Stalk rots are one of the most frequent and damaging issues for corn production. Corn plants produce sugars (carbon) through photosynthesis. These sugars are used as energy for growth and tissue maintenance. Extra sugars are stored as free sugars and starch in the roots and stalk. Plants move their sugars to areas where they are needed most or to put it another way, they move from a supply (e.g. foliage) to a demand (e.g. rapidly developing tissues). Keep in mind that sugars are the finances of the plant. Just like our finances, they are finite and limiting in supply. Plants therefore budget their sugars to preserve, protect, and grow tissues. Stalk rots are favored by conditions that alter the overall sugar budget and carbon allocation patterns in the plant. Anything that significantly alters the sugar budget of the plant can ultimately have some impact on late season stalk strength. I will describe why this is the case later in this article.
Common factors that can reduce the carbon budget, ordered by level of importance, include: 1) water stress, particularly during grain fill, which limits photosynthesis and nutrient update. Even minimal stress are sufficient to result in issues with stalk rots; 2) nutrient deficiency, which can result in inefficient photosynthesis; 3) pest and pathogen damage, which can reduce leaf area (pests and necrotrophic pathogens) or pull carbon and nutrients from tissues (rusts). Plants can increase their photosynthetic rates to accommodate pest and damage up to a certain point. Once that point is met, carbon budgets may be affected.
The ear is the major sugar sink after pollination. This means that the plant starts to move sugars from sources (foliage, particularly the ear leaf) to the developing ear. Consequently, the shift in sugar allocation comes at the expense of root and stalk tissue maintenance and defense. If a stressful event occurs, such a drought, the carbon budget is reduced. To meet the resource demands of the ear, additional sugars are diverted and even remobilized from the roots and stalk. This results in stressed roots, which can decay and be colonized by stalk rotting fungi. The colonization further inhibits water uptake, and the plant may not recover, even if plants receive subsequent water. An example of this is evident in research conducted by Schneider (1983) where corn hybrids were exposed to water stress pretassel, post pollination, and during grain fill. Half of the plants had their root zones inoculated with a stalk rotting fungus. When the corn was irrigated several days after the water stress event, those plants that were inoculated with the stalk rotting fungus had twice as much resistance to water movement as uninoculated controls, and behaved as if they were still under drought stress.
After colonizing the roots, stalk rotting fungi may develop and grow into the stalk. The stalk may already be weakened due to a lack of sugar to maintain tissues. As a result, the fungi can easily grow up the length of the stalk and degrade the internal pith and supportive tissues. This effectively turns the stalk from a rod to a tube. This is when you start to see issues with lodging and other factors at harvest.
The best time to scout for stalk rot is 40-50 days after pollination. Suspect plants may have “droopy” ears and leaves that may appear flaccid or grey. Over time, you will see the plant prematurely senesce, and the outer rind of the lower stalks will turn a yellow to brown color, whereas surrounding healthy plants will still be green. It is at this time that you may observe the inner stalk pulling away from the rind and weakened stalks.
To determine the lodging potential due to stalk rot, scout multiple sites per field; one site for every 10 acres is a good rule of thumb. Randomly select 10 or more plants per site and either push these plants 30 degrees from vertical or pinch the stalk near the lower node. Alternatively, you could use a pipe or bar of a given length (e.g. 5 ft) and push down a length of plants 30 degrees from vertical, then record the number of lodged plants observed within that length. A final and more time consuming way to assess your plants is to actually split stalks and look for decayed tissues within the stem. Fields with 10-15% of stalks that lodge should be the first fields you harvest.
What can you do to minimize stalk rots?
The first thing is to minimize stress, particularly water and nutrients. This means: 1) planting at a recommended and not excessive plant population; 2) using supplemental irrigation if possible; 3) ensuring a balanced nutrient program. The next thing is selecting hybrids with excellent stay green characteristics and good resistance ratings to common, residue borne diseases such as grey leaf spot and northern corn leaf blight. There are also hybrids with resistance to the anthracnose stalk rot pathogen that can be considered. These hybrids by their very nature will have stronger, more durable stalks due to their genetics. In addition, the resistance to grey leaf spot and northern corn leaf blight mean that these diseases, if they develop, will do so very slowly and therefore have minimal impacts on the overall carbon budget. Other diseases, such as common rust, typically arrive late after black layer and are not a major concern in our area. However, you can always minimize risk by avoiding hybrids rated poorly for any foliar disease that we encounter in Delaware and Maryland. 4) manage insects and foliar diseases if needed. Although fungicides can occasionally, and unpredictably impact lodging potential under minimal disease pressure, you will get the most “bang for your buck” when they are applied and prevent movement of diseases to the ear leaf and above by R5. It’s always a good idea to include at least one untreated strip if possible to help with your future management decisions. 5) Facilitate residue decomposition and rotate. This will help minimize the initial amounts of residue-borne diseases that potentially impact your corn, thereby reducing potential impact to the plant, carbon budget, and ultimately, stalk rots.