The relationship between leaf blotch caused by Rhynchosporium secalis and losses in grain yield of spring barley

SUMMARY Two methods were used to investigate the loss in grain yield associated with specific levels of leaf blotch. Yields from plots sprayed with fungicide were compared with those from unsprayed plots and yields of varieties of different susceptibility to the disease were compared with one another. A disease assessment key is presented, which was used to assess the percentage laminar area of the top two leaves affected by the disease. A linear relationship between disease on the upper two leaves and yield was established. Results from nine trials showed a consistent relationship between the disease level, at growth stage 11·1 (Feekes scale), and loss in yield. The loss in yield expressed as a percentage of the yield of an uninfected crop was equivalent to approximately two‐thirds of the percentage of the flag‐leaf area visibly infected, or one‐half of the infected area on the second leaf. The predicted loss in yield is the average of these two estimates.