Constraints on breeding for disease resistance in commercially competitive wheat cultivars

Abstract

Breeding of arable crops such as wheat has led to substantial improvements in yield, quality of produce, traits of agronomic value and resistance to disease and abiotic stress. Resistance to many diseases and pests of wheat has improved through a combination of innovation by breeders, driven by competition, and independent assessment of cultivars in Recommended List trials. In addition to pleiotropic effects of resistance genes, two further limitations on breeding for disease resistance are linkage drag and competition drag. Linkage drag, the slow rate of decline of linkage disequilibrium between closely linked genes, can be especially significant in wheat, particularly after the introgression of chromosome segments from related wild grasses. For example it slowed the deployment of the Pch1 gene for resistance to eyespot in leading cultivars because Pch1 is on a chromosome segment that also reduces yield. Linkage drag also inhibited the use of Pm16 for resistance to powdery mildew. Competition drag can be described as the delay in the deployment of a useful gene because of the additional time needed to raise yield and quality to the standard of the current market‐leading cultivars. This is exemplified by the near‐absence of resistance to Soil‐borne cereal mosaic virus in most modern varieties, despite no evidence for detrimental pleiotropic effects. Changes in agricultural practice and regulation of pesticides are increasing demand for durable resistance to disease but wherever possible, improvement of resistance should not constrain selection for yield which remains the most significant trait in leading wheat cultivars for the UK.

Durable Resistance of Crops to Disease: A Darwinian Perspective

“…tritici ) in the Midwestern United States in the early 2000s (92), and unusually heavy epidemics, such as that of brown (leaf ) rust of wheat in the United Kingdom in 2007 (54), cause financial loss by reducing yield or requiring greater spending on fungicides. For such a farmer, durable resistance is useful when it helps to achieve the primary goals of maximizing yield and quality (127). For a breeder who supplies these farmers, durable resistance has several benefits: It sustains sales of successful cultivars over several years, reduces the need for investment in new sources of resistance, prevents the loss of desirable parents because they have become disease susceptible, allows selection to be focused on the main objectives of yield and quality, and enables the marketing budget to be invested in promoting new cultivars that should have a long commercial lifespan.…”

Unraveling the molecules hidden in the gray shadows of quantitative disease resistance to pathogens

“…In plant breeding programs focusing on QDR, one of the limitations to be considered is the genetic linkage between the genes conferring resistance and closely linked undesirable genes, a phenomenon called linkage drag (Summers and Brown, 2013). Undesirable genes may affect the commercially accepted gene pool and, therefore, modify the quality and crop yield.…”

“…Through high throughput genotyping and the use of haplotype analysis of the introgressed region (QTL), linkage drag in seedlings can be detected and tracked in order to subsequently backcross these individuals to resistant varieties lacking drag. This strategy was applied to detect and remove the linkage drag around the Rpv12 gene and confer resistance to powdery mildew in wine grapes (Vitis vinifera L.) (Venuti et al, 2013); alternatively, the marker-assisted recurrent selection (MARS), combined with genomic selection (GS) (Heffner et al, 2009), can also contribute to solving the linkage drag problem for QDR (Summers and Brown, 2013). GS selects plant material carrying whole genome molecular marker that are associated with resistance to a specific pathogen through the prediction of the phenotype using breeding values (BV) (Falconer and Mackay, 1996).…”

Evaluation of Genetic Diversity amongthe Pakistani Wheat (Triticumaestivum L.) Lines through Random Molecular Markers

“…Wheat nourishment is superior in protein 11.2%, 6.8% pentosans, 1.7% ash, and 70% of cases, and having a higher percentage of carbohydrates than other crops [ 3 ]. Wheat production facing serious problems in semi-arid regions in the world due to changing environmental conditions [ 4,5 ] several pathogenic diseases [ 6 ] and its greater nutritional value [ 7,8 ]. The lower wheat yield in Pakistan is the result of limited diversity in the genome, which is used in breeding programs [ 56 ].…”