CN-MR-3-MissingCornPlantby Daniel Hudson, UVM Extension Agronomist
For the most part, the corn crop is off to a slow start in the Northeast and beyond. It is good to finally see fields with tidy rows of corn plants making the most of sunny days and warmer temperatures. At this time of year, it is not uncommon to get calls about missing corn plants. If you have missing plants or an uneven stand, investigating the matter sooner than later will increase the likelihood that you will find useful indicators of the cause of the problem.
Several years ago I was attending a seminar and a corn farmer was giving a presentation having to do with certain practices on his farm. One of the slides showed a nice picture of his corn field with the plants each about 10 inches tall. The farmer glanced at the slide, probably to remind himself of what he was supposed to say next. Clearly off script, he did a double-take, half-crouched and pointed at a gap in the row of corn in the picture on the screen and said, “All I want to know is what happened to that corn plant!” The inference was, ‘I paid about a 4/10 of a penny each for those seeds, a BUNCH of money for the equipment and field operations that were necessary to put it in the ground, and I was careful….it had better come up and keep growing!’
Sparse plant populations are disturbing and cause a fair amount of anxiety for different farmers each year, especially when things are behind schedule as they have been in 2014. Reasons for plants being missing can include planter problems, cool/variable soil temperatures, compaction, pathogens (damping off diseases), wire worms, cutworms, seedcorn maggots, and sometimes injury from excessive rates of seed-placed fertilizer.
If you dig where a missing plant is supposed to be and find an unemerged seedling with a mesocotyl  (the part between the coleoptile and the seed) that changed direction more than once underground, the phenomenon is commonly known as ‘corkscrewing.’ In some cases, the seedling even ‘leafs out’ below the soil surface. While the leaf occasionally makes it to daylight anyway, the seedling often dies underground. Bob Nielson of Purdue notes several potential factors that can contribute to corkscrewing:
• Compacted soils
• Variable soil temperatures, where the top layer of soil is very warm during the day, but drops dramatically during very cool nights.
• Herbicide injury (seedling growth inhibitors such as acetochlor) in certain soil conditions
• Kernel position in the soil
In one field where I observed corkscrewed corn seedlings last week, torrential rain after planting had caused an extremely dense layer of soil to form over the seeds. While the plant takes gravity into account when deciding which direction to send different plant parts, large fluctuations in daily soil surface temperatures also likely ‘confused’ the coleoptile about which way was actually up. It is not clear how extreme or prolonged these temperature differences need to be for this to happen.
Damping off diseases
Damping off diseases can afflict corn seedlings before or after emergence. If you dig up the top two inches of soil in a gap where a corn plant ought to be, you will often find that it is not simply a ‘skip’ due to the corn planter. You will often find a seedling that is dead, dying, or still in the process of emerging. If you find a seeding that is in the process of emerging and the mesocotyl and seminal roots are a nice white color and crisp, you probably just need to wait longer. This is not to discount the reality of the yield losses that uneven emergence can cause. However, if the aforementioned tissue is brown and/or mushy, the seedling probably succumbed to one of several pathogens, which is not uncommon when soils are cool and moist for long periods of time. Damping off can also occur when pathogens attack the mesocotyl (pictured). If an emerged seedling (V6 or younger) looks sickly and the exhumed mesocotyl is brown or deteriorating, the issue is post-emergence damping off. This is because young corn seedlings rely on nutrients coming from the seed and seminal roots (via the mesocotyl). After V6 the nodal root system begins to be the primary source of nutrients for the plant.
If you find a small white maggot living in/on the seed, it is very likely a seedcorn maggot. This happens most frequently in situations where seed is planted without an insecticide and organic material (manure, cover crop, etc) has been incorporated within the past month or so. The fly of the seedcorn maggot is attracted to the scent of decaying organic matter. The eggs hatch within a few days of deposition and the resulting maggots feed for up to two weeks before pupating. In systems where insecticide is not used and in fields with challenging soils this problem can be minimized by waiting to plant until soils are very warm (rapid emergence and plant development) and/or waiting for several weeks after substantial amounts of organic material have been applied or incorporated. For non-organic farmers, an appropriate insecticidal seed treatment is almost always very helpful.
In a corn field with missing plants and along with plants that came up and then died (generally before V6) wireworms are likely culprits. The most telling indicator of wireworms is the presence of a hole in the crown of the plant, just below the soil line. This generally only happens when the corn plants are still small and the growing point is still below the soil surface. Wireworms are also most common in situations where a sod or cover crop has been terminated. These pests are effectively controlled by soil-applied insecticides unless plant development is so slow that the insecticide loses efficacy.
While missing corn plants, thin stands, and uneven emergence can be very expensive and discouraging, learning to identify the cause of the problem this year can pay dividends in the future. If you need help identifying the cause of a sparse or uneven stand, do not hesitate to contact your local Extension Agronomist.