Impact of brassicaceous seed meals on the composition of the soil fungal community and the incidence of Fusarium wilt on chili pepper

Abstract

Although brassicaceous seed meals (BSMs) have been evaluated for their ability to control various soil-borne diseases, establishing the relationship between disease severity and the soil fungal taxa specifically inhibited or enriched by the seed meal amendment has not been carefully assessed. The purpose of the present work was to elucidate the relationship between the glucosinolate concentration and composition in BSMs applied as soil amendments, the diversity and composition of the resulting soil fungal community, and the incidence of Fusarium wilt disease. These relationships were examined using chili pepper (Capsicum annuum ‘Ben Villalon’) as the bioassay plant grown in pots amended with different BSMs in the presence of the Fusarium wilt pathogen. We evaluated three BSMs differing in the species from which the seed meal was prepared and hence the glucosinolate content. These included Camelina sativa ‘Crantz’ (CAME), Brassica juncea ‘Pacific Gold’ (PG), and a mixture of PG and Sinapis alba cv. ‘IdaGold’ (IG) (PG+IG, 1:1 ratio). The effect of the BSMs on the fungal community was evaluated after the soil had been incubated for 25 days separately with each seed meal (incubated soil) and again after growing with chili pepper in pots for 35 days (rhizosphere soil). A soil treatment (no BSM amendment, comparable N, P, K added) was used as control. The composition of the soil fungal community in the incubated soil and the rhizosphere soil were examined using DNA based methods including quantitative real-time PCR and high throughput pyrosequencing. Our results indicated that the fungal abundances in the incubated soils were significantly increased by all BSM amendments, however the proliferation became less pronounced during the period of chili pepper growth. Of significance, the BSMs differed in their effect on the fungal community composition and on disease severity. For example, PG and PG+IG specifically enriched Chaetomium species, the abundance of which was negatively correlated with disease severity, and simultaneously specifically inhibited Hypocreales including the genus of Fusarium, the abundance of which was positively correlated with disease severity. PG and PG+IG exhibited good disease control efficacies in the pot experiment suggesting that 2-propenyl (allyl) glucosinolate, the primary glucosinolate component in the PG seed meal, played an important role in the changes in abundance of specific fungi during the 25-day incubation. CAME specifically enriched members of the Mortierellales, including the genus Mortierella, the abundance of which was not correlated disease severity. Moreover, our data indicated that BSM type was a more important determinant of fungal community composition than the plant host, given that the composition of the fungal communities after 35 days growth in the rhizosphere of chili pepper were not substantially different from the incubated soil communities before planting. Our results suggested that some species in the genus of Chaetomium may be important for disease management and deserved further evaluation as potential biocontrol agents.