A comprehensive and balanced fertility program is essential to create high wheat yields. To maintain high yields, plants need to be protected from foliar and head diseases later in the season, if weather conditions are favorable. Most foliar diseases accelerate senescence of the top two leaves, bring harvest date forward and reduce yields and grain quality. Fungicides applied late in the growing season help prevent this green leaf area loss, protecting yields and profits.
Canopy expansion accelerates as temperatures warm in the spring and large upper leaves emerge. For high yields, the optimum green area of leaves and stems (measured as Leaf Area Index or LAI) should be around 3:1 or 4:1. This means there is three to four times the leaf area compared with the area of ground that the crop is growing on. Maximum LAI of a wheat crop is reached as the heads emerge, just before grain fill begins. Wheat yields have not been found to increase beyond an LAI of 4:1.
The critical time to scout fields for disease is when the flag leaves emerge. Depending on the region, rainfall and previous crop, common yield-reducing problems can include leaf rust, stripe rust and Septoria nodorum (also known as leaf blotch or glume blotch, depending on the stage of infection).
All of these diseases are variety-specific. Varieties that are susceptible to one or more of these diseases, for example, will likely experience a significant yield reduction when the disease pressure is high, but varieties resistant to specific diseases might not suffer significant yield loss.
One additional disease that is common in the eastern half of the U.S. is Fusarium head blight, or scab. Scab is generally prevalent east of a line stretching from eastern Texas to central North Dakota. It has been found to be a significant problem across most of this area for three to four years out of every 10.
Scab is not a disease that you can scout for. The only way to protect the grain is by applying a foliar fungicide before the disease invades the crop. Triazole fungicides such as Prosaro and Caramba have been found to offer the best performance, especially when coupled with forward and backward nozzle configurations and 15 gal. per acre of water.
Fungicide time. The ideal window of application for maximum disease control in a cereal crop is relatively narrow, so adequate sprayer capacity is required to cover all acres in a timely fashion. I suggest that growers have sufficient capacity to spray all their wheat acres within a three- to five-day window.
The optimum spray timing for control of leaf and stripe rust, in addition to leaf blotch, is when the flag leaf has just fully emerged. If weather conditions are favorable for scab, the optimum suppression timing is at flowering, three to five days after head emergence, depending on temperature.
To maximize fungicide performance, fields should be carefully scouted to determine the order in which they need to be treated and when. If leaf diseases are not present or conditions have not been favorable for foliar disease development, a sound practice is to wait until flowering to suppress scab and protect the plant from head and leaf diseases.
Don’t wait until flowering if heavy disease pressure is present within the canopy at flag leaf emergence, as yield loss and grain quality reductions might occur at the expense of a second pass across the field with a fungicide.
Spraying too early, when the flag leaf is not fully emerged, results in insufficient spray coverage on the leaf and lower disease control. Most fungicides are only locally systemic, so while they might move down the leaf, they won’t move from one leaf to another. Waiting too long, however, is worse.