Control of soilborne potato diseases using Brassica green manures

Brassica crops used in crop rotations and as green manures have been associated with reductions in soilborne pests and pathogens. These reductions have been attributed to the production of volatile sulfur compounds through a process known as biofumigation, and to changes in soil microbial community structure. In this study, selected Brassica crops, including canola, rapeseed, radish, turnip, yellow mustard, and Indian mustard, were evaluated for control of various soilborne potato pathogens and diseases in culture, in greenhouse trials, and in field trials on commercial potato farms. In in vitro assays, volatiles released from chopped leaf material of Brassica crops and barley inhibited growth of a variety of soilborne pathogens of potato, including Rhizoctonia solani, Phytophthora erythroseptica, Pythium ultimum, Selerotinia sclerotiorum, and Fusarium sambucinam, with Indian mustard resulting in nearly complete inhibition (80-100%). All Brassica crops and barley reduced inoculum levels of R. solani (20-56% reduction) in greenhouse tests, and radish, rapeseed, and Indian mustard reduced subsequent potato seedling disease by 40-83%. In an on-farm field trial at a site with a substantial powdery scab problem, Indian mustard, rapeseed, canola, and ryegrass grown as green manure rotation crops reduced powdery scab in the subsequent potato crop by 15-40%, and canola and rapeseed reduced black scurf by 70-80% relative to a standard oats rotation. At another field site where common scab was the primary disease problem, an Indian mustard green manure reduced common scab by 25%, and rapeseed, yellow mustard, and 'Lemtal' ryegrass also reduced black scurf relative to a standard ryegrass rotation. Disease reductions were not always associated with higher glucosinolate-producing crops, and were also observed with non-Brassica crops (barley and ryegrass), indicating other mechanisms and interactions are important, particularly for control of R. solani. Overall, Indian mustard was most effective for reducing powdery scab and common scab diseases, whereas rapeseed and canola were most effective in reducing Rhizoctonia diseases. These results indicate that Brassica crops have potential for use as green manures for the control of multiple soilborne disease problems. Published by Elsevier Ltd.