Espécies biológicas e filogenéticas do complexo Fusarium fujikuroi

Abstract

A group of species known as Fusarium fujikuroi species complex - FFSC derives from the now extinct name Fusarium moniliforme and the former Section Liseola. There are about 50 distinct species accepted in the FFSC, defined by morphological markers, mating populations and multigene phylogenetic analyses. Many of these species are pathogens of important crop plants and some are mycotoxin producers. During the last two decades, a considerable number of new species have been formally described, whereas other lineages continue poorly understood, inclusively in Brazil. The objective of this study was to evaluate the diversity of species of the FFSC that occur in association with plants like Cajanus, Crotalaria, Brachiaria, Musa, and Saccharum, and characterize isolates according to modern concepts of morphological, biological and phylogenetic species delimitation. In all studies, the isolates were characterized by phylogenetic analyses of the three-locus dataset, EF-1α, tub2 and RPB2, sexual crossing, morphology, and pathogenicity tests. In the first part (Chapter 1) the hypothesis was tested that the etiological agent of wilt on Crotalaria is Fusarium udum, as in 1934 the name F. udum f. sp. crotalariae was applied. Isolates from Crotalaria from Brazil group with isolates obtained from Cajanus in India and Africa in a unique clade in a Bayesian analysis of sequences from three gene regions. In laboratory crosses isolates from Brazil and India produce fertile perithecia. In pathogenicity tests all evaluated isolates induced symptoms in plants of Crotalaria ochroleuca. Morphological markers of the sexual and asexual phase are in accordance with the original descriptions of the pathogen. Based on these results it is concluded that the causal agent of Fusarium wilt on Crotalaria and Cajanus is F. udum. The species represent a new, distinct mating population within the FFSC. This is the first report of this species in Brazil and may contribute to the correct diagnosis and monitoring of the disease. In Chapter 2, isolates obtained from symptomless seeds of Brachiaria spp. and sugar-cane plants (Saccharum officinarum) showing symptoms of “pokkah boeng” were characterized by morphological markers, laboratory crosses, multi-gene phylogenetic analyses and pathogenicity tests. On Brachiaria the species F. andiyazi F. proliferatum, F. thapsinum, F. verticillioides were identified, and still two new distinct lineages belonging to the African clade of the FFSC. In the case of sugar-cane the isolates studied grouped together with reference material in a unique clade and formed fertile perithecia when crossed with known tester strains of Fusarium sacchari. The species isolated from Brachiaria spp. caused rot on stems of maize, sorghum and millet. Species from Brachiaria may act as a potential reservoir of inoculum of species which cause disease on maize, sorghum and millet. Crown rot of banana (Chapter 3) can eventually be observed in Brazil, nevertheless, no consistent information about its etiology is available. Isolates from Brazil grouped with reference isolates of Fusarium musae in a unique clade. When crossed with itself and with reference strains of F. musae, the production of fertile perithecia was observed. Selected isolates were inoculated in eight different varieties of banana and induced disease in all of them. Indeed, three genotypes showed some resistance to the pathogen. Other species associated to bananas were identified, like F. proliferatum, F. subglutinans, F. equiseti and F. camptoceras, but its pathogenicity was not confirmed. This is the first report of F. musae in Brazil. The findings will contribute to the correct identification of the pathogen, the future monitoring of the disease and the development of breeding programs searching for resistant varieties. Besides the characterization of the pathogens, evidence was generated that grasses may harbor species which are known as pathogens of important crops and mycotoxin producers. This issue should be investigated in the future with attention under a wider angle. Probably the most relevant aspect of this study was the observation that the diversity of Fusarium species in Brazil occurring as pathogens or endophytes is much bigger as initially expected.