COLUMBIA – “It’s raining, it’s pouring, the old man is snoring” is a child’s chant that rings far too true for Missouri farmers today.

Many Missouri field crops face fallout from flooding and excessive rain, said University of Missouri Extension corn and small grains specialist Greg Luce.

Survival of flooded corn and soybean seedlings depends on how long the flood lasts, floodwater temperatures and how fast fields dry, Luce said.

Flooded plants deplete oxygen in 24 to 48 hours. Moving water, which allows some oxygen to get to plants, results in less damage than still water, he says. Young plants can survive about two days when temperatures exceed 70 F. When temperatures fall to the mid-60s or below, they can survive as long as four days. Survival also depends on how much of the plant is submerged.

Plants should show new leaf development three to five days after water recedes, says Luce. Examine seedlings for disease. Look for rotted or discolored seedlings, roots and damping-off symptoms.

Generally, soybean tolerate flooding longer than corn—up to 48 hours. After 48 hours, expect stand reduction, loss of vigor and lower yield. Root damage also occurs. This impairs the plant’s ability to take up water and tolerate drought stress.

Abnormally high temperatures reduce survivability by 50% or more. Plants survive better in lower temperatures because metabolic processes slow.

Flooded plants also can face disease pressure. Cool, wet fields create favorable conditions for soil pathogens. They also delay plant development and growth. This puts some plants at greater risk of soil-borne diseases that attack seeds and seedlings.

Seed treatments can help prevent diseases, but they typically only provide protection for a couple of weeks under cool, wet conditions.

If these conditions persist longer than that, crop stands are at risk from Pythium, a parasitic disease that damages seedlings of soybean and corn. Phytophthora also can damage soybean seedlings or start infections in the early summer that may develop and kill soybean plants later in the summer.

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