Fusarium-suppressive effects of green manure of turnip rape

• Fungal diversity decreased under barley monoculture.
• The effect of allelochemicals of turnip rape in controlling pathogens in soil was weak.
• Turnip rape amended in soil, as a source of nutrient and organic matter led to high fungal diversity.
• High fungal diversity resulted in low Fusarium spp. relative abundance.

Allelochemicals of Brassica can inhibit a wide range of soil-borne pathogenic fungi. In the present study, the effects of different turnip rape (Brassica rapa) cultivation treatments on the soil fungal community were investigated in two experiments in 2009 2010 and 2010 2011. The treatments used turnip rape as a green manure, as a mixed-crop with barley, or as a rotational crop after barley, with continuous barley as the reference. The composition of the soil fungal community was monitored with capillary-based length heterogeneity PCR using primers ITS1F/ITS4. The operational taxonomic units (OTUs) identified as the pathogens Botrytis cinerea, Rhizoctonia solani and Sclerotinia sclerotiorum did not respond to different treatments. Fungal diversity was the lowest and the relative abundance of Fusarium spp. the highest in barley monoculture. Incorporation of turnip rape plants into the soil as green manure was associated with a low relative abundance of Fusarium spp. Turnip rape as a mixed crop or rotational crop did not substantially decrease the relative abundance of Fusarium spp. Aqueous and non-polar extracts of allelochemicals from turnip rape had very little effect on the growth of Fusarium culmorum in vitro. We conclude that high fungal diversity, resulting from either application of fertilizer or incorporation of organic matter of turnip rape, was more important than allelochemicals in affecting the growth of Fusarium spp. in field soils.