The flag leaf of the wheat plant, the last leaf to emerge, is responsible for about 70 percent of the effective leaf area that contributes to grain fill.
For this reason, researchers say protecting the flag leaf is imperative to getting maximum crop quality and yield.
"Whereas the lower leaves contribute to growing the plant, the flag leaf and to some extent the flag minus one leaf are the leaves that put the photosynthate into the grain," said Dr. Eugene Milus, a plant pathologist at the University of Arkansas. "So the longer you can keep those leaves healthy, the better off you are."
Research shows that because the flag leaf is in the very top of the canopy, it is in the primary position to receive most of the light within the canopy. As light strikes the crop, the leaves on top receive more light than the leaves on the bottom because of the shading effect.
Diseases that infect the flag leaf threaten yield, and the length of infection plays a key role in how great the yield loss is. Early and sustained disease pressure tends to result in great yield loss.
This is because the plant is less able to develop the spikelets needed to produce maximum yield. Essentially, the plant would lack the needed number of grain heads. However, if disease strikes the flag leaf after these grain heads have formed, the plant would lack the ability to fill those grain heads with kernels, which not only affects yield potential but also test weight and quality.
"Usually diseases that come in later would have a disproportionate effect on test weight, meaning that there would be a lot of kernels there, but they wouldn't fill," Milus explained. "Whereas if the disease came in earlier, you might not have the kernels there in the first place."
Researchers agree that an application of a fungicide, such as Quilt, can positively impact protein levels, test weight and quality of the grain.