Diversidade de Fusarium spp. causadores de podridão radicular do feijoeiro comum

Abstract

Ninety-six Fusarium isolates retrieved from soil samples collected in the Brazilian states of Paraná, Minas Gerais, São Paulo and Goiás were investigated, in order to identify the species complex that causes root rots in common bean. The first study consisted of pathogenicity tests and identification at the species level of 24 isolates from each state, which comprise the main growers of this crop in Brazil. Root infection was assessed by the inoculum layer method, in a completely randomized design with five replications. In addition to disease incidence, root rot severity was estimated according to a disease rating scale and McKinney index. Statistical analysis consisted of Shapiro-Wilk test, analysis of variance, and mean separation by Scott-Knott test, both of them at 5% of significance. All isolates caused root rot symptoms in common bean seedlings, with two different groups of disease severity. The species identification was carried out in carnation leaf-agar medium and optical microscopy, and by sequencing of the Tef-1α gene. Sixty five isolates were identified as Fusarium solani, 27 as F. oxysporum and three as F. proliferatum, according to morphological descriptors and DNA similarity above 95%, in comparison to reference sequences deposited in GenBank and Fusarium-ID database. The distribution of species and disease severity of were not linked to the geographical origin of the isolates. A second study aimed to estimate the physiological diversity of F. solani and F. oxysporum isolates according to the assimilation profile of 95 different carbon sources, assessed in FF Biolog microplates. Each Fusarium isolate was evaluated in duplicate, with 100 l of a suspension of 2 x 105 conidia ml-1 adjusted in phytagel sterile solution deposited in each microplate well. Each isolate was evaluated in duplicate, after 72 hours of incubation in the dark at 25°C, with absorbance assessed at 750 nm. This test followed a completely randomized design, with a 2 × 4 × 95 factorial arrangement, for evaluation of individual factors (species, isolate origin state and C source) and their interactions. The results were submitted to analysis of variance, with averages compared by Scott-Knott test (5%). Metabolism of C sources was also analyzed by descriptive statistics and by principal component analysis, to identify the main causes of variation and correlation between variables. The results showed differences between the metabolic profile of F. solani and F. oxysporum and between the geographical origin of the samples, with isolates from Goiás forming a clearly distinct population from the other states.Therefore, it was found that 100% of the isolates were pathogenic to common bean, and that the three species (F. solani, F. oxysporum and F. proliferatum) form a pathogen complex distributed in different Brazilian states. The physiological differences between F. solani and F. oxysporum suggest that these species may inhabit different soil niches, and that carbon sources assimilation profile can be used to identify species and populations adapted to different environmental conditions.