Morphological and Molecular Characterization of Cercospora beticola Causing Cercospora Leaf Spot on Sugar Beet

Abstract

Cercospora leaf spot, caused by Cercospora beticola Sacc., represents the most prevalent and destructive foliar disease of sugar beet (Beta vulgaris L.) in temperate and semi-arid production regions worldwide, inducing characteristic circular necrotic lesions with reddish-brown borders and pale gray centers that, under epidemic conditions, coalesce to cause extensive defoliation, repeated compensatory regrowth, and significant reductions in extractable sucrose content and root yield. Despite the longstanding recognition of C. beticola as the primary causal agent of this disease, comprehensive integrative characterization combining detailed morphological assessment with contemporary multilocus molecular phylogenetics across diverse pathogen populations remains essential for resolving taxonomic uncertainties, detecting cryptic diversity, and establishing reliable diagnostic frameworks to support resistance breeding and fungicide stewardship programs. This study conducted morphological and molecular characterization of C. beticola isolates recovered from sugar beet plants displaying typical Cercospora leaf spot symptoms across surveyed production fields spanning multiple agro-ecological zones. Symptomatic leaf tissues were collected systematically, and fungal isolates were established through standard single-conidium isolation procedures. Morphological characterization encompassed colony pigmentation, growth rate, stromatal development, conidiophore fasciculation patterns, conidial dimensions, septation frequency, and hilum morphology examined under light and scanning electron microscopy. Pathogenicity of representative isolates was confirmed through spray inoculation of healthy sugar beet seedlings under controlled greenhouse conditions in accordance with Koch's postulates. Molecular identification and phylogenetic placement were resolved through sequence analyses of the internal transcribed spacer region, the cercosporin toxin biosynthesis gene cluster, ACT, CAL, GAPDH, and TEF1-α loci using maximum likelihood and Bayesian inference methods. Significant morphometric variation was detected among isolates from geographically distinct populations, while multilocus phylogenetic analyses robustly supported the placement of all recovered isolates within C. beticola.