Effect of organic matter removal treatments and addition of aluminum-containing substances on incidence of Fusarium wilt of lettuce

Abstract

In order to determine the mechanism of lettuce root rot suppression in topsoil of andosols we investigated the effect of removing humus from a topsoil of andosol and adding aluminum substances to a disease-conducive subsoil of andosol on the incidence of lettuce root rot. Both topsoil of andosols heated at 250°C and treatment with hydrogen peroxide sustained disease suppressive effect, and exhibited similar levels of disease suppression as the untreated topsoil of andosol. The disease suppressive effect was lost by heating topsoil of andosol at 400°C. Next, both subsoil of andosol amendment with aluminum-containing humic acid (Al-humic acid) and aluminum hydroxide gel (Al(OH)3) added the disease suppressive effect, but not with humic acid or crystalline structure aluminum hydroxide [crystalline Al(OH)3]. The addition of Al-humic acid and Al(OH)3 to potato dextrose agar medium significantly inhibited growth of the causal agent of lettuce root rot, Fusarium oxysporum f. sp. lactucae, compared with the control. Based on the above results, we speculate that noncrystalline aluminum hydroxide, which forms complexes with humus, inhibits growth of pathogenic fungus and thereby suppresses incidence of lettuce root rot.