Fusarium udum revisited: a common, but poorly understood member of the Fusarium fujikuroi species complex
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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.
KeywordsBiological species Cajanus cajan Multigene phylogeny Wilt disease
This work is dedicated to the memory of Walter Gams, who introduced the first author to the secrets of the genus Fusarium, when he could not even imagine the importance that this act would have in his further professional life. Thanks are due to Edson Luis Rezende for skillful technical assistance and to Dr. Ivan Pereira for preparing the perithecial sections. We also thank Gilson Soares, Universidade Estadual do Maranhão, São Luiz, Brazil, Antonio Logrieco, ISPA, Bari, Italy and Ludwig M. Niessen, TU München, Germany, for providing field isolates and reference strains.
This research was supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq (Proc. 406335/2013-5). Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior CAPES provided a fellowship to MMC and SSC, and CNPq to MPM. LHP acknowledges a grant given by CNPq (Proc. 308291/2013-1).
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