Mustard biofumigation disrupts biological control by Steinernema spp. nematodes in the soil

Mustard green manures or seed meal high in glucosinolates, which produce a natural biofumigant upon incorporation into the soil, form an alternative to synthetic fumigants. However, the non-target impacts of these biofumigants in the field are unclear. We examined the effectiveness of soil incorporation of Brassica carinata seed meal both in controlling the plant-parasitic Columbia root-knot nematode (Meloidogyne chitwoodi), and on the biological control exerted by the entomopathogenic nematodes Steinernema feltiae and Steinernema riobrave on root-knot nematodes and the Colorado potato beetle (Leptinotarsa decemlineata). Singly, both the seed meal and Steinernema spp. reduced root-knot nematode damage to potato tubers and increased marketable tuber yields. However, there was a negative interaction between the two bioagents such that their combination did not further improve suppression of plant-parasitic nematodes. Thus, mustard seed meal applications harmful to the target root-knot nematode also disrupted the ability of Steinernema spp. to act as biocontrol agents. Further, we observed modest disruption of the biological control of potato beetles following biofumigation. But, the potato beetles were less likely to lay eggs on potato plants grown in mustard-amended soil, suggesting a counteracting benefit of mustard application. Multiple, complementary controls must be integrated to replace the very effective pest suppression typical of synthetic soil fumigants. Our study suggests significant interference between biofumigation and biocontrol agents in the soil, presenting challenges in combining these two environmentally friendly approaches to managing plant-parasitic nematodes and other pests.