Evaluation of Virulence of Phakopsora pachyrhizi and P. meibomiae Isolates

Abstract

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi and recently discovered for the first time in continental United States, has been of concern to the U.S. agricultural industry for more than 30 years. Since little soybean rust resistance is known, and resistance is often difficult to detect or quantitate, we initiated a project to develop a better, more quantitative, method. The methodology determined the average numbers and diameters of uredinia in lesions that developed on leaves of inoculated plants 14 days after inoculation. It was used to compare virulence of P. pachyrhizi isolates from Asia and Australia and P. meibomiae from Puerto Rico and Brazil, collected as many as 30 years earlier, with isolates of P. pachyrhizi recently collected from Africa or South America. Susceptible reactions to P. pachyrhizi resulted in tan-colored lesions containing 1 to 14 uredinia varying greatly in size within individual lesions. In contrast, on these same genotypes at the same time of year, resistance to other P. pachyrhizi isolates was typified by 0 to 6 small uredinia in reddish-brown to dark-brown lesions. Using appropriate rust resistant and rust susceptible genotypes as standards, examination of uredinia 14 days after inoculation allowed quantitative comparisons of sporulation capacities, one measure of susceptibility or resistance to soybean rust. The study verified the presence and ability to detect all known major genes for resistance to soybean rust in the original sources of resistance. It demonstrated that soybean lines derived from the original PI sources, and presumed to possess the resistance genes, in actuality may lack the gene or express an intermediate reaction to the rust pathogen. We suggest that a determination of numbers and sizes of uredinia will detect both major gene and partial resistance to soybean rust.

Resistência de genótipos de videira à ferrugem

“…avenae Fraser & Led.) (Chaves et al, 2004), e ferrugem asiática da soja (Phakopsora pachyrhizi) (Bonde et al, 2006;Azevedo et al, 2007).…”

Variabilidade genética de Phakopsora pachyrhizi avaliada por meio de marcadores microssatélites

“…Contudo, no ano seguinte, esta resistência foi quebrada. Essa cultivar apresenta o gene denominado Rpp (FT2) (Yamanaka et al, 2008) A variabilidade genética em populações de P. pachyrhizi tem sido relatada de acordo com a variabilidade na virulência do patógeno (Sinclair & Hartman, 1999;Yamaoka et al, 2002;Bonde et al, 2006) e por análise molecular de regiões ITS (Freire et al 2008). Anderson et al (2008) (Soares et al, 2008), o que mostra que há variabilidade na patogenicidade do fungo no Brasil.…”

“…Foi constatado que o gene Rpp1 confere resposta imune da soja ao patógeno, pois, após a inoculação de certos isolados, as plantas não apresentaram sintomas visíveis . A resistência conferida pelos genes Rpp2 a Rpp5 limita o crescimento e a esporulação do fungo, por meio de reação de hipersensibilidade que provoca lesão marrom-avermelhada visível, descrita como "reddish-brown" (RB) (Bonde et al, 2006;Garcia et al, 2008). A interação entre a ferrugem-asiática-da-soja e as plantas suscetíveis resulta em lesões de coloração bronzeada denominadas "Tan" (Bromfield & Hartwig, 1980;Bromfield, 1984;Miles et al, 2006).…”

“…Diversos trabalhos relatam a existência de variabilidade genética em populações de P. pachyrhizi (Sinclair & Hartman, 1999;Yamaoka et al, 2002;Bonde et al, 2006;Freire et al, 2008). As fontes identificadas de resistência, no entanto, são todas específicas a determinados isolados do fungo (Bonde et al, 2006).…”

“…As fontes identificadas de resistência, no entanto, são todas específicas a determinados isolados do fungo (Bonde et al, 2006). Yamaoka et al (2002) (Soares et al, 2009).…”