Cercospora species associated with soybean diseases in Russia

Soybean diseases induced by Cercospora spp. exhibit a global prevalence worldwide. Cercospora kikuchii causes both Cercospora leaf blight (CLB) and purple seed stain (PSS), whereas Cercospora sojina is a causal agent of frogeye leaf spot (FLS). Eighteen Cercospora isolates originating from soybean plants exhibiting CLB, PSS, and FLS symptoms were obtained from continental Russia, the Crimea Peninsula, and South America. The identification was based on the Consolidated Species Concept and involved multi-locus phylogenetic analysis, assessment of cercosporin production capacity, and pathogenicity testing. Ten isolates were identified as C. sojina; the eight remaining ones were categorized into seven distinct species. Two isolates of C. kikuchii were obtained from South America, along with a single isolate each of Cercospora cf. sigesbeckiae and Cercospora sp. Q from the Russian Far East and South America, respectively. Three isolates from the Russian Far East were identified as Cercospora cf. alchemillicola and Cercospora celosiae. A single isolate formed a distinct monophyletic clade that did not include ex-type or representative Cercospora strains and is, therefore, considered a candidate for a new Cercospora species. Cercosporin production in vitro is not a stable and reliable feature for species identification; it could vary and depends on factors such as the nutrient medium composition and the specific lighting conditions during the culturing process. In Russia, multiple Cercospora species are associated with PSS: at least C. cf. alchemillicola, C. cf. sigesbeckiae, and C. celosiae, which are new records for Russia. Cercospora kikuchii and Cercospora sp. Q emerge as causal agents of PSS in South America. PSS and CLB symptoms evident on soybeans are intricate features; thus, they can no longer be definitively regarded as unequivocal signs for the presence of C. kikuchii.