I spent last week out in the field assessing corn emergence. The warm weather we finally received last week really helped corn jump out of the ground. However, uneven emergence due to the cool wet conditions over the last 2 weeks was evident in most fields I was in. Cool wet conditions while the corn is trying to germinate can cause the seeds to rot, failure of the radicle root to emerge, the mesocotyl to corkscrew, or the plant to leaf out underground. I saw all these symptoms in the fields I walked but none of these symptoms appeared to be severe enough to force a replant.
When assessing corn emergence and seedling health in your field, first check the corn seed for firmness by squeezing it with your fingers. The corn seed should be firm to the touch and not mushy. A mushy seed likely means a rot has set in and emergence is likely not going to take place. If the radical root and mesocotyl have emerged, these should be creamy white in color and the mesocotyl and coleoptile should be intact. The mesocotyl is the portion of the seedling that pushes the coleoptile (the portion that contains the first true leaves) to the surface. If either the mesocotyl or coleoptile are severed, this ensures death of the seedling. If both the mesocotyl and coleoptile are damaged, the plant could still survive and emerge. It’s likely this year to see some mesocotyl corkscrewing or leafing out underground before the plant emerges. From what I’ve seen, a combination of surface crusting along with cool soil temps and wet conditions are the root cause for this. When conditions are cool & wet, damage to the cell membranes of the seedling disrupt normal function and energy transfer within the seedling causing the leaves to prematurely emerge or mesocotyl to corkscrew. If the seed imbibes (takes up) cold water within the first 24-36 hours, damage to the membranes within the seed could occur preventing the radical root from emerging and the seed not to germinate at all. I saw this be the case where water had stood for a few days. Seedling injury could also be the result of herbicides that leach into the seed zone shortly after planting. Mesocotyl corkscrewing and leafing out underground are the common symptoms of herbicide injury during germination and emergence. Typically the herbicides that cause this are the growth regulators (2,4-D & Dicamba) or the group 15’s (acetochlor, metolachlor). If used at high enough rates or if the product does not contain a safener, injury could occur if heavy rain pushes the herbicide into the seed trench and cool conditions prevail preventing metabolism of the herbicide. An open seed trench while the herbicide is being applied is a great way for herbicide injury to expose itself. I have not seen any injury from herbicides yet this year but with the amount of rain most have received accompanied with cool conditions and the potential that some fields were planted wetter than ideal means it could show up this year.
Unless water has stood over a field for an extended period (1 week or longer) it appears that most corn is emerging just fine with the expectation that’s some of it is uneven. Unfortunately, many around my area received substantial rain over the weekend with more rain in the forecast for this week. Try to stay positive and remember to always stay safe!
1:Picture showing the mesocotyl and coleoptile of a corn seeding
2:The seed imbibed cold water shortly after planting forcing the radical to abort failed germination
3:This plant leafed out underground due to the cool wet weather and surface crusting