Screening Wheat Seedlings for Leaf Rust Resistance

The 365 wheat accessions were evaluated in two independent experiments for seedling response to LR. For the leaf rust screening, wheat seedlings at the two-leaf stage were evaluated for their reactions to described races in the biosafety level 2 (BSL 2) facility at Dalrymple Research Greenhouse Complex, North Dakota Agricultural Experiment Station (AES), Fargo. Briefly, five seedlings per each accession along with susceptible checks were used for phenotypic screening for each LR race. Plants were grown in a 50-cell tray containing PRO-MIX LP-15 (www.pthorticulture.com ) sterilized soil mix and maintained in a rust-free greenhouse growth room set to 22°C/18°C (day/night) with 16 h/8 h day/night photoperiod. At two-leaf stage, the seedlings were inoculated with fresh urediniospores suspended in SOLTROL-170 mineral oil (Philips Petroleum) at a final concentration of 10⁵ spores mL^-1 using an inoculator pressurized by an air pump. The inoculated seedlings were placed in a dark dew chamber at 20°C overnight and then transferred back to the growth room. The infection types (IT) were scored about 12 to 14 days after inoculation, using 0-4 scale, where ‘0’ = no visible uredinia, ‘;’ = hypersensitive flecks, ‘1’ = small uredinia with necrosis, ‘2’ = small to medium-sized uredinia with green islands and surrounded by necrosis or chlorosis, ‘3’ = medium-sized uredinia with or without chlorosis, ‘4’ = large uredinia without chlorosis (Stakman et al., 1962 ). For each IT, ‘+’ or ‘-’ was used to represent variations from the predominant type. A ‘/’ was used for separating the heterogeneous IT scores between leaves with the most prevalent IT listed first. For plants with different ITs within leaves, a range of IT was recorded with the most predominant IT was listed first. The IT scores were converted to a 0-9 linearized scale referred as infection response (IR) (Zhang et al., 2014 (link)). Genotypes with linearized IR scores of 0-4 were considered as highly resistant, 5-6 as moderately resistant, and 7-9 as susceptible.