Effects of the fungicides azoxystrobin and tebuconazole on Didymella exitialis, leaf senescence and grain yield in wheat

The effects of the fungicides azoxystrobin and tebuconazole on Didymella exitialis, and on senescence and yield of wheat, were studied in 10 field trials with low (eight trials), medium-low (one trial) or high (one trial) levels of visible disease over three growing seasons. Fungicides were applied at three alternative growth stages between flag leaf emergence and anthesis. Didymella exitialis was commonly isolated from symptomless seedling and adult leaves, with the frequency of isolation in each leaf position increasing until leaf senescence. Pycnidia of the Ascochtya anamorph of D. exitialis commonly formed on dead areas of seedling leaves, and perithecia on adult leaves, especially on senesced flag leaves. Azoxystrobin reduced the rate of isolation of D. exitialis from symptomless flag leaves and the area of senesced flag leaves with perithecia, while tebuconazole either had no effect or increased levels in symptomless and senesced flag leaves, depending on the trial. There were no consistent effects of crop growth stage of fungicide application on D. exitialis levels. Both fungicides maintained green leaf area for longer than in untreated plots-azoxystrobin on average for 4 days and tebuconazole for 2.3 days across all of the trials. There were also no consistent effects of application growth stage on green leaf area retention. Tebuconazole increased yield in nine of the trials by an average of 5.3% and azoxystrobin in all of the trials (average 6.1%). The best fungicide x timing combination in each trial resulted on average in a 9% yield increase (range 2-27%). This study suggests that D. exitialis is only a weak pathogen of wheat and has a minimal effect on flag leaf senescence. (C) 2004 Elsevier Ltd. All rights reserved.