Yearly changes in virulence of Phakopsora pachyrhizi isolates in Paraguay

Asian soybean rust (ASR), caused by the pathogen Phakopsora pachyrhizi, is one of the most notable soybean diseases in Paraguay because of the associated productivity losses and expenditures related to fungicide use. There is also concern regarding the environmental impact of excessive fungicide use. In addition, the decreased sensitivity of ASR pathogens to various fungicide groups has been reported in recent years. To effectively control ASR, it is necessary to combine fungicides with resistant varieties. To introduce ASR resistance genes (Rpp: Resistance to P. pachyrhizi) into Paraguayan soybean cultivars, the effectiveness of Rpp resistance genes must be determined, as a few are rendered ineffective due to virulence changes in the pathogen population. In this study, the effectiveness of 12 different soybean genotypes with different Rpp genes was evaluated using 18 P. pachyrhizi samples obtained in the 2016/17, 2017/18, 2018/19, and 2019/20 seasons and compared with the previous eight seasons from 2007 to 2015. Yearly changes in virulence were observed in a few differentials; however, these changes were not always from avirulent to virulent. Further, all differential varieties with a single resistance gene showed resistant phenotypes in less than 50 % of the samples in their least effective years. Considering these observations, the introduction of a single resistance gene poses a risk in breeding for ASR resistance. The differentials carrying a single gene, Rpp1-b or Rpp5, tended to show high frequencies, specifically 86.9–89.8 % resistance in the tested samples from 2007–2019, and that of the Rpp-pyramided line carrying Rpp2 + Rpp4 + Rpp5 was 97.0 %. Therefore, the introduction of two or more resistance genes, via combination with Rpp1-b and/or Rpp5, or the introduction of Rpp2 + Rpp4 + Rpp5 would be effective and durable for mitigating the Paraguayan ASR pathogen.