Corn Top Dieback

With corn harvest soon upon us, it is especially important this year that we spend time scouting our corn fields and prioritizing which fields need to be harvested first and which ones can wait. In walking corn fields, we are seeing a fair amount of senescing or dying leaves at the top of the plant. If the plant is showing senescence or leaf death at the top and bottom of the plant, then that can be a typical dry down pattern of some hybrids, especially under stress. When the plant is showing leaf death from the top down, that is called top dieback or top kill.

Figure 1 Top dieback from anthracnose

Top dieback is when leaves begin to die or senesce from the top down earlier than normal. The most common stresses that cause top dieback are drought, insect damage or disease. Top dieback caused by drought or hybrid-specific top senesce will be fairly uniform across a field. Top dieback caused by insect damage or disease will be more random across the field. Damage from tunneling insects like corn borer are less common today due to the wide use of Bt traits.

A common cause of top dieback is anthracnose. Anthracnose is a fungal disease that infects the plant before tassel and sits dormant until late season stress triggers it to break dormancy. This typically shows up as scattered plants and the flag leaf turning yellow, purple, or brown. Symptoms of anthracnose include black lesions that are visible on the outer stalk tissue behind the leaf sheaths. If you split the stalk, the pith will be discolored or rotted in the upper nodes. Conditions favorable for infection include cloudy days with high humidity and plant stress following pollination. Figure 1 shows top dieback from anthracnose.

Corn plants with top dieback are usually pushed to black layer sooner and typically give up some kernel depth. Fields that are showing dieback from heat or drought stress may still have decent stalk integrity, while top dieback from anthracnose could have stand issues from stalk rot. It is a good idea to evaluate stalk integrity to get an idea of which fields need to be prioritized for a timely harvest.

To evaluate stalk integrity, walk across a portion of the field and perform Pinch, Push, Shove and Slash tests along the way looking at both green, healthy plants and plants that are yellow/tan or stunted:

Pinch the stalks 4 to 8 inches off the soil level. If it collapses, you have a crown or stalk rot, or both.
Push the stalks so the top of the plant touches the row across from it. If it buckles under that pressure, odds are you have a stalk rot.
Shove a few stalks down the row and see how much pressure it takes to break them. The less pressure it takes to break them, increases the probability of standability issues.
Slash or cut the stalk 10 to 12 inches above the soil line with a sharp knife down to or through the roots. (Remember to be safe and cut away from yourself.) It is a good idea to dig up the plant first before cutting. The crown and stalk should be a white or creamy color. Figure 2 shows a healthy stalk and root on the left, whereas the plant on the right has a crown or stalk rot.

Figure 2 Healthy stalk and root (left) and rotted stalk and root (right)

The more plants that fail these tests in a field, the more likely you will need to harvest that field early. Remember that these affected plants will be more susceptible to wind damage the longer the harvest window gets. Having and following a harvest order decreases the likelihood of harvesting down corn, and increases the likelihood of capturing more yield, but also having a safer and less stressful harvest season.