Hail, high winds and above average rainfall has caused extensive crop damage in many areas of Wisconsin. Many growers are wondering about what effects these storms will have on the corn crop.

Recent rains have caused periods of flooding and ponding in many cornfields. The extent to which flooding injures corn is determined by several factors including:

plant stage of development when flooding occurs,
duration of flooding, and
air-soil temperatures.

Prior to V6 (6 visible leaf collars) the growing point is near or below the soil surface. Corn can survive only 2 to 4 days under flooded conditions. The oxygen supply in the soil is depleted after about 48 hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions; e.g. nutrient and water uptake is impaired, root growth is inhibited, etc. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival.

Once the growing point is above the water level, the chances of survival improve greatly. Even if flooding doesn't kill plants outright, it may have a long-term negative impact on crop performance. Excess moisture during the early vegetative stages retards root development. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Flooding and ponding can also result in losses of nitrogen through denitrification and leaching.

If flooding in corn is less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. Also look for new leaf growth 3 to 5 days after water drains from the field.

Disease problems that may become greater risks due to flooding and cool temperatures are corn smut and crazy top. There is limited hybrid resistance to these diseases and predicting damage is difficult until later in the growing season.

Hail Loss

Yield loss from hail this early in the season is minimal. In fact, hail charts for the National Crop Insurance Service do not start until V6. Defoliation consists of damage to leaves, and is measured in terms of the exposed leaf area destroyed at the date of loss (Table 1). Leaf area destroyed is that portion of the leaf that has been removed by hail, plus portions on the plant that are no longer green because of hail. Live green tissue on the plant, even though mutilated, should not be considered leaf area destroyed.

Table 1. Corn yield loss due to leaf defoliation at various development stages.
Leaf defoliation	Growth stage
Leaf defoliation	V6	V8	V10	V12
%	% yield loss
0	0	0	0	0
25	0	0	1	2
50	0	3	6	9
75	3	6	9	16
100	7	11	16	28
derived from National Crop Insurance Services No. 6102

Often when stand loss and defoliation takes place other hidden stalk and ear damage may also occur. Additional second losses may reduce yield again later in the season. Furthermore, plants are predisposed to pests (insects and diseases) that may further reduce yield Assessment of these types of damage must be done later usually closer to harvest.