Colletotrichum truncatum (Schwein.) Andrus & W.D. Moore IN SOYBEAN SEEDS: OCCURRENCE, PATHOGENICITY, AND ANTAGONISTIC POTENTIAL OF Bacillus spp.

Abstract

Soybean (Glycine max (L.) Merr.) is an important global crop that is often affected by phytopathogenic fungi. These fungi compromise seed germination and early seedling development, resulting in significant yield losses. This study examined the fungi naturally associated with soybean seeds, the physiological performance of various seed lots, the impact of Bacillus spp. treatments on the incidence of fungi associated with seeds and seed germination, and the ability of Bacillus isolates to antagonize Colletotrichum truncatum. Four soybean cultivars (OLIMPO, 82i78, NR8383RR, and SYN16871PRO) were evaluated using assays of seed health, germination, emergence, pathogenicity, seed treatment, and in vitro antagonism. The most prevalent fungi identified were Aspergillus sp. (5.5%), Cladosporium sp. (7.5%), and Colletotrichum truncatum (0.25%). The pathogenicity of the C. truncatum isolate was confirmed by symptom development and subsequent re-isolation of the pathogen from inoculated plants. However, cultivar 82i78 could not be evaluated due to the absence of plant development. The incidence of fungi associated with seeds and germination varied depending on the treatment with Bacillus species, cultivar, and surface disinfection conditions. In vitro confrontation assays demonstrated strong antagonistic activity against C. truncatum. All Bacillus isolates showed high mycelial growth inhibition, and isolate MOL27 (Bacillus sp.) reached the highest inhibition value (84%). Overall, these results demonstrate that Bacillus spp. can influence the incidence of fungi naturally associated with soybean seeds and exhibit strong antagonistic activity against C. truncatum in vitro. This supports further investigation of these isolates as potential biological control agents.