When freezing temperatures injure corn plants in a field, some plants may survive and recover, while other plants will die. Corn plants die immediately when growing point tissue is frozen. Corn plants not killed immediately may still succumb to various physical or biological factors that prevent recovery, including:
Plant starvation – Leaf loss due to frost injury reduces photosynthetic area available to produce carbohydrates for new plant growth and recovery.
Plant disease – Injured plants have reduced levels of resist-ance to secondary pathogens invading damaged tissues.
One of the first steps in diagnosing frost injury is to check the health of the internal growing point. Plants can be split vertically and the growing point region inspected visually for damage (Nielsen, 1999). This is typically manifested as mushy, discolored tissue at the top of the mesocotyl. If the growing point tissue is obviously damaged, plants will not recover. However, even if the growing point appears healthy immediately after the frost, plants still may die.
Cool weather after the frost can delay visible deterioration of damaged tissue on plants. Those plants not directly killed by the frost can succumb to the other factors described above. Therefore, it is usually best to wait until three to five days after the frost or freeze damage occurred to make an assessment of recovery potential (Nielsen and Christmas, 2001).
Crop damage can appear very severe immediately after the injury, but plants often recover if the growing point is not damaged. Nevertheless, each situation is different and decisions regarding whether to keep existing stands or replant often need to be made over a longer period of time on a field-by-field basis. The next sections describe observations from the extensive experience of Pioneer agronomists in assessing frost damaged corn.
Assessing Recovery Potential – Growing Point Below Ground
Recovery from early season death of above ground tissue depends on:
Stage of growth – Potential for recovery is higher at VE-V2 than for V3-V4 stages because energy reserves still exist in the seed to support growth.
Amount of green tissue remaining – The more green tissue there is for the plant to live on until there is enough re-growth for photosynthesis, the higher the potential for recovery, especially at V3-V4 stages when seed reserves are nearly depleted.
Weather conditions during re-growth – Dry, warm condit-ions are more favorable than cold and wet weather.
Number of frost events – Plants at this stage will often recover from one episode, but reserves can be depleted with multiple frosts while damaged seedlings are recovering. The probability that plants will not re-cover or that the stand will contain non-competitive “runts” is increased with repeated frost damage/recovery cycles.
Experience has shown that seedlings with tissue damage within 0.5 inches or less of the growing point will most likely not recover. Even if plants survive, potential to produce competitive plants with acceptable yield potential is compromised. Seedlings less severely damaged, with more than 0.5 inches of healthy tissue above the growing point often will survive and develop into plants with high yield potential. Figure 3 (taken 24-48 hours after damage occurred) shows progression from most to least damage for corn seedlings affected by frost within a field when plants were at V1-V2 stages.
Figure 3. Corn seedlings with a range in damage from frost injury when plants were at V1-V2 stages.
The two seedlings on the left represent those that will likely not recover, even though the growing point was not damaged by the frost. Recovery and yield potential of the seedling in the middle is uncertain. For the seedling second from the right, recovery is likely but three to four days of warm temperatures are needed to verify this. The seedling on the far right has a very high probability of survival with little impact on yield potential. As mentioned previously, in most instances it is best to wait several days before making the final decision regardless of how hopeless or promising seedling damage appears soon after the injury.
After a few days with high temperatures near or above 70 °F, look for green tissue close to the soil surface. If evidence of progressive increases in green tissue are not observed in this portion of the plants, the seedlings are unlikely to recover and produce healthy plants. Fields with visible re-growth should still be monitored for several more days for continued progress. New green plant growth is an important first step but watch for developing leaves and nodal root growth.
Figures 4 to 7 show plants damaged by a frost at the V2 stage, which killed all above-ground leaf tissue to within 0.25 inches of the soil surface.
Two days after the frost, green tissue has emerged, but dead tissue is restricting the new growth. The growing point is a healthy white/yellow color and the mesocotyl is white (Figures 4 and 5).
Figure 4. Corn seedling damaged by frost at the V2 stage, two warm days after damage occurred.
Figure 5. Corn seedling from Figure 4 dissected to show growing point.
Eight days after the frost, plants with this level of damage show significant growth but the two largest leaves are tied at the tip (Figure 6). A small leaf (not visible in the image) is emerging from the whorl, free from the “knot”. This plant is on the way to healthy recovery and should be counted in a post-frost stand assessment. After seven or more days of post-frost warm weather have transpired, any plants that are both smaller than this and still tightly knotted should not be counted in a stand assessment. If conditions are cool after the frost, more than seven consecutive days may be required to develop this level of re-growth.
The plant to the left of the stake in Figure 7 is the same plant as in Figure 6, 20 days after the frost. By this time, the plant has six visible green leaves.
Figure 6. Corn seedling in same field and with similar level of damage as in Figures 4 and 5, eight warm days after damage occurred.
Figure 7. Corn plant left of the stake is same plant as in Figure 6, 20 warm days after damage occurred.
Because of the microclimate factors mentioned above, fields next to each other and plants within fields will respond differently to both the damaging temperatures and the recovery process. When damage has been severe/repeated or if recovery conditions have not been favorable, diligent scouting and close observation for two to three weeks after frost events are often needed before assuming that an accurate, final assessment has been made.
Assessing Recovery Potential – Growing Point Above Ground
Many of the considerations are the same as for earlier stages. It is important to evaluate the health of the growing point and not to assume that plants will recover, even if the growing point was not damaged directly by the frost (Figure 8). Amount of green tissue remaining, growing conditions during re-growth, and whether frost damage involved single or repeated episodes are important factors that will influence recovery potential.
Figure 8. Growing points of dissected corn plants after frost at V5-V6 stage. Left: Growing point is brown indicating plant death. Middle: Growing point is discolored indicating probable death. Right: Growing point is healthy.
Similar to earlier stages, the most reliable way to determine frost impact is to wait until after three to five days with daily high temperatures above 70 °F and check for evidence of new growth. Remove dead whorls and look for erect, lime green leaf growth inside the corn plant. Another sign of active growth is a “rippled” leaf effect within the whorl when the plant is cut lengthwise. The “rippled” leaves indicate that new growth occurring after the frost is backing up behind the damaged, knotted whorl.