Relationship between powdery mildew infection, green leaf area and grain yield of barley

Barley plants (cv. Julia), grown in a greenhouse in large boxes of John Innes compost at a spacing equivalent to 300 plants/m², were exposed to powdery mildew from growth stage (G.S.) 2 (Feekes scale) until maturity. Treatment with ethirimol was used to provide five different epidemic patterns, viz. (i) no mildew, (ii) early mildew, (iii) late mildew, (iv) continuous mildew and (v) continuous mildew but delayed by seed dressing. None of the treatments affected the number of spikelet primordia differentiated on primary shoots, nor were the numbers and sizes of leaves affected. From graphs of mildew and green leaf area (GLA), plotted from G.S. 2 to G.S. 10.5, areas under the curves were determined. There was a strong negative correlation (r= -0.926) between areas under the mildew and GLA curves and it was also clear that reduction in GLA lagged behind disease. Grain yield was strongly correlated with total area under the GLA curve (r= 0.994 for primary shoots and 0.986 for tillers). For values of GLA at specific growth stages the best correlations were found at G.S. 9 (r= 0.967 for primary shoots and 0.992 for tillers). The correlation between total yield of primary shoots and area under the mildew curve was also high (r= 0.953) but for mildew estimates at specific times only those at G.S. 7 were significant (r= 0.980); at G.S. 10.5 there was no significant correlation. It may be postulated from the data that retranslocation of stored photosynthate, produced before anthesis, plays an important role in grain filling and that mildew post anthesis may have little effect on yield. The implications of the results are discussed in relation to methods of loss appraisal and strategies for mildew control.