Fungal Colonization of the Anatomical Parts of Soybean Seeds Supplied with Different Nitrogen Rates and Inoculated with Bradyrhizobium japonicum

The soybean [Glycine max (L.) Merr.] plays an important role in human and animal nutrition due to its high protein and oil content. The present study was undertaken to determine the effect of different mineral nitrogen (N) rates and inoculation with Bradyrhizobium japonicum bacteria on fungal colonization of the anatomical parts of seeds (APSS) of two soybean cultivars (Aldana and Annushka). Fungi were identified with the use of the macroscopic method and the polymerase chain reaction (PCR) assay. The study demonstrated that fungal colonization was higher on soybeans cv. Annushka than cv. Aldana. The obtained results indicate that fungal colonization intensity was highest in the cotyledons, lower in the seed coat, and lowest in the embryonic axis. The APSS were colonized by pathogenic fungi belonging mostly to the genus Fusarium, as well as saprotrophic fungi represented by Alternaria alternata, Cladosporium cladosporioides, Penicillium spp., and Rhizopus nigricans. Fungal colonization intensity was highest in soybean seeds inoculated with HiStick®Soy and in control seeds, whereas the number of fungal isolates obtained from the APSS was lower in the remaining treatments: 60 kg N ha−1 + HiStick®Soy, 30 kg N ha−1 + HiStick®Soy, Nitragina, and 60 kg N ha−1. In addition, the statistical analysis revealed that fungal abundance and the biodiversity indicators of fungal communities, including relative frequency (Rf), dominance (Y), and species richness (S), differed across the analyzed APSS and years of the study, which indicates that these parameters were significantly influenced by weather conditions. The abundance of pathogenic and saprotrophic fungal species did not differ significantly between the examined soybean cultivars. Spearman’s rank correlation coefficients were calculated to assess the strength of the relationship between weather conditions and the diversity of fungal communities colonizing soybean seeds. The analysis revealed that the development of pathogenic fungi on soybean seeds was determined by temperature and precipitation on 11–30 June and 1–10 August, whereas the prevalence of saprotrophic fungi was influenced only by precipitation on 1–10 and 21–30 July and 1–10 August. The qPCR analysis demonstrated that the colonization of soybean seeds by F. graminearum and P. verrucosum was affected by all experimental factors.