Isolation, Characterization, and Phytotoxic Activity of Streptomyces spp. from Diseased Potato Scab Tubers

Biological herbicides serve as highly efficient and eco-friendly weed control agents, which typically consist of natural secondary metabolites, such as phytotoxic metabolites produced by Streptomyces sp. However, the isolation of phytotoxic compounds from pathogenic Streptomyces strains has been insufficiently investigated. Here, a total of 50 strains of the Streptomyces genus were isolated from the potato tubers with typical common scab (CS) symptoms using the culture-dependent method. The radish seedling test indicated that 32 fermentation broths of potato common scab-associated Streptomyces produced phytotoxic metabolites, which inhibited the normal growth of radish seedling radicles. Of note, two candidate pathogens (NKY-4 and L7-2) were discovered via a combination of radish seedling assay, potato tuber slice assay, and txtAB pathogenic genes PCR detection. Moreover, Petri dish phytotoxic assays demonstrated that the fermentation broths of 31 strains exhibited phytotoxic activity against at least one of the tested weeds (Echinochloa crusgalli, Digitaria sanguinalis, and Abutilon theophrasti). Furthermore, six compounds were isolated from Streptomyces bottropensis AMCC400023 (1-4) and Streptomyces sp. SCA10 (5-6). Among them, thaxtomin A (2) exhibited strong phytotoxic activity against E. crusgalli, D. sanguinalis, and A. theophrasti under Petri dish assay conditions, with IC50 values of 0.99, 0.78, and 1.95 μg/mL, respectively. Based on these findings, phytotoxic metabolites from potato scab pathogens may offer potential lead structures for further phytotoxin screening.