Phenotyping adult plant resistance to leaf rust in wheat under accelerated growth conditions

Leaf rust (LR), caused by Puccinia triticina, is among the most important diseases of wheat (Triticum aestivum L.) crops globally. The most sustainable method for controlling rust pathogens is deployment of cultivars incorporating durable forms of resistance, such as adult plant resistance (APR). However, phenotyping breeding populations or germplasm collections for LR resistance in the field is dependent on weather conditions and limited to only once a year. In this study, we report a protocol for phenotyping APR to LR incorporating ‘speed breeding’ technology, which utilizes controlled temperature regimes and 24-hour light to provide accelerated growth conditions (AGC) – enabling up to 6 plant generations of wheat per year. A panel of 22 genotypes, including disease standards carrying known APR genes along with a diversity panel comprising 300 accessions (including winter types and landraces) were characterized for resistance to LR under AGC and in the field. Analysis of genotypes displaying APR revealed that disease response expressed on flag–2 leaves under AGC was highly correlated with field-based measures (R2 = 0.76). Analysis of the diversity panel indicated that APR was expressed by plants that had obtained the stem elongation stage (i.e. GS≥30) prior to inoculation. Despite the high degree of genetic diversity in the panel, strong correlations between LR response under AGC and the field were observed, and were further improved when field response was adjusted based on growth stage (R2 = 0.81). The diversity panel was also screened with DNA markers for known APR genes (Lr34, Lr46 and Lr67), which identified 22 accessions carrying potentially novel sources. This method integrates assessment at both seedling and adult growth stages and requires only seven weeks to complete, enabling up to seven consecutive assays annually. When coupled with ‘speed breeding’, this approach could also accelerate introgression of resistance genes into adapted wheat cultivars.