Effect of preceding Guinea grass cropping on the incidence of clubroot disease in Hanana (nabana)

Pot and field experiments were conducted to evaluate the effectiveness of preceding cropping with several potential “bait crops” for suppressing clubroot disease in Hanana (Brassica rapa var. nippo-oleifera), which is a semi-traditional vegetable cultivated in Kyoto, Japan. Among the tested species, Guinea grass (Megathyrsus maximus; syn. Panicum maximum) exhibited a consistent suppressive effect on disease incidence, comparable to that of the known bait crop ha-daikon (Raphanus sativus var. longipinnatus), in pot experiments. Conversely, under field conditions, M. maximus did not reduce disease incidence compared with the untreated control. These findings suggest that the suppressive effect of M. maximus is confined to controlled environments and may depend on soil and management conditions. However, M. maximus confers advantages over ha-daikon as a bait crop in terms of vigorous growth and reduced pest damage during summer cultivation.