Root injury: If seedlings are growing poorly from the start of the season, always check the roots for damage. It is not too late in the season to check the roots. The seed remnant can still be easily found, which will indicate planting depth. From the remnant seed, you can check the radicle, which is the larger root growing from the tip of the seed. A healthy radicle should be a few inches long and will have many lateral roots. These lateral roots should be slender and are often 0.5 to 1 inch long.

Also look at the mesocotyl, which is the section of shoot between the seed and nodal or crown roots. The mesocotyl should still be white and healthy at this time of the year. The crown roots at the base of the plant should be growing rapidly at this time of the season. They should also bear slender lateral roots.t this time of the season. They should also bear slender lateral roots.

The herbicide modes of action that can directly affect corn root growth include 1) growth regulators, 2) ALS inhibitors, and 3) seedling root growth inhibitors.

Growth regulators like 2,4-D, dicamba, and the clopyralid in Hornet would not carryover from last year to injure corn. A preemergence application of 2,4-D or dicamba could potentially cause growth regulator injury. Shallow planting depth, open seed furrow, rain, and coarse soil are factors that would contribute to the risk of injury. Growth regulators can inhibit or prune roots. With more severe cases, corn roots can be shortened and thickened.

There are ALS inhibitors that can persist and carryover to injure corn. The risk may depend on the rate (over-application or over-laps), soil pH (high pH increases the persistence of sulfonylureas and low pH increases the persistence of imidazolinones), a dry summer, or late season applications. Certain ALS herbicides (e.g. Harmony GT) degrade rapidly and would not carryover. Python or Hornet applications to corn also pose an injury risk, which may be increased with over application, shallow planting, depth, or OP insecticide use. Regardless of the ALS herbicide, the root symptoms would be similar. I always start by checking the radicle for pruning of the lateral roots. Classic symptoms are lateral roots that are still slender, but only about 0.25 inches long or shorter. These pruned laterals are sometimes called bottlebrush roots. These bottlebrush roots may also be seen on the other seminal (seed) roots and on the crown roots if the injury is more severe.

The seedling root growth inhibitors (e.g. dinitroanilines) are pendimethalin (Prowl) and trifluralin (Treflan). These herbicides could persist and carryover if over-applied or if conditions were droughty. Pendimethalin applied preemergence to corn could also cause injury if the corn was shallow planted or if the seed furrow was not closed. These herbicides cause pruned, thick, club-shaped root tips.

The following herbicide modes of action can be dismissed as direct causes of corn root injury.

Glyphosate, glufosinate (Liberty), ACCase inhibitors (Assure, etc.), cell membrane disruptors (Gramoxone) - these herbicides lack sufficient soil residual activity to cause root injury.

Pigment inhibitors (Callisto, etc.), photosynthetic inhibitors (atrazine, etc), PPO inhibitors (Flexstar, Authority, etc) - these herbicides require the energy from sunlight for injury to occur, which is difficult to achieve in roots.

Shoot growth inhibitors (Dual, Harness, Outlook, etc.) - the main activity of these herbicides would be seen on the corn seedling's shoots.

Pruned roots, irregardless of the cause, can lead to phosphorus (purpling) or potassium (yellowing) deficiency symptoms. Other causes of pruned or poor root growth include compacted soil, root disease, and starter fertilizer burn and must be checked for as well.

Shoot injury: A damaged root system will translate into stunted shoot growth. However, other herbicide modes of action need to be considered if the corn shoot is injured.

Glyphosate - Drift, tank contamination, or burndown application at spiking can injury corn. The degree of injury will depend on the rate. Sublethal doses can cause stacking, white leaves, or chlorotic regions on leaves emerging from the whorl. The ACCase inhibitors also cause these symptoms, but glyphosate causes injury more often because of its more frequent use.

Pigment inhibitors - Root uptake of pigment inhibitors can bleach the first corn leaves. Although not frequently used, Command can carryover to corn, especially if over-applied. Mesotrione (Callisto, etc.) seems to have a low risk of injury from preemergence applications. On occasion, postemergence applications of mesotrione can cause some bleaching (yellow thumb print to some whitening) of new leaves, but the plant's growth is not distorted as with glyphosate. Preemergence Balance Pro applications have a greater risk of injury than mesotrione, but it is not registered for use in Wisconsin. As a result, Balance Pro should not be responsible for any bleached corn in the state.

ALS inhibitors - Raptor, Lightning, and Pursuit may be the herbicides of greatest concern of the ALS inhibitors. Each of these herbicides may cause purpling and stacking of corn when drift, tank contamination, or mis-application occurs.

Liberty - Drift or misapplication will cause corn to yellow. In most cases, corn should have close to full recovery from the low rates contained in drift.

Photosynthetic inhibitors - As these herbicides are taken up by the roots, they move with water to the leaves. If injury occurs, it would appear as yellowing or burning of the leaf tips of the oldest leaves. Atrazine and Princep are not likely to cause injury. Sencor should not carryover from soybeans, but the Sencor component in Axiom could be cause occasional leaf tip burn.

The shoot growth inhibitors (Dual, Harness, etc.) can cause leafing out or bugging whipping. It seems like when a growth regulator is tank mixed with a shoot growth inhibitor, the risk of leaf deformities increase. Shoot growth inhibitors and root growth inhibitors don't have postemergence activity and will not cause injury other than potential speckling.

Dead corn: There are few commonly used field crop herbicides that will actually kill corn directly. This list would include glyphosate, Lightning, Raptor, ACCase inhibitors, full rates of Liberty, and Gramoxone. Other herbicides are more likely to only cause some type of damage or stunting.

Check areas: When checking to see if this year's herbicide application is the reason for injury, be sure to thoroughly scout the field for spray skips. This will take some time, but the effort is worth it. If corn is injured in the skip, this season's herbicide application can be ruled out as the cause of the injury. Carryover or other environmental or management factors will need to be investigated further.