Effects of powdery mildew and leaf rust on the apical development and yield of barley (Hordeum vulgare L.)

Two barley trials were conducted to study the effects of natural and fungicide modified epidemics of powdery mildew and leaf rust on yield and the development of yield potential. The determination of disease effects on yield and the development of yield potential requires a meaningful disease assessment method. The non-random distribution of disease on leaves of unequal size necessitated the use of a weighted mean to fully describe the level of disease present. Furthermore, a carry-over effect of early disease on leaf area per se can be better accounted for by expressing disease as the total green leaf area per shoot. This suggested that absolute green leaf area should be used for all studies in which the mechanism of yield reduction is to be analysed. The effects of early and late diseases were studied by analysis of the primary and secondary components of yield. Apical development and the formation of grain potentials were studied in both healthy and infected plants. Whilst the total spikelet primordia number was unaffected, tip degeneration of spikelets was enhanced from an early stage and was the major cause of loss in final grain number, especially in plants infected early in the growth season. Late epidemics, on the other hand, induced grain loss by increasing the later determined aspect of yield, the number of infertile spikelets. Undeveloped spikelet number was significantly increased in plants subjected to combined powdery mildew and leaf rust epidemics and severe early epidemics of powdery mildew. A multivariate statistical technique, factor analysis, revealed that early diseases acted directly by increasing spikelet degeneration or indirectly via an effect on green leaf area developed at a later stage resulting in grain number reductions. Powdery mildew reduced the total number of ear-bearing tillers by decreasing tiller survival. Leaf rust at mid-anthesis and early dough had both a direct and indirect effect, via green leaf area, on the mean grain weight, causing a major loss in final yield. These results showed that, at least for the conditions prevailing in Canterbury, New Zealand, early disease was at least as important as late disease infection in causing yield losses. There was no evidence for compensation in later determined components for the reductions in yield potential due to the effects of early disease. Detailed analysis during the growth of the barley crop of tiller and apex development aided the identification of the mechanisms of yield reduction. Thus the causes of yield loss could be used in the development of models describing the relationship between disease and yield reduction. Such models, especially if they could be quantified, could then be used as the basis for prediction of losses and, perhaps more importantly, as a basis for disease control recommendations. Further studies emphasising the crop rather than the pathogen will lead to the development of better disease management programmes.