The Effect of Septoria glycines and Fungicide Application on the Soybean Phyllosphere Mycobiome

Septoria brown spot, caused by Septoria glycines, is Illinois’ most prevalent soybean disease. It is common to use foliar fungicides to control Septoria brown spot and other late-season diseases of soybean. The effects of fungicide on nontarget organisms in the phyllosphere are unknown. To study the effect of S. glycines and fungicide application on the soybean phyllosphere mycobiome, we conducted a replicated field trial and collected samples at three soybean developmental stages. Then, we sequenced full-length internal transcribed spacer and a partial large subunit region using Oxford Nanopore technologies. Sequencing and data analysis produced 3,342 operational taxonomic units. The richness of the fungal community increased with the host development. There were differences in mycobiome diversity between soybean lines at the early developmental stage but not at the reproductive stages. Inoculation with S. glycines did not affect the α diversity but some significant changes were observed for the β diversity. At the beginning seed stage (R5), fungicide application changed the composition of the fungal community. The fungicide treatment decreased the proportion of several fungal taxa but it increased the proportion of Septoria. The core mycobiome in the phyllosphere was composed of genera Gibberella, Alternaria, Didymella, Cladosporium, Plectosphaerella, Colletotrichum, and Bipolaris. Network analysis identified significant interactions between Septoria and Diaporthe, Bipolaris, and two other taxonomic units. In this study, we set Septoria as the target organism and demonstrated that metabarcoding could be a tool to quantify the effect of multiple treatments on the mycobiome community. Better understanding of the dynamics of the phyllosphere microbiome is necessary to untangle the late-season diseases of soybean.