Yield losses in wheat from yellow spot: comparison of estimates derived from single tillers and plots

Abstract

Losses in wheat yield associated with yellow spot (Pyrenophora tritici-repentis) have been examined in a field experiment where development of crop and disease were promoted with sprinkler irrigation. Different amounts of infected wheat stubble were applied to initiate epidemics in four treatments, while fungicide sprays were used to reduce the severity of yellow spot in a fifth treatment. The relationship between severity of yellow spot and the amount of infected stubble at first appeared to be linear but became more noticeably logarithmic as the epidemics progressed. Under conditions favouring disease development, a loss in grain yield of c. 49% was measured in the most severely diseased treatment relative to the sprayed treatment, with grain number per unit area and grain size both being reduced. The percentage loss in grain yield was less for main stems than for later heads. Regression analyses of disease severity with grain yield and its components using 50 main stems in each plot gave different estimates of yield loss, depending on the growth stage at which disease severity was assessed. These estimates of yield loss and those provided by a previously developed disease-loss relationship severely underestimated the overall loss in grain yield. However, there was better agreement between estimates derived from the regressions and loss in grain yield on main stems. Possible reasons for the discrepancies in estimates of loss in grain yield are discussed.