Partial Resistance to Asian Soybean Rust in South Brazilian Soybean Cultivars: Genotypic Variation and Implications for Management

The state of Rio Grande do Sul (RS) is the third-largest soybean producer in Brazil. Over the past fifteen years, the cultivated area in the state has expanded by 65%, particularly in its southern region. To assess the partial resistance levels of different soybean cultivars to Asian soybean rust (ASR), this study was conducted under field conditions during two growing seasons (2013/14 and 2014/15), evaluating 22 cultivars in southern RS. Although all cultivars were susceptible to the pathogen, disease severity varied significantly among genotypes and between crop years. The highest ASR severity at the R5.3 growth stage was 76.7% in 2013/14 and reached 100% (complete defoliation) in 2014/15. In contrast, the lowest severity was observed in the cultivar TMG 7062 IPRO, with 4.3% in 2013/14 and 8.3% in 2014/15. These findings highlight substantial differences in the levels of partial resistance to ASR among soybean cultivars. In conclusion, the use of cultivars with higher levels of partial resistance represents a viable and reliable strategy for managing ASR in southern RS. However, in seasons with highly favorable environmental conditions for disease development, additional integrated disease management practices may be necessary for most cultivars.