Thanks to the ever-challenging growing environment in the Eastern Corn Belt in 2011, many fields are beginning to show symptoms of stalk rots. Pollination and grain fill puts a tremendous demand on the corn plant. Fields that have been put under a number of stresses are having a hard time keeping up with the photosynthetic demands of the ear. Plants that are unable to keep up with the demand will resort to pulling stored carbohydrates from the stalks and roots and moving it to the developing ears. The reallocation of carbohydrates is the driving factor to stalk rots moving into many corn fields.
The pathogens that cause stalk rots are weak and opportunistic pathogens. Being weak and opportunistic means that stalk rots very seldom affect healthy, non-stressed corn, but instead attack corn plants that have a weakened defense system or are under some other stress. Some typical stresses that can predisposition corn to stalk rot pathogens are excessive heat, drought conditions, excessive rainfall, denitrification, leaf diseases, compressed grain fill period and high yield environments. No matter the stress or even the type of stalk rot, the corn plant will provide the grower a warning as the stalk rots begin to invade the corn plant.
Often the first visual sign of stalk rots moving into a cornfield is premature top die back. Premature top die back is easily identified by the top of the corn plant turning brown while the middle of the corn plant is still green. Premature top die back is not a guarantee of stalk rots, but a strong indicator. A guaranteed visual sign of stalk rots being present in a cornfields is when corn plants appear to be frosted. Very rapidly the corn plants will turn from a healthy green to an ugly grayish color. The change in color may only take a day or two to completely change.
Soon after the leaves on the corn plants begin to wilt and turn gray, the corn stalks will begin to change colors. The stalks will turn from a greenish color to a yellowish tint and then ultimately a brownish color.
The last visual symptoms of a stalk rot invasions is the changes in the color and integrity of the stalk pith. Normally the pith of a healthy plant is white and moist and is attached to the outer rind. On a wilted plant the pith begins to desiccate and lose the bright white color. The pith also pulls away from the outer rind. When the pith pulls away from the outer rind approximately 30% of the stalk strength is lost. The loss of stalk strength leads to lodging and standability concerns.
Infected stalks are easily identified by pinching the lower stalk. If the stalk is easily pinched, it is infected. Also, cutting the stalk open and examining whether or not the pith is pulled from the rind can be used to determine if stalk rots are present.
Even though most growers only notice the above ground symptomology of stalk rots, the entire process begins below the soil surface with the roots.
Root rots always, always, always precede stalk rots. Even before any above ground symptomology appears, the corn roots have begun rotting. The roots are the primary vessel of entry for stalk rot pathogens. Wilted plants can be easily pulled out of the ground due to their deteriorating root system. Upon closer examination of the infected corn plants roots, they will be brown, hollow and dead. The opposite is true for the root systems of a healthy corn plant. Healthy corn plants are securely anchored and difficult to pull out of the ground and will have white, robust roots.
Scouting fields in question is the first step to determine the severity of the problem. The second step is to push 10 plants in a row over at a 45-degree angle. If the plant springs straight back up, the stalks are in good shape, but if the plant continues falling over, stalk quality is compromised. Repeat this procedure in several different areas of the field to accurately determine overall stalk quality.
The next step when dealing with stalk rots is balancing harvest timing, harvest moistures and overall field standability. If 25 to 35% of the tested plants are showing signs of stalk rots, an early harvest plan should be implemented to ensure harvest occurs prior to severe stalk lodging and loss of yield due to down corn.