Capture the Flag (Leaf)
Posted: April 23, 2013
In my area, south-central PA, wheat has started the stem elongation stage which means it’s time to get ready to scout for disease and be prepared to apply a fungicide if necessary.
Powdery mildew infection will first be seen on the lowest leaves of wheat and may gradually move upwards on the plant if conditions are favorable for it. Yield losses occur when it damages the upper leaves. Above all, (no pun intended) the top or flag leaf needs to be kept free of disease infection. For that reason, fungicide applications are most effective when applied soon after the flag leaf has begun to emerge.
Since wheat leaves appear by emerging from within the stem, how do you know when the top leaf that you see on a wheat plant is, in fact the flag leaf? Here are some ways to tell if the flag leaf is emerging instead of one of the lower leaves.
Collect a few wheat plants. Dig them out so the lowest parts of the stems remain intact. Pick out the more advanced stems. The first thing to locate is the first node. This should be somewhere between 1 and 3 inches above the soil line. Strip off the lowest leaves to reveal the first node. If you strip off the leaf that was located just above this node to fully reveal it, then there will be three more leaves left on the plant between the first node and the flag leaf. If you only see three leaves, then the flag leaf has not yet emerged from the whorl. When you see the fourth leaf emerging then you know that this is the flag leaf.
Another method that you can combine with counting leaves is to look for the formation of the third node. You will be able to feel the third node just above the second node if the flag leaf is beginning to emerge. If you cut open the stem just above the third node, you will be able to see the early stages of the head beginning to develop.
If the disease has progressed up the plant and reaches action threshold guidelines, a fungicide application should be made when the flag leaf is from half to fully emerged from the whorl. Avoid applying Strobilurin fungicides at or after flowering as they have been reported to increase DON production. However, some fungicides applied for head scab control may also give you good control of late season leaf diseases. Be sure to use all fungicides according to their label.