What impact will recent weather conditions have on corn that's already been planted? Some fields were planted several weeks ago before soils were saturated by persistent rains. In past years, we have observed that early planted corn that was in the process of germinating or as far along as the V1 stage (one leaf collar visible) survived freezing temperatures in late April with little impact on crop performance or plant stand.
Agronomists generally downplay the impact of low temperature injury in corn because the growing point is at or below the soil surface until V6 (six leaf collars visible), and thereby relatively safe from freezing air temperatures. However when dry corn seed absorbs cold water as a result of a cold rain or melting snow, "imbibitional chilling injury" may result. Cold water can cause similar injury to seedling structures as they emerge during germination.
Such physiological injury was widely observed in 2005 when early planted corn in various stages of germination and emergence was subjected to a period freezing rain and snow followed by temperatures at or below 50 degree F for about 10 days. What we've experienced thus far in 2007 is mild in comparison to 2005.
To assess the impact of these freezing temperatures on emerged corn, check plants about 5 days after the freezing injury occurred (and preferably when growing conditions conducive for regrowth have occurred). New leaf tissue should be emerging from the whorl. You can also observe the condition of the growing point (usually located 1/2 in to 3/4 in below the soil surface) by splitting seedlings lengthwise. If the growing point appears white to light yellow and firm several days after the frost, prognosis for recovery is good.
Of greater concern with regard to the viability of germinating and emerging corn is how long soils will remain saturated. Cool temperatures and wet weather provide the right conditions for the development of seedling blight diseases. Cold temperature injury can play a significant role in predisposing plants to root infection and blight. Under normal conditions plants can continue to grow and produce new roots, but when other injuries occur, new roots cannot develop rapidly and Pythium and other soil fungi can kill stressed plants.
Seed treatment fungicides generally remain effective from 10 to 14 days but under saturated conditions the duration of protection may be shorter.