The rust fungus, Puccinia psidii, is a devastating pathogen of introduced eucalypts (Eucalyptus spp.) in Brazil where it was first observed in 1912. This pathogen is hypothesized to be endemic to South and Central America and to have first infected eucalypts via a host jump from native guava (Psidium guajava). Ten microsatellite markers were used to genotype 148 P. psidii samples from eucalypts and guava plus five additional myrtaceous hosts across a wide geographic range of south-eastern Brazil and Uruguay. Principal coordinates analysis, a Bayesian clustering analysis and a minimum-spanning network revealed two major genetic clusters among the sampled isolates, one associated with guava and another associated with eucalypts and three additional hosts. Multilocus genotypes infecting guava differed by multiple mutational steps at eight loci compared with those infecting eucalypts. Approximate Bayesian computation revealed that evolutionary scenarios involving a coalescence event between guava- and eucalypt-associated pathogen populations within the past 1000 years are highly unlikely. None of the analyses supported the hypothesis that eucalypt-infecting P. psidii in Brazil originated via host jump from guava following the introduction of eucalypts to Brazil approximately 185 years ago. The existence of host-associated biotypes of P. psidii in Brazil indicates that this diversity must be considered when assessing the invasive threat posed by this pathogen to myrtaceous hosts worldwide.

Rust caused by Puccinia psidii is one the most destructive diseases of Eucalyptus. Management of the disease is achieved through selection of resistant host genotypes. Recently, eucalypt plants from clone BA6021, resistant to P. psidii isolate race-1, were infected by rust in Brazil. Microsatellite profiles of infected plants confirmed that the host was indeed clone BA6021. In pathogenicity tests, the resistant clones BA6021 and G21 (which carry the resistance gene Ppr-1) were found susceptible to the newly discovered isolate EUBA-1, indicating a new biotype of the pathogen. These results show that the isolate EUBA-1 and other potentially unrecognized pathogen races should be given strong consideration for eucalypt breeding programs aimed rust resistance.

RESUMO -A ferrugem, causada pelo fungo Puccinia psidii, é uma das doenças mais frequentes nos plantios de eucalipto no Brasil. Atualmente, o plantio de clones resistentes constitui a principal estratégia para o controle da doença no campo. Para selecionar clones resistentes, é fundamental inocular e avaliar a resposta fenotípica de diferentes materiais genéticos, o que demanda tempo e recursos. Para facilitar e acelerar essa etapa do programa de melhoramento genético, o objetivo deste trabalho foi avaliar a resistência em paralelo à etapa de multiplicação dos clones de eucalipto pela técnica de micropropagação. Para isso, seis clones foram multiplicados em meio MS, modificado nas fases de multiplicação, alongamento e enraizamento. Após 60 dias de incubação, os explantes foram inoculados com suspensão de esporos do patógeno ajustada para 2x10 4 urediniósporos mL -1 . Os explantes foram incubados a 24 ± 2 ºC, fotoperíodo de 14 h de luz com intensidade de 20 µmol.s . Após 7, 11 e 14 dias da inoculação, avaliou-se a incidência da doença. Observou-se que as reações dos genótipos avaliados em condições de micropropagação foram altamente correlacionadas com os fenótipos determinados pelo procedimento-padrão de inoculação. Assim, o uso desse protocolo permite avaliar grande número de genótipos, com maior rapidez e precisão.Palavras-chave: Puccinia psidii, Resistência, Melhoramento genético, Indexação e Controle de doenças. EVALUATING RESISTANCE OF EUCALYPTUS CLONES TO RUST IN MICROPROPAGATIONS CONDITIONS