Research at K-State and other regions continue to demonstrate that it is often possible to achieve high levels of foliar disease control with a single fungicide applied between flag leaf emergence and heading growth stages. The yield response to this later fungicide application is influenced by the level of disease risk (amount of disease and predicted weather conditions), variety resistance to the most threatening fungal diseases, crop yield potential, foliar fungicide efficacy, and other factors.
Fungicides can also be applied as an early application made between “spring green-up” and jointing. This application may provide some yield benefits in some fields and years. Early fungicide applications may result in small yield advantages due to a reduction in early disease establishment in the lower canopy. This may be particularly true for “leaf spot diseases” such as tan spot or Septoria leaf blotch that survive in wheat residue and can establish early in the year. Yield benefits are most likely in wheat fields planted back into wheat stubble and when weather conditions are wet enough to favor fungal disease development.
Many active ingredients have use restrictions, during a single season. It is important to ensure that early fungicides do not limit options for fungicide applications at flag leaf (which have the potential for higher yield protection in conducive disease years).
Advantages and limitations of split applications -there is no additional cost for application if the fungicide is tank mixed with other products, such as liquid nitrogen fertilizer or herbicide. However, the optimal timing for an early fungicide application is not until after the wheat has jointed – with one or two joints present. This is usually sometime in mid-to-late March in southern Kansas and later in northern Kansas. Top-dressed nitrogen and many postemergence herbicides should be applied before this stage to be most effective and, in many cases, to be within label restrictions, so the optimal timing of both applications may not match. A separate trip for an early fungicide application adds to the cost of production.
Provides suppression of early-season disease caused by tan spot, powdery mildew, and septoria leaf blotch that overwinter locally in Kansas. The benefits of fungicides applied at green-up are more sporadic for diseases like leaf rust and stripe rust, which are less likely to survive the winter in Kansas.
The limitations of early-season fungicide application include. Leaves not present at the time of application will not be well protected. Therefore, these applications will not control leaf rust or stripe rust epidemics that come in from the south at later stages of growth. The early applications are most effective when combined with a second, later fungicide application.
Additional product costs may not pay off under some conditions, when the crop's yield potential may be limited by drought. Remember, the second application does the heavy lifting in the dual-application approach. If capital resources are limited because of low prices, investing your money where you are likely to see the largest yield response may be best.
The main conclusions from recent studies in Kansas and Oklahoma are:
· In K-State studies, the greatest average profit has come from the flag leaf application of fungicides. Fungicides applied prior to jointing are less likely to result in a positive profit.
· The likelihood of profit for an early-season fungicide application is greatest for susceptible varieties in continuous wheat systems with a high level of surface wheat residue.
· Fields with hot spots of tan spot, septoria leaf blotch, and stripe rust prior to flag leaf emergence are candidates for an early fungicide application, provided environmental conditions are conducive for further disease development and yield potential of the crop. These applications are often most effective when made around the jointing stages of growth.