Factors involved in disease suppression of violet root rot, caused by Helicobasidium mompa, with a fertilizer paste applied to soil surface around the base of apple trunks.

Abstract

The mechanism underlying suppression of apple violet root rot caused by Helicobasidium mompa was studied using a fertilizer paste (product name: TP, Taki Chemical, Hyogo) in kuroboku soil (soil texture: L). In pot experiments, mycelial growth in soil treated with urea (UR), the major ingredient of TP, was strongly suppressed, but treatment with H₃PO₄ or KCl was not effective. Mycelial growth was not suppressed in potato-dextrose broth containing 10 mg/mL of urea or nitrates (NaNO₃, KNO₃), but was strongly suppressed by 1 mg/mL of ammonium carbonate (AC) and 0.1 mg/mL of nitrites (NaNO₂, KNO₂). In pot experiments, amendments with TP, UR or AC inhibited mycelial growth for at least 29 d. During this time, the level of NH₄-N and NO₂-N was higher than the threshold to suppress mycelial growth in culture. The soil microbial community changed with treatments; the species of fungi decreased, but bacteria increased. The suppressive effect of soil treatments was barely detectable 62 d after treatment, when both NH₄-N and NO₂-N decreased, and populations of fungi and bacteria became equivalent to that in the untreated control soil. Disease severity on apple rootstocks (Malus prunifolia var. ringo) in pots treated with UR, AC or nitrites was low as in the case treated with TP, while nitrates failed to suppress disease. These results suggest that AC and NO₂-N induced by UR are effective for suppressing violet root rot of apple. In addition, the suppression mechanism may be ascribed to biological factors.