations. Cucumber seedlings were inoculated with F. oxysporum at the three euphylla stage. After inoculation, p-hydroxybenzoic and cinnamic acids were separately incorporated in cucumber rhizosphere. The growth status of fungi was observed 60 h after inoculation. Mycelial growth of F. oxysporum was obviously inhibited after incorporation of cinnamic acid at the concentrations of 0.2 g. L⁻¹ and 1.0 g.L⁻¹. But the mycelial growth of F. oxysporum was not affected by p-hydroxybenzoic acid at the concentrations used in this experiment. Cucumber rhizosphere soil and non-rhizosphere soil were treated with p-hydroxybenzoic and cinnamic acids at concentrations of 0, 25, 50, 100 and 200 mg^.kg⁻¹ respectively. The allelochemicals promoted the growth of soil fungi, bacterium and Fusarium oxysporum and inhibited the growth of soil actinomycete in non-rhizosphere soil and reduced the occurrence rate of Fusarium wilt disease. In cucumber rhizosphere soil, the assayed allelochemicals inhibited the growth of soil fungi, bacterium and actinomycete and promoted the growth and infection rate of Fusarium oxysporum. These results illustrate that the assayed allelochemicals under different environmental conditions have different effects on F. oxysporum and soil microbes. The soil diseases in continuous cropping were correlated with the presence and absence of the assayed allelochemicals and their concentrations in cucumber rhizosphere.