RESUMO
A população de Puccinia triticina, agente causal da ferrugem da folha do trigo, é extremamente dinâmica na região do Cone Sul da América do Sul, onde o surgimento de novas raças é freqüente. A cultivar de trigo "BRS 194" foi a segunda variedade com maior disponibilidade de sementes para plantio no Rio Grande do Sul e em Santa Catarina em 2005, por apresentar características como rusticidade, elevado rendimento de grãos e resistência qualitativa a todas as raças de P. triticina ocorrentes no Brasil e em outros países da América do Sul. Em 2005, pústulas de ferrugem da folha foram observadas sobre plantas desta cultivar, tanto em lavouras quanto em campos experimentais, indicando a possível superação de sua resistência. O objetivo deste trabalho foi identificar a raça de P. triticina presente em amostras oriundas de "BRS 194" e verificar se esta se tratava de uma nova virulência do patógeno, a qual teria superado sua resistência. Oito amostras de ferrugem provenientes da cultivar "BRS 194" foram enviadas em 2005 à Embrapa Trigo, Passo Fundo, Rio Grande do Sul, para isolamento e identificação por meio da Série Internacional de Hospedeiros Diferenciais específica para este patógeno. Todas as amostras apresentaram a mesma combinação de virulência, a qual correspondeu ao código MFP-CT, conforme o Sistema Norte-Americano de nomenclatura do patógeno. Esta foi a primeira vez que este código foi detectado no Brasil, caracterizando o surgimento de uma nova raça de P. triticina virulenta à "BRS 194". Outras cultivares de trigo, também amplamente semeadas, foram inoculadas com a nova raça, sendo que algumas foram suscetíveis e outras resistentes. Entre as cultivares resistentes estão "Fundacep 30" e "IPR 84", as quais permanecem também resistentes a todas as raças do patógeno ocorrentes no Brasil.
The population of Puccinia triticina, the causal agent of wheat leaf rust, is extremely dynamic in the South Cone region of South America, where the appearance of new races is frequent. The wheat cultivar 'BRS 194' was the second variety in availability of seeds for planting in Rio Grande do Sul and Santa Catarina States in 2005, because it has good agronomic characteristics as rusticity, high yield and resistance to all races of P. triticina occurring in Brazil and other countries in South America. In 2005, pustules of leaf rust were observed in this cultivar on farms and in experimental fields, indicating the probable breakdown of its resistance. The objective of this research was to identify the race of P. triticina present in samples from 'BRS 194' and verify if it was a new virulence of the pathogen able to overcome its resistance. Eight samples of leaf rust from 'BRS 194' were sent to Embrapa - National Wheat Research Center, Passo Fundo, Brazil for isolation and identification in International Sets of Differential Hosts specific for this pathogen. All samples showed the same virulence combination, which corresponded to MFP-CT according to the North-American System of nomenclature for the pathogen's races. That was the first time that this letter code was detected in Brazil, characterizing the emergence of a new race of P. triticina virulent to 'BRS 194'. Other popular varieties were inoculated with the new race, some were susceptible and others were resistant. Among the resistant cultivars were 'Fundacep 30' and 'IPR 84', which also remain resistant to all races of the pathogen occurring in Brazil.
RESUMO
A genetic analysis of adult plant resistance to leaf rust (Puccinia triticina) was performed in in vitro obtained double haploid progenies from a cross between the Brazilian wheat cultivar Trigo BR 35, which, under the high inoculum pressure of the southern region, has been resistant to leaf rust for more than 12 years, and the susceptible cultivar IAC 13-Lorena. Haplodiploidization via in vitro gimnogenesis was done by somatic elimination of the pollen donor genome after maize pollination of the F1 plants. The advantages and usefulness of double haploids (DH) for genetic analysis of complex inherited traits like durable adult-plant resistance to wheat leaf rust were evident: it was possible to analyze inheritance patterns in this cross by using only the 35 DH homozygous segregant lines obtained by in vitro embryo culture of F1 flowers pollinated by maize, this number being equivalent to 1,225 conventional F2 lines because of lack of heterozygosity. After being infected with MCG and LPG races, the results indicated that Trigo BR 35 has two resistance genes. One of the genes expressed resistance only after the intermediate stage of plant development (5-6 leaves).