Contribution of adult plant resistance gene Yr18 in protecting wheat from yellow rust

Yellow, or stripe, rust of wheat (caused by Puccinia striiformis), an important disease in many wheat growing regions of the world, is best controlled through genetic resistance. The Yr18 gene is known to confer slow rusting resistance in adult plants. This study was conducted to quantify the effectiveness of Yr18 in reducing losses in grain yield and other traits under high yellow rust pressure. Fungicide-protected and nonprotected plots of two near-isogenic lines, Jupateco 73R with the Yr18 gene and Jupateco 738 without it, were sown on two planting dates during two crop seasons. Yellow rust epidemics were initiated by artificial inoculation. The area under the disease progress curve and final rust severity were significantly higher for both plantings of nonprotected plots of Jupateco 73S, confirming that the Yr18 gene does confer slow rusting in Jupateco 73R. Comparison between protected and nonprotected treatments showed that yellow rust infection caused grain yield losses of 31 to 52% in Jupateco 73R and 74 to 94% in Jupateco 73S. This indicates that the slow rusting resistance conferred by Yr18 protected grain yield in the range of 36 to 58%, depending on the year and sowing date. Grain yield losses in both cultivars were mainly associated with reductions in kernel weight and kernels per m(2) however, reductions in spikes per m(2) and kernels per spike also contributed to yield loss in Jupateco 73S. A reduction in plant height was observed in late plantings of Jupateco 738, suggesting that early yellow rust infection may affect stem elongation on susceptible cultivars. Although deployment of Yr18 alone is not recommended in areas with high yellow rust pressure, previous studies have shown that the Yr18 pyramided with other slow rusting genes, a combination commonly known as the ''Yr18 complex,'' should provide effective control of yellow rust.