Control of tan spot (Drechslera tritici-repentis) using cultivar resistance, tillage methods and fungicides

Tan spot caused by Drechslera tritici-repentis has become a more common disease in winter wheat in Denmark in recent years and non-inversion tillage and intensive winter wheat production were found to be the major factors influencing its severity. In order to optimise disease control measures, cultivar resistance, tillage methods, and fungicides were investigated under Danish conditions. Relatively few of the commonly grown cultivars had high levels of resistance to tan spot; Stakado, Legron and Senat were among the most resistant cultivars. The ranking of cultivars for resistance to tan spot in three different seasons was fairly consistent. In cultivars with similar maturity disease severity differed between the most susceptible and the most resistant cultivar by a factor two to four depending on the leaf layer. This led to a difference of 10 days in senescence of the flag leaf. Field experiments verified that non-inversion tillage was found to be a major factor increasing the severity of tan spot compared with conventional ploughing. Yield increases from fungicide treatments in crops with non-inversion tillage varied from 0.8 to 1.7 tonne ha−1 for effective control measures. In experiments where large amounts of wheat straw was left on the surface, yield increases of up to 4.4 tonne ha−1 were obtained from the most effective fungicide treatments. Under conditions favourable for the development of severe tan spot a three-spray fungicide strategy may be required, but in most cases a one or two-spray strategy gave sufficient control and the best margin over fungicide cost. Pyraclostrobin, picoxystrobin, propiconazole and prothioconazole were the most effective fungicides for control of tan spot. Reduced rates of the fungicides generally provided adequate control, and the margin over fungicide cost was maximised when 25–50% of the recommended rates were used. As strobilurin resistance is known to occur within the tan spot population, triazoles are currently considered the most reliable fungicides for control of this disease. A threshold model for tan spot control based on visual assessments carried out in the field was developed as part of the decision support system Crop Protection Online (CPO). The model is specifically directed at farmers growing winter wheat after winter wheat and practicing non-inversion tillage. The model was validated for three seasons and has so far shown reasonable control scenarios using thresholds depending on the growth stage and susceptibility of the cultivar.