THE EFFECT OF LEAF-RUST (PUCCINIA-HORDEI) ON YIELD RESPONSE IN BARLEY(HORDEUM-VULGARE L) CROPS WITH DIFFERENT YIELD POTENTIALS

The effect of leaf rust was investigated on field-grown crops, in two consecutive seasons, each at a single location, using a non-replicated design. The experiments involved three crops and 10 distinct epidemic s in the 1987/8 season, and four crops with seven epidemics in the 198 8/9 season. The epidemics were concurrent in each season in subplots w ithin each crop, and the yield range was from 2.2-9.0 tha(-1) in nil-d iseased plots. Variation in the experiments, assessed as the percentag e coefficient of variation, was in the same range as similar, but repl icated, experiments with fewer treatment combinations. Early sowing an d high agronomic inputs were associated with greater leaf areas, green area durations and yields than in comparable crops sown later or with lower inputs. Disease intensity was described both by per cent diseas e severity and by the effect on green area. Disease reduced yield most ly via reduced number of fertile ears (mean 31% for all crops) and gra in weight (21% mean reduction). Reductions in these components were gr eater in the 1987/8 season than in the 1988/9 season, but there was no consistent effect associated with yield potential. The number of grai ns per ear was affected much less (2.5% mean for all crops) than the o ther components. There was little evidence for compensation between yi eld components.