Variability among Phaeoisariopsis griseola isolates / Variabilidade entre isolados de Phaeoisariopsis griseola

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Common bean (Phaseolus vulgaris L.) angular leaf spot, caused by the fungus Phaeoisariopsis griseola (Sacc.) Ferraris, is responsible for extensive yield losses of common bean in Brazil. Yield loss can reach up to 80% under disease favorable environmental conditions. Breeding for disease resistance is the most effective strategy to control the bean angular leaf spot disease. However, the successful development of a strategy to control and or reduce the impact of P. griseola requires an understanding of the levels of variability and previous knowledge of the population structure of the pathogen. Due to the importance of common bean angular leaf spot in Minas Gerais State and to the high magnitude of pathogenic variability of this fungus, monitoring the pathogenic variability of its causal agent becomes an important tool for the development of adequate strategies for a breeding program aimed at the development of resistant cultivars. Traditionally, the high pathogenic and genetic variation has been evaluated by differential cultivars and molecular markers. Besides, the anastomosis occurrence among hyphae can be taken into consideration as a trait for population studies. The aims of this work were: i) to investigate the pathogenic variability among P. griseola isolates collected in Minas Gerais state, Brazil; ii) to study the genetic diversity and populationâs structure among P. griseola isolates collected in Minas Gerais and Goias state, Brazil, by RAPD markers and iii) to identify the variation among isolates of P. griseola fungus collected in Minas Gerais state, Brazil, by anastomosis groups. Ten pathotypes were identified among forty-eight isolates of P. griseola demonstrating the wide pathogenic variability of this fungus in Minas Gerais State. Pathotypes 55-15, 63-15, 63-25 e 63-27 had not been reported in this state. Pathotypes (63-7, 63-15, 63-23, 63-25, 63-27, 63-31, 63-47, 63-55 e 63-63) identified in this study, except the pathotype 55-15, induced compatible reactions with all andean cultivars and were classified as mesoamerican pathotypes. . Pathotypes 63-31 and 63-63 were the most frequent, whereas pathotype 63-63 was the most widespread among the places studied. Patothype 63-63 overcame resistance genes present in all differential cultivars. Anastomosis bridges were observed in H form among 20 isolates studied. Anastomosis were observed for all the isolates ones, however three isolates presented 85% compatibility, in other words, were compatible with 17 isolates. This is the first report of the anastomosis occurrence among hyphae for the P. griseola fungus. Sixteen anastomosis groups were observed. Isolates from different groups can present anastomosis among hyphae. High variability for anastomosis occurrence among hyphae and ausent clustering among isolates for formation of anastomosis among hyphae demonstrating the existence of genetic variability for loci envolved with control this trait. The genetic similarity, obtained from RAPD markers, among the seventy isolates of P. griseola, varied from 0.301 to 0.993, with average of 0.746. The descriptive analyses revealed tendency of differentiation of isolates for origin areas. The Shannon diversity index revealed that ViÃosa, MG and Ijaci, MG counties presented, respectively, the largest and the smallest genetic diversity. The genetic differentiation among the populations was 0.1979 (GST value). Therefore, 80.21% of the genetic variation observed in this study was due to differentiation within populations. AMOVA demonstrated that 77.51% of the variation was contained within counties and 22.49% among counties. Pairwise comparisons of 76 polymorphic RAPD loci gave disequilibrium values that were all significantly different from zero for studied populations, showing that P. griseola maintains a genetic structure consistent with asexual reproduction.

ASSUNTO(S)

genetic diversity feijÃo estrutura genÃtica mancha-angular diversidade genÃtica pathogenic variability anastomosis common bean genetic structure angular leaf spot anastomose genetica molecular e de microorganismos variabilidade patogÃnica

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