Virulence Pattern of Pyricularia grisea Isolates from Farmers’ Fields on Newly Released Upland Rice Cultivars

The virulence pattern of the isolates of Pyricularia grisea from commercial fields of the upland rice (Oryza sativa) cultivars ‘Primavera’ and ‘BRS Bonança’ was analyzed. A hundred and seventy monoconidial isolates of the pathogen virulent to ‘Primavera’ and 139 to ‘BRS Bonança’ collected from eight fields, during two years (2001-2003) were tested, under greenhouse conditions, on six newly released rice cultivars. Differences in virulence pattern were observed in pathogenic populations of ‘Primavera’ and ‘BRS Bonança’. Isolates with virulence to improved cultivars were common in samples from farmers’ fields in the absence of aloinfection. The virulence frequency of P. grisea isolates collected from ‘Primavera’’ to cultivars ‘BRS Vencedora’, ‘BRS Colosso’, ‘BRS Liderança’, ‘BRS Soberana’, ‘BRS Curinga’ and ‘BRS Talento’, was high in descending order. On the other hand, in the fungus population of ‘BRS BRS Bonança’ virulence frequency was high in ‘BRS Talento’, followed by ‘BRS Curinga’, ‘BRS Vencedora’, ‘BRS Liderança’, ‘BRS Colosso’ and ‘BRS Soberana’. While virulence to ‘BRS Talento’ was rare among isolates from ‘Primavera’, it was most frequent in isolates of ‘BRS Bonança’. The six improved rice cultivars permitted to differentiating agriculturally important virulences in the pathogen population which can be utilized in selecting breeding lines for specific resistance, in rice blast improvement program. Additional keywords: Oryza sativa, Magnaporthe grisea, rice blast, physiologic races, resistance. RESUMO Padrão de virulência de isolados de Pyricularia grisea provenientes de lavouras de cultivares de arroz de terras altas recém lançadas Foi analisado o padrão de virulência de isolados de Pyricularia grisea provenientes de lavouras de arroz (Oryza sativa) de terras altas das cultivares ‘Primavera’ e ‘BRS Bonança’. Foram testados, em casa de vegetação, 170 e 139 isolados monospóricos virulentos às cultivares Primavera e BRS Bonança, respectivamente, coletados de oito lavouras, durante dois anos, 2001 e 2003, em seis cultivares recentemente lançadas. Foram observadas diferenças no padrão de virulência nas populações de ‘Primavera’ e ‘BRS Bonança’. Isolados virulentos para novas cultivares foram comuns nas lavouras na ausência de aloinfecção. A freqüência de virulência de isolados de P. grisea coletados de ‘Primavera’ foi alta em ordem decrescente para as cultivares ‘BRS Vencedora’, ‘BRS Colosso’, ‘BRS Liderança’, ‘BRS Soberana’, ‘BRS Curinga’ e ‘BRS Talento’. Por outro lado, na população do fungo da cultivar BRS Bonança, a freqüência de virulência foi alta em ‘BRS Talento’, seguido pela ‘BRS Curinga’, ‘BRS Vencedora’, ‘BRS Liderança’, ‘BRS Colosso’ e ‘BRS Soberana’. Entre isolados coletados da cutivar Primavera, a virulência para ‘BRS Talento’ foi encontrada em baixa freqüência; porém foi freqüente para isolados coletados da cultivar BRS Bonança. As seis cultivares melhoradas permitiram estabelecer diferenças importantes em virulência nas populações do patógeno, que podem ser usadas na seleção de linhagens resistentes no melhoramento para esta característica. Palavras-chave adicionais: Oryza sativa, Magnaporthe grisea, brusone, raças fisiológicas, resistência. 623 Fitopatol. bras. 30(6), nov dez 2005 INTRODUCTION Rice blast, caused by Pyricularia grisea (Cooke) Sacc. [= Magnaporthe grisea (T.T.Hebert) Yageshi & Udagawa] is the major yield constraint in risk prone upland rice (Oryza sativa L.) in Brazil. One successful means of controlling plant diseases has been the development of cultivars with major or vertical resistance genes, because this type of resistance can be easily manipulated in a breeding program and is effective until the pathotypes of the pathogen to which it does not confer resistance become established (Flor, 1945). Despite the limited durability of this type of resistance, considerable progress has been made in reducing losses caused by rice blast, by planting cultivars with major resistance genes. Several upland rice cultivars such as ‘BRS Colosso’, ‘BRS Curinga’, ‘BRS Liderança’, ‘BRS Soberana’,