Diagnosing stalk rots

By John Brien, CCA, AgriGold agronomist

The diseased stalk on the left is much more prone to lodging than the healthy stalk on the right.

There have been numerous cases of anthracnose stalk rot moving into corn fields throughout the area. Unfortunately, stalk rots are often misunderstood. Many times when stalk rots move into a field, an assumption is typically made that the particular hybrid had bad health and poor standability, but in reality that assumption is not always correct. To understand stalk rots and why they affect certain hybrids, fields or even certain plants, an investigation into the entire stalk rot cycle must occur.
One of the most important facts about stalk rots is that they are opportunistic pathogens. Being opportunistic means 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. There are many different issues that can cause a corn plant to be susceptible to stalk rots, but this article will focus on the stresses causing the stalk rots this year.
When evaluating why a field has been infected by stalk rot, a grower must take into consideration the entire growing season. Stalk rots do not occur because of one event, but rather there are combinations of issues that compound and cause stalk rots. To gain an appreciation for the complexity of stalk rots we need to start at the beginning of the growing season.
When a corn seed is planted, its sole purpose is to reproduce and multiply. Between planting and tasseling, the corn plant is busy building the foundation for grain production. The plant is building an energy factory of green leaves.
The plant determines the amount of grain it can support depending greatly on its environment early in the growing season. Finally, the corn plant has a fully functioning defense mechanism intact, meaning there are plenty of sugars to maintain and grow roots as well as keeping the leaves healthy. By the time tasseling occurs, a corn plant has the largest root structure it will have and all leaf production is complete. At tasseling a total switch of priorities occurs; the plant transforms from vegetative in nature to reproductive. So, what does this mean?
When a corn plant switches from vegetative to reproductive, the most important difference is the redistribution of sugars. During the vegetative process, sugars are used to build leaves, maintain and build roots and help fight off diseases or infections. During the reproductive portion of a corn plant’s life, the sugars are used first for ear and grain development and the remaining will be utilized by the rest of the plant.
Once the ear is pollinated and the kernels begin to fill, the ear becomes a “sink” or a major user of sugars. The ear demands a certain amount of sugars every day no matter how much sugar the plant produces. Under normal conditions and yield levels, a corn plant is able to produce all of the sugar that the ear requires and often has a small amount left over for the plant. If any event occurs that limits the amount of sugar being produced, the corn plant must then look at other sources for its needed sugar.
Photosynthesis is the process in which plants turn sunlight and carbon dioxide into energy or sugars. Plants use the chlorophyll in the plants green tissue, i.e. leaves, to run photosynthesis. As mentioned earlier, under “normal” growing conditions corn plants are able to produce enough sugars via photosynthesis to meet the kernel’s needs. Anything that limits the amount of green leaf area or the photosynthetic rate can cause a strain on the system. Items such as leaf diseases (Gray Leaf Spot, Common & Southern Rust, Northern Corn Leaf Blight, etc), hail damage, leaf shredding, drought conditions, lack of nitrogen and cloudy, overcast days can all limit the amount of photosynthesis that can take place in a corn plant.
The other factor that can cause a strain on the sugar supply system is large kernel counts. Early in the growing season, favorable weather and ample nutrients can lead to the plant setting large kernel counts on each ear; this is also known as high yield potential. So, how does a high kernel count affect the sugar supply system? If you take into account that every kernel needs the same amount of sugar to grow and mature, an ear with more kernels will require more sugar or energy to fully develop. Large kernel counts put an additional strain on the corn plant by often requiring more energy than a plant can produce.
When the loss of photosynthetic potential and high kernel counts are combined, the stress on the corn plant greatly increases. When a corn plant is lacking in sugar production, the plant begins pulling sugars away from other parts of the corn plant. The defense mechanisms are shut down, energy for growing and repairing roots is eliminated, and sugars from
stalks and roots are cannibalized for grain production. Earlier in the discussion, stalk rots were categorized by being opportunistic. The opportunity stalk rots are looking for these particular conditions.
Stalk rots are always preceded by root rots. Even before any above ground symptoms appear, the corn roots are rotting. Plants infected with stalk rots can be easily pulled out of the ground. Upon closer examination, the roots will be brown and dead. The opposite is true for a healthy corn plant. Healthy corn plants are securely anchored, difficult to pull out of the ground and will have white robust roots.
Stalk rots aim to attack those plants most susceptible and do not necessarily target a particular hybrid. Granted, there are hybrids that seem to be more susceptible to stalk rots than others, but in reality those hybrids tend to have larger kernel counts and are under more energy stress than other hybrids. To rationalize why certain hybrids have stalk rots, a grower must examine the yield potential and overall stress a hybrid has gone through before eliminating a high yield potential hybrid.
Scouting fields and harvesting early when 25%-35% of the plants are showing signs of stalk rots are the best options to prevent lodged corn later in harvest.

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