Blast samples were collected during [2004][2005][2006] in different rice growing regions in São Paulo State, Brazil. The 71 monosporic isolates of Pyricularia grisea obtained were grouped into physiologic races based on the reaction type on a set of eight international and nine japanese differentials. Twenty-one pathotypes were identified among the 71 blast isolates according to the international set. The IB, ID and IG groups were most frequently detected (23, 21 an 17 times each, respectively). The virulence frequency of isolates to the genes Malavolta, V.M.A.; Carqueijo, A.P.; Mendes, L. Pathogenic variability of Pyricularia grisea in the State of São Paulo, Brazil. Summa Phytopathologica, v.35, n.1, p.49-51, 2009 Keywords: blast, physiologic races, resistance genes, Oryza sativa.in the international differencials varied from 0 (Raminad Str.3 variety) to 90.1 % (Sha-tiao-tsao). Based on the reaction of japanese differential set, 36 pathotypes or races were identified. None of the 71 isolates were capable of overcome the resistance of the gene pita 2 , and only one isolate the resistance gene pi-z t . On the other hand, 63.4%, 59.1% and 53.5% among these isolates were virulent respectively on varieties Aichi Asahi (gene pi-a), Yashiro-mochi (gene pi-ta) and Ishikari-shiroke (gene pi-i).A brusone, principal doença do arroz, é causada pelo fungo Pyricularia grisea (Cooke) Saccardo [teleomorfo Magnaporthe grisea (Hebert) Barr], microrganismo que apresenta alta variabilidade patogênica, com existência de grande número de raças fisiológicas. A utilização de cultivares resistentes é o método ideal de controle da doença, porém a alta variabilidade e natureza complexa do patógeno tem dificultado a obtenção de cultivares com resistência durável, que perdure mais que 2-4 anos. Diversas séries diferenciais têm sido empregadas nos estudos de caracterização de patótipos do fungo, sendo a primeira e mais utilizada até hoje a série diferenciadora internacional (2). A caracterização das raças fisiológicas de P. grisea ocorrentes em determinada região é essencial para entender a dinâmica da virulência do fungo e de grande importância para orientação dos trabalhos de melhoramento, possibilitando adoção de estratégias adequadas para aumentar a durabilidade da resistência à brusone. NOTAS CIENTÍFICAS
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