Potato (Solanum tuberosum) is a crucial global food crop, yet its production is threatened by fungal pathogens such as Pythium ultimum and Colletotrichum coccodes, which cause significant yield losses and impact tuber quality. In this study, we investigated the antifungal properties of various Streptomyces and Bacillus isolates against these pathogens. The Streptomyces isolates, known for their prolific secondary metabolite production, were selected for their varying efficacy against these fungi. Despite the anticipated potential, Streptomyces isolates exhibited limited antifungal activity against Pythium ultimum and only moderate activity against Colletotrichum coccodes. In contrast, Bacillus isolates, particularly B1, demonstrated strong inhibition of C. coccodes, although this activity diminished after the removal of the live bacteria and after fractionation. The study has several potential factors influencing these outcomes, including the degradation of bioactive compounds during the fermentation, concentration, storage, and fractionation processes. The 10-day fermentation period was chosen based on the typical peak of secondary metabolite production in Streptomyces; however, the transition from solid to liquid media may have affected metabolite stability. Furthermore, the study suggests that repeated subculturing over six years may have led to genetic drift or loss of metabolite production in Streptomyces isolates. Additionally, the presence of Bacillus contaminants in an old DMSO stock added complexity to the results, particularly in early bioassay replicates. This study highlights the challenges of maintaining bioactive compound efficacy through various stages of microbial processing and suggests that alternative extraction and fractionation methods, as well as a focus on preserving synergistic compound interactions, may enhance the development of effective biocontrol agents for agricultural use.

Fungal diseases, Pythium ultimum (Pythium leak) and Helminthosporium solani (silver scurf) have detrimental effects on potato tuber (Solanum tuberosum) quality and yield. Tubers are the world’s fourth largest agricultural food crop and are crucial for feeding a growing population. Bacteria from the genus Streptomyces are known for producing a wide variety of secondary metabolites with antifungal properties. Isolates of Streptomyces have recently shown inhibitory effects towards P. ultimum and H. solani in Petri dish assays. These data suggest that Streptomyces may work as a biocontrol to protect tubers from P. ultimum and H. solani. We tested talc-based powder formulas for their ability to maintain viable Streptomyces spores in storage. The formula that maintained spores the longest was then used to coat varying Streptomyces isolates onto a tuber surface that contained, or would be exposed to P. ultimum or H. solani. Tests were conducted in a lab, greenhouse, and field setting. We found a powder formula that kept 50% of the added Streptomyces spores viable for a period of three to six months depending on the isolate. Isolates with inhibitory effects towards H. solani were applied as a powder on seed tubers infected with H. solani and grown in a greenhouse. Upon completion of the experiment, we found that progeny tubers from neither the treatment nor the control groups contained H. solani. Instead, we found a similar surface pathogen, Colletotrichum coccodes (Black dot), on many of the progeny tubers. While not the target pathogen of this study, some isolates significantly limited C. coccodes compared to the control. This experiment was repeated in a field setting where C. coccodes was again the primary disease found on the progeny tubers. In the field, isolates showed no inhibitory effect towards C. coccodes. Isolates with inhibitory effects towards P. ultimum were applied as a powder onto wounded tubers. One hour later the tubers were exposed to P. ultimum. Isolates did not limit P. ultimum compared to the control after a week of incubation. A follow up experiment revealed that the Streptomyces isolate used needed at least 24 hours of growth to produce antifungal secondary metabolites. Our data suggest that Streptomyces bacteria can easily be stored in a powder and that there are beneficial effects as a biocontrol against C. coccodes. Our data also suggest that timing Streptomyces application for maximum secondary metabolite production may improve its efficacy as a biocontrol.