Biological mode of action of the fungicide, flusulfamide, against Plasmodiophora brassicae (clubroot)

Flusulfamide (2', 4-dichloro-alpha,alpha,alpha-trifluoro-4'-nitro-m-toluene sulfonanilide) was investigated for its mode of action against Plasmodiopho ra brassicae Woronin. Seedlings of Chinese cabbage (Brassica rapa L. subsp. pekinensis) were grown for 14 and 21 days in soil infested with P. brassic ae and then transplanted into soil containing flusulfamide (0.9 mu g a.i. g (-1) dry soil). Clubroot was not suppressed by this treatment, indicating t hat the fungicide is ineffective against P. brassicae established within co rtical cells of the host root. Where seedlings were grown in soil infested with resting spores which had previously been treated with flusulfamide, ro ot-hair infection and club formation were suppressed. This indicates that f lusulfamide directly acts against resting spores. When placed in root exuda tes of Chinese cabbage, untreated resting spores germinated at a high frequ ency while flusulfamide-treated resting spores hardly germinated at all. Us e of the Evan's blue staining assay indicated that flusulfamide-treated res ting spores remained viable. Flusulfamide was detected by high performance liquid chromatography on resting spores treated with flusulfamide for 30 mi n. This indicates that the chemical is adsorbed onto resting spores. These results suggest that flusulfamide suppresses clubroot disease by inhibiting germination of P. brassicae resting spores through adsorption onto their c ell walls.