Streptomycetes against Colletotrichum circinans: a new way to control anthracnose in onion?

Colletotrichum circinans is an important plant pathogenic fungus that causes anthracnose, or “smudge”, in onion. Bulbs for pickle production have very high standards for the aesthetic quality of the products. Anthracnose causes symptoms that are not compatible with industrial processing, leading to significant losses to producers. The number of available chemical molecules for its control is limited, and hardly would give acceptable results in the control of soil-borne diseases. Additionally, their use is restricted by the need to consume the product at early stages of growth reducing the interval from treatment to consumption. The use of alternative and environmentally friendly soil treatments is therefore necessary to control the disease. A set of Streptomyces spp. strains (n= 30) was tested for their activity against four strains of Colletotrichum circinans. In vitro antibiosis assays were coupled with in vivo greenhouse and field trials with natural disease occurrence. Overall, four strains showed promising growth-inhibitory activity against C. circinans, reducing its growth by up to 56%. In in vivo controlled conditions, streptomycetes applied in the soil showed the ability to reduce the incidence of the disease by more than 70%. In field trials, the disease incidence was reduced by up to 57%, and disease severity decreased from 31.33% to 12.33% when using the DEF48, DEF39, and DEF58 strains compared to the untreated control. Influence on plant growth was also observed on onion plants treated especially with Streptomyces spp. DEF19 and DEF58. Overall, the use of streptomycetes to control soil-borne diseases is promising. Optimization of the delivery methods for the strains may further improve their efficacy.