Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
August is at a close and many corn fields are rapidly approaching harvest. If you are out scouting green fields, you may see plants with accelerated senescence due to crown and/or stalk rots (Figure 1). After pollination, the ear becomes the major sink of sugars produced by the plant. If a stress event occurs, plants will divert or remobilize sugars from the stalk and roots to meet the needs of the developing ear. Often the pathogens that cause stalk rots are opportunistic and take advantage of plants that have been weakened by potential stress events (drought, flooding, hail, insect damage, foliar disease damage, etc.). Crown rot is a disease that has been re-emerging over the past few seasons and may be tied more to early season moisture or compaction.
Figure 1. Corn stalk with accelerated plant senescence.
While infection is thought to have occurred early in the season, crown rot symptoms are usually observed at dough through black layer. In fields I have been scouting the past 1-2 weeks, I often see that plants with crown rot will still have some green left in the leaves, while the stalk rot plants are already completely senesced. The closer we get to harvest, the harder it is to tell these differences. The plants that die early from crown rot are often referred to as “ghost” plants. Once split open, you can see discoloration of crown tissue and roots may be brown to black (Figure 2). A few pathogen groups can be associated with crown rot, but Fusarium species seem to be the most frequent. Fusarium species are also able to cause stalk rots, but the symptoms of crown rot and stalk rot are seen in different sections of the stalk. Many of the plants with crown rot can also have stalk rot issues and it is difficult to distinguish specific yield losses from crown rot. This article from Crop Protection Network provides additional Q&A if you have interest in learning more about crown rot.
Figure 2. Corn plants with discolored tissue from crown rot.
Common stalk rots include Anthracnose top dieback or stalk rot, Diplodia stalk rot, charcoal rot, and Gibberella stalk rot. Anthracnose stalk rot is caused by the fungus Colletotricum graminicola (the same organism that causes anthracnose leaf blight), but the disease phases are considered to be separate. When stalk rot symptoms appear in the upper canopy first, this is called anthracnose top dieback (Figure 3). Cloudy, warm humid weather after silking can favor this disease, I have not seen much of it around so far this year. With Diplodia stalk rot, there is no red or pink coloration in the tissue. Black specks (pycnidia) can be observed embedded in the stalk tissue (Figure 4). Diplodia stalk rot is generally caused by Stenocarpella maydis, but Stenocarpella macrospora, the causal agent of Diplodia leaf streak, can also infect ears and stalks (Figure 5). Charcoal rot is caused by the fungus Macrophomina phaseolina, which is the same organism that causes charcoal rot in soybeans. This disease is favored by hot, dry weather, and may be around this season. The fungus moves through the roots and lower stems into the stalk where many small black structures called microsclerotia give the inside of the stalk a speckled appearance. Another common stalk rot in our area is Giberella stalk rot. Affected plants may wilt and have premature loss of green tissue. Plants often lodge at the nodes and there will be reddish-pink discoloration within the stalk (Figure 6). Small black structures (perithecia) can form at the internodes. Unlike the structures associated with Diplodia, these are superficial and can be easily scraped away from the stalk surface (Figure 7). Red root rot is another disease that can look similar. With red root rot, discoloration is usually darker (Figure 8) and any black structures at the base of the plant (pycnidia) will be embedded in the stalk tissue.
Figure 3. Corn plant with anthracnose top leaves are still green.
Figure 4. Diplodia stalk rot with embedded pycnidia in lower stalk tissue.
Figure 5. Spores recovered from corn stalks with Diplodia Stalk Rot. S. maydis (left) is most commonly associated with Diplodia stalk rot. S. macrospora (right) causes Diplodia leaf streak, but can also infect stalks and ears.
Figure 6. Pink discoloration in the stalk from Gibberella stalk rot.
Figure 7. Giberella stalk rot with easily removed black perithecia at the base of the stalk.
Figure 8. Red root rot (Phoma terrestris) symptoms on corn roots.
Once plants are around black layer, you can scout for stalk rots by walking the field in a W pattern and randomly checking stalks with either the pinch or push test. (Aim to check 10-20 plants for every 10-20 acres). For the pinch test, pinch the stalk between the lowest two internodes to see if it can withstand the pressure, if the stalk collapses, it fails. To complete a push test, push the stalk 30-40 degrees from vertical (around 8 inches) and see how many return to upright and how many snap and lodge. In cases where more than 10% of plants are lodging, you may want to consider harvesting at higher moisture and drying grain after harvest to avoid yield loss due to lodging. Since stalk rots are linked to stress, the best management strategies are to reduce stress by planting optimal stand populations, monitoring field moisture (in cases where irrigation is possible), managing insect pests and foliar diseases, and using a balanced nutritional program. Planting hybrids with some level of foliar disease resistance can also help to reduce plant stress and encourage strong stalk development.