Over the last few years we have observed several new trends in the timing of foliar fungicide applications in wheat. In 2010, we conducted several field trials at multiple locations to evaluate the efficacy and benefit, if any, of these new fungicide programs. Our current recommendations are to apply a foliar fungicide at flag leaf emergence or heading to control foliar diseases if a susceptible variety is planted and weather conditions are favorable for disease development.
For head scab management, we recommend applying a triazole at flowering, along with planting the most resistant variety and rotating wheat with soybeans. These recommendations are based on years of research that consistently show they are the most effective and economical fungicide programs for disease management in wheat.
However, recently, some growers have expressed interest in, and others have already been using, one or more of several new foliar fungicide application programs, including applications at green-up, split applications and applications at flowering.
Quite often, the primary purpose of these new programs is to preserve or protect yield and not necessarily to manage diseases. Growth and development of the wheat plant can be divided into four distinct phases: tillering, jointing or stem extension, heading and ripening. The main yield components of the crop are head-bearing tillers/acre, spikelets per head, kernels per head and kernel size.
These components are determined at different stages of plant development, therefore the crop needs to be managed during each of the four stages to achieve the best yield and minimize yield and quality losses.
Seeding rate, planting depth, planting date, fertilizer application and weed and seedling disease control are all important for determining the number of tillers/acre, and as such, are commonly managed at planting or during the tillering stage of crop development.
The head begins to develop and the number of potential spikelets per head and head size are determined during the late-tillering and early jointing growth stages (Feekes 5 and 6). Nitrogen application is important at this stage of crop development, since it can affect the number of kernels per head. Stresses such as severe drought at Feekes 5 and 6 may also reduce the potential number of kernels per head. However, the number of kernels that ultimately develop per head and size of these kernels depend on pollination and grain fill.
The flag leaf (the uppermost leaf of the plant) contributes about 75% of the compounds needed for grain fill. As a result, it is very important to protect the flag leaf from damage caused by foliar diseases and insects, since these may substantially reduce grain yield and quality if the damage occurs before grain fill is complete.
Before the 2009-2010 growing season, most of the early (before flag leaf emergence) and late (after heading) foliar-fungicide application programs had not been tested in Ohio with currently available fungicides. However, results from previous studies showed 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, with the exception of powdery mildew, usually develop and reach the flag leaf after Feekes 8 and 9. In 2010, we evaluated the effects of single, split and double applications of several triazole combination 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 and 9.
A single full-rate application at this growth stage did just as well or better than the green-up plus flag leaf or the flag leaf plus flowering applications. 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.
However, it is rarely ever beneficial to make two foliar fungicide applications in wheat in Ohio (and some other eastern production areas). The yield gain is generally not sufficient to offset the cost of two applications. If foliar diseases are a concern, then one well-timed application between Feekes 8 and 10 should be sufficient to control powdery mildew, Stagonospora, Septoria and leaf rust. If head scab is of concern, a well-timed application at flowering will provide the best control of scab, while at same time provide protection against the late development of Stagonospora and rust. In general, conditions favorable for scab development are also favorable for Stagonospora.