The longer the young crop sits in standing water and bakes in 80-90 degree (Fahrenheit) temperatures, the more likely injury or even death is to occur, according to Peter Thomison, an Ohio State University Extension agronomist. The result could mean more soybean plantings as growers decide whether or not to make the switch in the coming weeks.

"We've had fields covered with water and flooding for a long duration mainly in parts of south central and southwest Ohio, but how widespread the potential problem is we don't know yet. Corn above the six leaf collar stage can survive a flooding period of 24 hours to 48 hours under moderate temperatures, say in the 60s and 70s," says Thomison, who also holds an appointment with the Ohio Agricultural Research and Development Center. "Unfortunately because of the slow growth of the corn plant, the growing point is still below the soil surface in many fields and that just makes the crop more vulnerable to flooding injury. Throw in the warmer temperatures and that just basically knocks the plant out."

Growers should begin assessing potential crop injury four to five days after any initial flooding has occurred, Thomison says.

"Growers should see evidence of new growth. They should also analyze the growing point of the plant, and if it's soft and dark in color, that means they've probably lost the plant. The growing point should be white to cream-colored. Flooding injury may not kill the plant outright, but the crop ceases to be a productive plant because the growing point is destroyed and just paves the way for disease development," says Thomison. "In some cases the crop will survive, but you'll have a sucker rather than a main plant."

Young corn plants are susceptible to flooding injury and ponding because debris, residue from other fields and weed seed mixes with silt and enters the whorl of the plant. Bacteria associated with the soil also find their way into the whorl and may kill the plant.

"Soil saturation is also an issue. There may not be any standing water, but if the soil is like soup, that's also hurting the plant," says Thomison. "If temperatures are warm during ponding, the crop may not survive 24 hours. The plant more quickly uses up oxygen in the root zone. Without oxygen, the plant can't perform critical life-sustaining functions."

Thomison offers the following tips to consider when evaluating possible ponding damage:

• The extent to which ponding injures corn is determined by three main factors: stage of plant development when ponding occurs, duration of ponding, and air/soil temperatures. "When the growing point is at or below the soil surface, corn can usually survive only two to four days of flooded conditions, and cooler temperatures prolong survival," said Thomison. "Once the growing point is above the water level, the likelihood of survival improves greatly."

• Even if ponding doesn't kill plants outright, it may have a long-term negative impact on crop performance, including retarding root development and resulting in greater losses of nitrogen through denitrification and leaching.

• If ponding in corn lasts less than 48 hours, crop injury should be limited.

• Disease problems that become greater risks due to ponding include pythium, corn smut and crazy top.

Thomison said that growers might decide to replace damaged or dying corn plants with soybeans, especially now that the optimum planting date in Ohio is slipping away.

"Growers really shouldn't be planting corn beyond the end of the week of June 13, unless you are in southern Ohio and planting corn for silage. Then growers might be able to get away with as late as a June 20 planting date," says Thomison. "If that is the case, they should plant a shorter-season corn hybrid, but the question remains if seed companies will have the variety they need at this stage of the game."

Soybeans may also face ponding injury due to the recent heavy rains, although soybeans are more tolerant to flooding conditions than corn.

"Growers need to be aware of the varieties they've planted that could be facing ponding conditions. When we have wet conditions, Phytophthora root rot goes to town, especially in heavy clay soils," says Thomison. "If a grower has planted a variety susceptible to the disease, he could be in trouble."

For the latest information on Ohio's field crops, log on to OSU Extension's Agronomic Crops Team Web site at agcrops.osu.edu.