THE EFFECT OF POWDERY MILDEW (ERYSIPHE-GRAMINIS F-SP HORDEI) AND LEAF RUST (PUCCINIA-HORDEI) ON SPRING BARLEY IN NEW-ZEALAND .1. EPIDEMIC DEVELOPMENT, GREEN LEAF-AREA AND YIELD

Powdery mildew and leaf rust caused large yield losses in spring barley grown near Christchurch, New Zealand, in two seasons. Disease present during early growth stages was as damaging to yield as disease late in the season. Moderate leaf rust severities after anthesis were most damaging when combined with earlier mildew epidemics. Later growth did not compensate for reduced yield potential induced by early infection. This was attributed, at least in part, to an effect on leaf size, and therefore on green leaf area, at later growth stages. There was a closer relationship, by regression analysis, of yield to green leaf area than to disease severity in three cultivars. The three cultivars, which differed in yield potential and disease resistance, were not equally sensitive to disease. It is proposed that high yielding cultivars may be the most sensitive to yield constraint by disease.