Field trials of a Rpp-pyramided line confirm the synergistic effect of multiple gene resistance to Asian soybean rust (Phakopsora pachyrhizi)

Asian soybean rust (ASR) is the most serious disease affecting soybean production in South America. Planting resistant genotypes is one way to control the disease; however, ASR populations in South America exhibit high pathogenic diversity. The soybean genotype No6-12–1 with three resistance genes (Rpp) to ASR has exhibited resistance to most of the South American ASR populations in laboratory trials. However, little is known about the resistance responses of No6-12-1 under field conditions. Here, we compared the resistance of six different genotypes of soybean to ASR under field conditions: (1) No6-12-1, a line with a pyramid of the Rpp2, Rpp4, and Rpp5; (2–4) lines with only Rpp2, Rpp4, or Rpp5, (5) PI 587880A, which harbors Rpp1-b, and (6) BRS 184, a susceptible genotype. Both fungicide-treated and untreated plots were grown in three cropping seasons, from 2014 to 2018, in the Brazilian state of Parana. We evaluated disease severity, area under disease progress curve (AUDPC), the number of uredinia per lesion (NoU), and urediniospore production of the six genotypes. Both fungicide treatments and genotype affected disease severity and AUDPC, and genotype affected NoU. No6-12-1, the pyramided genotype, showed lower disease severity and AUDPC than the other genotypes that harbored only one resistance gene, except for sprayed plots of PI 459025 in the 2017/2018 crop season, and PI 587880A in the 2016/2017 and 2017/2018 crop seasons. NoU and urediniospore production were lower in No6-12-1 than in the other genotypes. These results indicate that the synergistic effects of Rpp-gene-pyramiding observed in laboratory assays also occur, especially in NoU, under field conditions in Brazil.