Bioactive secondary metabolites of Streptomyces spp. in the control of Asian soybean rust (Phakopsora pachyrhizi)

Soybean ( Glycine max ) is one of the most important crops in the world; its production is severely affected by the asian soybean rust, caused by Phakopsora pachyrhizi. The search for sustainable alternatives to chemical fungicides has led to increased interest in microbial metabolites with antifungal properties or the ability to induce resistance. This research evaluated the potential of secondary metabolites produced by seven Streptomyces spp. isolates in P. pachyrhizi management. The isolates were cultured under different physical conditions (agitation and illumination) to enhance metabolite production. The presence of thaxtomin A and other metabolites in the culture media was evaluated using thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and liquid chromatography-tandem mass spectrometry (LC-MS). Biological activity was measured in urediniospore germination in vitro and in soybean plants in a greenhouse. Despite the absence of thaxtomin A, LC-MS results indicated the presence of compounds compatible with 2,5-diketopiperazines, a class of bioactive cyclic dipeptides. The isolates 1901 and 1909 exhibited the highest bioactivities, inhibiting urediniospore germination, and reducing disease severity in soybean plants. The evaluated isolates produced metabolites that reduced urediniospore germination and disease severity, highlighting their potential for future biocontrol applications.