Hail, high winds and above average rainfall has caused extensive property damage in many areas of western Wisconsin. It will take some time to clean up and restore damaged buildings and property. Many growers are wondering about what effects these storms will have on the corn crop?

Flooding

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.

"GREEN SNAP"

Strong winds that accompanied thunderstorms have caused stalk breakage in cornfields often referred to as "green snap" or "brittle snap". Corn plants are more prone to snapping during the rapid elongation stage of growth between V8 and tasseling.

Breaks in the stalk usually occur at nodes (along nodal plates) below the ear. When soil moisture and temperature conditions are favorable corn plants elongate rapidly but stalks are brittle. Stalk brittleness is greatest in rapidly growing corn under high temperature and high soil moisture conditions. There is speculation that rapidly growing plants because there has been little time for plants to develop lignified tissues at the nodes.

Green snap is a more serious problem in the western Corn Belt. In Nebraska, green snap has caused major stand losses. Vulnerability to green snap damage varies among hybrids. However, all hybrids are at risk from such wind injury when they are growing rapidly prior to tasseling. Based on studies in 1993 and 1994, Nebraska researchers observed that green snap often occurred on the most productive fields with the highest yield potential. The use of growth regulator herbicides such as 2,4-D or Banvel is also associated with stalk brittleness, especially if late application or application during hot, humid conditions occur. Once the crop tassels green snap problems generally disappear.

Stand Loss

To immediately assess yield impacts in cornfields, you must assess stand loss and defoliation. Other deferred losses (ties, damaged plants and cripples) are assessed later once an ear has developed on the plant.

To determine stand loss on corn prior to V10, count the number of original plants per acre. Next determine the number of totally destroyed plants per acre. Subtract totally destroyed plants from the original stand and consult Table 1.

Some plants called "ties" may need to be reassessed later during ear development. A tie is a plant with the leaves bound in the whorl, sometimes referred to as "roped" or "buggy whipped." Plants originally classed as ties may later fall into several categories: 1) plants completely recovered, 2) damaged plants non-competing, and 3) "cripples" individually evaluated. A non-competing damaged plant will not produce a harvested ear (may be bushy-topped or mushroom shaped) and should be counted as a dead plant. A cripple grows to approximately normal height and either does not produce a harvestable ear (but is not barren), or a harvestable ear is considerably less than normal size. Evaluate cripples separately from other normal corn.

Table 1. Corn yield loss due to stand loss prior to V10 in southern WI.
Plant population	Planting date
Plant population	May 1	May 10	May 20	June 1
Number/acre	% yield loss
30000	0	0	0	0
25000	4	4	3	1
20000	11	11	10	7
15000	22	22	19	14
10000	37	36	31	23
derived from UWEX A3353

Defoliation

Defoliation consists of damage to leaves, and is measured in terms of the exposed leaf area destroyed at the date of loss (Table 2). Leaf area destroyed is that portion of the leaf that has been removed by hail, plus portions on the plant which are no longer green because of hail. Live green tissue on the plant, even though mutilated, should not be considered leaf area destroyed.

Table 2. 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

Other losses

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) which may further reduce yield Assessment of these types of damage must be done later usually closer to harvest.