Role of Fungicides in Maximizing Grain Yield of Barley1

Application of fungicides to cereals normally increases grain yield, this usually being accounted for by control of well-defined diseases. Sometimes, however, yield increases are obtained when apparently trivial amounts of defined diseases are present or when their control seems insufficient to explain the observed benefit. Explanation for these ← unexpected → increases have been sought in: a) control of ← weak → pathogens or organisms not considered pathogenic but which may accelerate leaf senescence, and b) direct effects of fungicides on the plant's physiology. Whatever the precise explanation in individual instances, we may ask whether there is some general relationship linking fungicidal action, plant growth and yield. In barley, recent studies of controlled epidemics of powdery mildew have shown: a) that there is a strong correlation between severity and duration of mildew and reduction in green leaf area (GLA) integrated over time; b) that the grain yield of plants is highly correlated (r values often approaching unity) with values of GLA integrated for the period from seedling emergence to anthesis. The data also show that retranslocation of stored carbohydrate, produced before anthesis, plays an important role in grain filling, and indicate that mildew post anthesis may have little effect on yield. The main implication of these studies is that, in barley, fungicide treatments which increase GLA prior to anthesis are likely to enhance yield. Evidently this may be achieved by different means (control of pathogens, including ← weak → pathogens, or by directly extending the functional life of leaves); the end result, in terms of grain yield, will, however, be the same.