September 2018


Fungal diseases infecting aerial plant parts of wheat are widely found in South Africa. Leaf diseases which frequently occur on bread wheat include leaf rust, stem rust, stripe rust, powdery mildew, Septoria tritici blotch (leaf blotch) and Stagonospora nodorum blotch (glume blotch). Under favourable weather conditions, any of these diseases can cause significant yield loss on susceptible cultivars.

Although resistant cultivars provide effective disease control, they are prone to infections by frequently emerging new strains of these diseases, emphasising the need for additional measures which could contribute to sustainable control of important wheat diseases. Amongst available options, chemical fungicides can be considered in integrated control of wheat diseases. Hence, field studies were conducted to determine the profitability of fungicide applications on susceptible versus resistant cultivars. Field experiments were conducted over four seasons at Roodebloem and Malmesbury in the Western Cape. Two wheat cultivars namely SST 88 and SST 056, which differ in their resistance to wheat rusts, were used in all tests. SST 056 is moderately resistant to leaf rust and stripe rust and moderately susceptible to stem rust. SST 88 is susceptible to stem and leaf rust and moderately resistant to stripe rust. Different fungicide treatments were applied at seven leaf and flag leaf growth stages and finally, grain yields from fungicide treated and untreated plots were compared.

Averaged over seasons and localities, fungicides resulted in varying levels of yield increments compared to the untreated control on the two cultivars. On both SST 88 and SST 056, Epoxiconazole + Pyraclostrobin, Pyraclostrobin and Pyraclostrobin/Tebuconazole provided better yields than the remaining treatments (Table 1).

About 0,571 t/ha and 0,285 t/ha yield increments over the control were obtained by application of Epoxiconazole + Pyraclostrobin to SST 88 and SST 056, respectively. Most of the remaining fungicide treatments on the resistant cultivar SST 056 provided lower yield improvements of less than 0,200 t/ha. Most fungicides which increased yield, also reduced the severity of leaf and stem rust.

This suggests that the increased yield from fungicide treatments was largely due to the effect of fungicides on foliar diseases. This conclusion can be further supported by the higher yield response recorded from the susceptible cultivar SST 88 than from the resistant SST 056.

Economic analysis was performed to determine the benefits of the different fungicide treatments on the two wheat cultivars. For this purpose, average yields of each treatment over four seasons and two localities were used. The profitability of each treatment was calculated by subtracting costs of double fungicide application from the gross benefit obtained by application of each fungicide treatment. The gross benefits were calculated based on average wheat price of R3 705/ton during March 2018.

Fungicide treatments resulted in different levels of profitability (Table 1). On the susceptible cultivar SST 88, most of the fungicide treatments resulted in positive net profits ranging from R121/ha to R1 266/ha. However, a few fungicides gave negative net benefits of R17/ha to R379/ha. Averaged over all fungicides and seasons, application of fungicides to the susceptible cultivar gave an average profit of R362/ha. On the resistant cultivar, only three fungicide treatments out of eleven tested, gave positive net benefits of R26/ha to R207/ha whereas the majority of the fungicides showed varying levels of negative net benefits (losses). Averaged over four seasons, application of fungicides to the resistant cultivar resulted in a loss of approximately R250/ha.

These results showed that application of fungicide was profitable on the susceptible cultivar while there was little or no benefit in applying fungicides to the resistant cultivar. This indicates that when resistant cultivars are available, they can be used for the control of foliar diseases and protection of wheat yields and such resistant cultivars may not require fungicide application. The use of fungicides should be considered on susceptible, high yielding cultivars which could suffer serious yield losses when disease pressures are high.

Publication: September 2018

Section: Focus on integrated pest control