Fusarium udum revisited: a common, but poorly understood member of the Fusarium fujikuroi species complex

Fusarium udum is the causal agent of a wilt disease on pigeon pea (Cajanus cajan) in tropical regions. This species shares morphological characteristics with F. oxysporum, leading to misidentification when the diagnosis is solely based on morphological markers. The sexual stage of this fungus was observed on stems of Cajanus in India and was formally described as Gibberella indica. In Brazil, a wilt disease is reported on Crotalaria, but the etiological agent has not been identified correctly so far. In this study, we tested the hypothesis that the causal agent of a wilt on Crotalaria belongs to the same species pathogenic to C. cajan. Strains obtained from diseased Crotalaria spp. were characterized through molecular phylogeny of TEF, TUB and RPB2, laboratory crosses, morphological markers, and pathogenicity tests. Strains from Crotalaria from Brazil formed a well-supported clade with F. udum strains from Crotalaria and Cajanus from other countries. Strains from Brazil intercrossed among themselves and also with others from all geographic regions, and formed fertile perithecia, defining a distinct mating population inside the Fusarium fujikuroi species complex. One strain obtained from Cajanus cajan in India is indicated as epitype, and female-fertile tester strains of both mating types were selected for the identification of field isolates through sexual crosses. These results confirm that the species associated with wilt disease on Crotalaria and Cajanus is F. udum. Wilt symptoms caused by F. udum in Cr. ochroleuca are described and illustrated.