Effect of soil aeration on the occurrence of clubroot disease of crucifers

A clear difference in the occurrence of clubroot disease on cabbage was found in fields consisting of two different soils, i.e. high-humic Andisol and low-humic Andisol in Tsumagoi region in Gunma Prefecture in Japan. Soil physical properties and CO2 concentrations in the soil air of the two Andisols were studied in fields. The high-humic Andisol which was considered to promote the occurrence of clubroot disease displayed a low drainage ability and low gas diffusivity. The low-humic Andisol which was considered to inhibit the occurrence of the disease showed a high drainage ability and high gas diffusivity. The CO2 concentrations in the soil air of the high-humic Andisol were 2–4 times as high as those of the low-humic Andisol during the growing period of cabbage. Influence of CO2 concentration in the soil air on the occurrence of clubroot disease was examined in turnip in pot culture experiments under controlled soil water matric potential and CO2 concentration in the soil air. It was found that the increase in the CO2 concentration promoted the occurrence of clubroot disease in the high-humic and the humic Andisols at high soil moisture levels. In the field experiments and pot culture experiments in which crucifers were severely damaged by the disease, the decrease in the soil water pH induced by CO2 accumulation exceeded the value of 0.5 in the high-humic Andisol. The promotive effect of a high CO2 concentration on the occurrence of the disease is ascribed to the fact that the decrease in the soil water pH by CO2 accumulation in soil air may activate the germination of resting spores and impair the activity of crucifers' roots. On the other hand, the occurrence was negligible even at high soil moisture levels in the low-humic Andisol. This suppressive character of the low-humic Andisol is attributed to the physical and chemical properties of the soil surface, other than the high drainage ability and high gas diffusivity. However, in the field, the high drainage ability, high gas diffusivity and subsequently small decrease in soil water pH may also contribute to the suppression of the disease occurrence in the low-humic Andisol.