Morphological and Molecular Characterization of Fusarium oxysporum f. sp. phaseoli Causing Wilt on Common Bean

Abstract

Wilt symptoms observed on common bean (Phaseolus vulgaris L.) were investigated to identify the causal agent and characterize its morphological and molecular attributes. Field surveys in bean-growing regions revealed progressive yellowing, vascular discoloration, stunted growth, leaf chlorosis, and eventual plant collapse under warm soil conditions. Fungal isolates recovered from infected vascular tissues produced cottony white to pale violet colonies on potato dextrose agar, with abundant microconidia, macroconidia, and chlamydospores typical of Fusarium oxysporum species complex. Pathogenicity tests conducted on healthy bean seedlings under controlled greenhouse conditions successfully reproduced wilt symptoms, confirming virulence and fulfilling Koch’s postulates. Molecular identification using ITS rDNA and translation elongation factor 1-alpha (TEF1-α) gene sequencing confirmed the isolates as F. oxysporum f. sp. phaseoli, showing high sequence similarity with reference isolates from global databases. Phylogenetic analysis placed all isolates within a well-supported clade corresponding to the forma specialis phaseoli, indicating relatively conserved genetic structure among populations. However, minor nucleotide polymorphisms suggested limited intraspecific variability possibly associated with geographic adaptation and host-pathogen interactions. Disease incidence was strongly correlated with high soil temperature and poor drainage, which favored pathogen persistence and root infection. The study highlights the increasing importance of F. oxysporum f. sp. phaseoli as a major constraint in common bean production systems and emphasizes the need for accurate molecular diagnostics for early detection. Integrated management strategies, including resistant cultivars, crop rotation, and soil health improvement, are recommended to reduce wilt incidence and sustain bean productivity.