Brassica species are increasingly being used as cover crops to suppress soil-borne diseases in potato cropping systems. Experiments were conducted in controlled environments and in the field to ...evaluate the effects of cover crop root or shoot or a combination of root and shoot tissues on potato root and tuber health. In a lab assay we examined the extent to which volatile compounds released from tissues of two cover crop species, rye (Cereale secale L.) and oriental mustard (Brassica juncea L.), could inhibit mycelium growth of two important potato diseases, Rhizoctonia solani and Pythium ultimum. Twenty-four hours into the lab assay, volatile compounds from all residues suppressed fungal growth. After 48 h, marked suppression of hyphal growth continued in the presence of mustard residues but not in the presence of rye tissues or the control without tissues. A 75 L volume container experiment evaluated the effect of incorporating different quantities of mustard shoot and root tissues (none, comparable to field level and fourfold field level) into R. solani and P. ultimum infested soil on potato growth, root health and tuber disease. In the container study, incorporating mustard shoots at the highest dose increased potato yield by 54% and reduced disease rating to 2.3 compared to a severe rating of 4.4 in the control. In the field trial, potato growth, root health and tuber disease levels were evaluated in plots where disease management involved either incorporation of mustard or rye cover crop roots, shoots and whole plants (roots plus shoots) or standard farmer practice of a fumigated fallow as a control. White root tissue was used as a health indicator, and averaged 58 and 78% in the fumigated control and mustard cover crop treatments, respectively. The highest healthy root tissue status (91%) was recorded where whole plants of mustard were incorporated. In contrast to the visual assessment of root and tuber health, tuber yield in the field was not influenced by cover crop treatment. Across experiments, the incorporation of or exposure to whole mustard plants was consistently effective at suppressing soil-borne fungi and promoting healthy roots and tubers, especially at higher rates of biomass. Mustard should be managed so as to maximize incorporated biomass for effective biofumigation. Multipurpose management requiring removal of mustard shoots is incompatible with promoting potato rhizosphere health.
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