Early foliar fungicide applications

By Pierce Paul and Dennis Mills, Ohio State University Extension

Most of our wheat is now beyond Feekes 6, jointing, and spring N applications have been completed in most areas. Attention is now being shifted to foliar fungicide applications, with questions again being asked about what to apply, when to apply. Another question that should be asked is whether a fungicide should be applied. With current wheat prices and the wet conditions we have had so far this spring, producers are concerned about protecting their valuable wheat crop. When conditions are as wet as they have been this spring, foliar disease development is certainly a reason for concern.

Septoria blotch is usually one of the first to show up, and it already has been reported in some fields. This disease is favored by cool (50 to 68 degrees F), rainy conditions, and although it usually develops early in the season, it really does not cause yield loss unless it reaches and damages the flag leaf before grain fill is complete. Like many other foliar diseases such as Stagonospora, Septoria reduces grain fill and the size of the grain. It usually does not affect the number of spikelets per spike, an important yield component that is defined very early in the development of the plant. As a result, a foliar fungicide application at green-up or jointing is less likely to be as beneficial for Septoria and Stagonospiora control as an application made at or after flag leaf emergence. An early application will certainly control Septoria and powdery mildew, another disease that usually shows up early under cool conditions, but the residual effects of the fungicide will not adequately protect the flag leaf. If the weather conditions continue to be rainy and favorable for foliar disease to develop, spores will continue to be produced or blown in from other areas, and new infections will occur, even after early applications have been made. In addition, frequent rainfall may also reduce the residual effects of the early fungicide applications, making them even less effective against mid- and late-season foliar disease development. Results from previous studies have shown that the greatest benefits from foliar fungicide applications were obtained when applications were made between Feekes 8 and 10. This is largely because most of our major foliar diseases usually develop and reach the flag leaf after Feekes 8-9.

During the 2010 growing season, we tested several early (before flag leaf emergence) foliar fungicide application programs in wheat to provide answers to question about the benefit of early applications. The month of May was wet and highly favorable for the development of diseases such as powdery mildew, Septoria and Stagonospora blotch, and head scab. It was an excellent year for testing fungicides. We evaluated the effects of single, split, and double applications of several triazole (Prosaro, Caramba and Folicur), strobilurin (Headline), and combination (Twinline, Quilt, and Stratego) fungicides on powdery mildew, Stagonospora, head scab, and grain yield. Applications were made at green-up, flag leaf emergence, boot, and flowering. Among the single application programs, applications made at flag leaf emergence or boot did better than green-up applications in terms of foliar disease control and yield. A single application of a triazole at flowering provided the best control of head scab. Among the programs with double or split applications, we observed the best results with those treatments that included an application at full rate at Feekes 8-9. A single full-rate application at flag leaf emergence did just as well or better than the green-up and flag leaf.

Comparing a single application at flag leaf emergence with a single application at flowering, all of the tested fungicides resulted in better control of powdery mildew when applied at flag leaf emergence than when applied at flowering, and comparable levels of Stagonospora control were achieved with the two programs. This is largely because powdery develops early and as such applications made at flowering are generally too late to provide the best control of this disease. Stagonospora, on the other hand, usually develops later in the season, and in a wet growing season like 2010, foliar fungicides may still provide very good control when applied at flowering. In fact, because of the high levels of powdery mildew, Stagonospora, and head scab we had in 2010, the fungicide programs that provided the best overall control of all three diseases and resulted in the highest yield gain were those with a triazole applied at flag leaf emergence followed by a second application at flowering.

So, to answer questions regarding whether to apply a foliar fungicide, what to apply, when it should be applied, here are a few things to consider. Should a fungicide be applied? Yes, if the variety is susceptible and weather conditions are favorable for disease development. When should the fungicide be applied? It depends on the disease.

Early (before flag leaf emergence) applications will not provide adequate protection against Stagonospora. Application at flag leaf emergence or at heading will control both Stagonospora and powdery mildew, but will not control head scab. Fungicides must be applied at flowering to achieve the best results against head scab. An application made at flowering will also control late-season Stagonospora development.

Which fungicide should be applied? This also depends on the disease being controlled as well as the cost of the fungicide. Choose the fungicide that is the cheapest and most effective against the disease being controlled. There are lots of excellent triazole (Prosaro, Caramba and Folicur), strobilurin (Headline, Quadris), and combination (Twinline, Quilt, and Stratego) fungicides labeled for use in wheat and they are all effective against foliar diseases. However, the triazoles provide the best results against head scab. In addition, the Strobilurins are usually not recommended for head scab management because they have been shown to result in vomitoxin increase in the grain. A fungicide efficacy chart can be found on our Web site:

http://www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/OFCDwheatfungicides.pdf.