Quantitative resistance of barley cultivars to Rhynchosporium secalis

Quantitative resistance in spring barley cultivars with different genetic b ases of resistance to scald was investigated in field trials conducted in 1 993, 1995, and 1996. The variables measured included area under the disease progress curve (AUDPC), apparent infection rate (AIR), infection frequency (IF), lesion length (LL), lesion width (LW), time to disease onset at 5% s everity (T-5), final severity (FS), and sporulation (SP). Grain yield and t housand-kernel weight (TKW) of each cultivar in plots protected with Tilt(R ) were compared with those in nonprotected plots. The eight cultivars evalu ated were statistically separated into susceptible and resistant groups acc ording to the variables measured. 'Argyle,' 'Harrington,' 'Klages,' and 'Ja ckson' were placed in the susceptible group and 'AC Stacey,' 'Johnston,' an d 'Leduc' were placed in the resistant group. 'CDC Guardian' became progres sively more susceptible each growing season, indicating that its resistance was overcome by new pathotype(s) of the scald pathogen. Scald caused signi ficant yield and TKW losses based on a combined analysis of the three-year data. Grain yield and TKW of susceptible cultivars were reduced more than t hose of resistant cultivars. AUDPC, FS, AIR, Tg, IF LL, and LW were signifi cantly correlated to each other. Percent TKW relative to plots protected wi th Tilt(R) was negatively correlated with AUDPC, FS, AIR, IF, LL, and LW, p ositively correlated with T-5, but percent yield relative to plots protecte d with Tilt(R) was not correlated with any of these variables. 'Leduc' reta rded disease development, as evidenced by a smaller AUDPC, lower AIR, reduc ed IF, longer T-5, and lower FS. 'Leduc' was consistently tolerant to scald , having the lowest reduction in yield among the eight cultivars. Therefore ,'Leduc' can be used as a source of durable resistance by breeding programs in western Canada.