The potential of Brassicaceae biofumigant crops to manage Pleiochaeta setosa in sustainable lupin cultivation

Lupin is prone to infection by Pleiochaeta setosa (Kirchn.) Hughes causing brown spot and root rot in Lupinus species. The control of these diseases is crucial to limit yield losses, but an increased public awareness and European legislative restrictions have resulted in a drastic reduction of allowed pesticides and many research groups invest in alternative biocontrol strategies. In the current study, the inhibitory effect of volatiles, released by two members of the Brassicaceae family (white mustard (Sinapis alba) and fodder radish (Raphanus sativus)), on brown spot and root rot in Lupinus species was evaluated in vitro and in vivo. The in vitro trial showed that root tissue from Brassicaceae was more effective in reducing the vegetative growth of Pleiochaeta setosa than leaf and stem tissues. Furthermore, R. sativus was more effective than S. alba. The GC-MS analysis revealed that dimethyl disulfide and dimethyl trisulfide were predominantly emitted by the tissues of both Brassicaceae. Additionally, the ground root tissue from R. sativus emitted 4-isothiocyanato-1-(methylthio)-1-butene, whereas the stem + leaf tissue emitted benzyl isothiocyanate. Benzyl isothiocyanate was also released by the ground S. alba root tissue, whereas the leaf + stem tissue emitted 3-butenyl isothiocyanate. In vitro trials with pure dimethyl disulfide and dimethyl trisulfide confirmed the antifungal effect of the volatiles released by these sulfides. For dimethyl disulfide 7.95 mg per Petri dish was necessary to (almost) completely inhibit mycelium growth, whereas for dimethyl trisulfide 1.20 mg per Petri dish was sufficient to prevent fungal growth. In addition, a trial with allyl isothiocyanate showed that for this compound even lower doses (0.50 mg per Petri dish) stopped the growth of the fungus. The results obtained with the pot trial illustrated that the amendment of Brassicaceae species significantly reduced the P. setosa infection level. Also in the field, incorporation of Brassicaceae species suppressed P. setosa. From this study, it can be concluded that Brassicaceae species are a powerful tool to manage P. setosa in lupin cultivation. However, it should be combined with other control practices since the amendment of these biofumigant crops could not completely inhibit symptom development.