More on the chemical control of wheat rust

TAREKEGN TEREFE and PETRUS DELPORT, ARC-Small Grain Institute

Fungal diseases such as leaf and stem rusts, leaf and glume blotches as well as powdery mildew are commonly found on bread wheat in the major wheat growing regions, such as the Western Cape.

Under favourable weather conditions, these diseases can significantly reduce the yield and quality of susceptible wheat. Therefore, disease control practices are mostly required in order to prevent yield losses.

Fungicides are one of the most important components of a disease control strategy in wheat. Particularly when resistant cultivars are not available or when the resistance in existing cultivars becomes ineffective due to the emergence of new races, fungicide intervention provides an effective and practical means of reducing disease outbreaks.

The ARC-Small Grain Institute understands the value of fungicides in the management of wheat diseases and has, over the past 20 years, continually been testing fungicides that are entering the market in order to identify superior products.

The results have assisted producers in the profitable control of wheat diseases using fungicides. As a continuation of this long-term project, field trials were conducted at Malmesbury and Tygerhoek in the Western Cape during the 2013/2014 season to determine the response of two wheat cultivars to fungicide applications at seven leaf and flag leaf growth stages (Table 1).

The two cultivars evaluated were SST 88 (susceptible to leaf and stem rust and moderately resistant to stripe rust) and SST 056 (moderately resistant to leaf and stripe rust and moderately susceptible to stem rust). The results of this study are summarised in the article.

Leaf and stem rust were frequently observed in the experimental plots mainly on the susceptible cultivar SST 88 both at Tygerhoek and Malmesbury. Untreated plots of SST 88 at Tygerhoek displayed over 75% and 30% final leaf and stem rust severity, respectively.

At Malmesbury, the average leaf and stem rust severity on fungicide untreated SST 88 was over 65% and 10%, respectively, suggesting that disease pressure was slightly lower at Malmesbury than at Tygerhoek.

Cabrio applied at seven leaf stage followed by Folicur application at flag leaf stage and Abacus and Folicur applied at seven leaf and flag leaf stages, consistently resulted in a relatively lower level of leaf and stem rust on SST 88. Disease severity on SST 056 was very low (≤ 5%) at both localities, indicating that this cultivar has still retained its resistance to leaf and stem rust.

Fungicide application significantly increased the yield of SST 88 and SST 056 at the two localities. At Malmesbury, SST 88 sprayed with Cabrio at seven leaf stage followed by Folicur at flag leaf stage, Cabrio and Abacus applied at both stages, and Prosper Trio and Prosaro applied at seven leaf and flag leaf stages, respectively, resulted in a significantly higher yield than the untreated control (Graph 1).

Cabrio and Folicur applied to SST 88 at seven leaf and flag leaf stages, respectively, resulted in the highest yield increase of 0,943 ton/ha over the untreated control.

A significantly higher yield for SST 056 was obtained from Acanto, Abacus and a mixture of Acanto and Capitan applied at seven leaf and flag leaf stages (Graph 1).

At Tygerhoek, Abacus, Cabrio, Folicur (each applied twice, i.e. at seven leaf and flag leaf stages), and Cabrio applied at seven leaf followed by Folicur at flag leaf stage, resulted in significantly higher yields of SST 88 and SST 056 compared to controls (Graph 2). At this locality, Cabrio and Abacus increased the yield of SST 88 and SST 056 by 0,850 ton/ha and 0,502 ton/ha, respectively (Graph 2.)

At both localities, yield response to fungicide application was greater for the rust susceptible cultivar SST 88 than for SST 056. The increased yield of SST 88 following fungicide application may have been partly due to the effect of the latter in reducing the level of foliar diseases.

On the other hand, although disease severity on SST 056 was insignificant, fungicides significantly increased its yield over the untreated control at the two localities. This suggests that certain fungicides could increase grain yield not only by controlling diseases, but also through other mechanisms, probably due to their effect on the physiology of the wheat plant. However, the yield increment achieved with fungicide application on the resistant cultivar SST 056 appeared to be relatively low.

Some of the fungicides evaluated here, including Acanto, Cabrio and Flint, have not been registered for use on wheat in South Africa. However, they are registered for the control of various diseases on other crops. Acanto is registered for the control of some barley diseases, whereas Cabrio and Flint are registered for use mainly on fruit crops.

In this study, Cabrio applied individually or in combination with Folicur, performed better than most of the remaining fungicides. Thus, Cabrio appears to be a potentially useful product for the control of wheat diseases. However, additional tests are required to confirm these results before recommending Cabrio for use on wheat in South Africa.

Conclusion

Fungicides can significantly increase wheat yields by effectively reducing disease development. They can provide profitable control on susceptible cultivars grown under favourable weather conditions where disease pressure is likely to be high.

Among fungicides registered for use on wheat in South Africa, and tested in this study, Abacus and Folicur appear to be promising in terms of controlling leaf and stem rust with positive yield responses. Of those fungicides not registered for use on wheat, Cabrio appears to be the best performer. However, this result needs to be verified with additional tests before this fungicide can be recommended for the control of wheat diseases. The resistant cultivar included in this trial showed a smaller yield response to fungicide application, suggesting that it may not be necessary to apply fungicides when resistant cultivars are planted.

For more information, contact Dr Tarekegn Terefe at 058 307 3400.