Heavy rains in some parts of Kansas over the past week or two have subjected some fields of corn to saturated soils or even flooding for a day or two, or even more. Producers should know what might happen to their corn as a result of early-season wet conditions so they can correctly diagnose any future problems that may occur as the season progresses.
Saturated soils inhibit root growth, leaf area expansion, and photosynthesis because of the lack of oxygen and cooler soil temperatures. Yellow leaves indicate a slowing of photosynthesis and plant growth. Leaves and sheaths may turn purple from accumulation of sugars if photosynthesis continues but growth is slowed.
Corn plants can recover with minimal impact on yield if the plants stay alive and conditions return to normal fairly quickly. Although root growth can compensate to some extent later in the season, a saturated profile early in the season can confine the root system to the top several inches of soil, setting up problems later in the season if the root system remains shallow. Corn plants in this situation tend to be prone to late-season root rot if wetness continues throughout the summer, and stalk rots if the plants undergo mid- to late-season drought stress. Plants with shallow root systems also become more susceptible to standability problems during periods of high winds. Overall, shallow root systems are more prone to drought and nutrient stresses, due to the diminished capacity of the plant to explore the entire soil profile.
Young corn plants typically can tolerate full submersion for up to 48 hours with minimal impact on yield. If flooding occurs before V6, when the growing point is at or below the soil surface, flooding that lasts more than 2 to 4 days can impact season-long plant growth and grain yield or cause significant plant mortality. Chances of plant survival increase dramatically if the growing point was not completely submerged or if it was submerged for less than 48 hours. Research has demonstrated yield reductions from early-season flooding ranging from 5 to 32 percent, depending on soil nitrogen status and duration of flooding.
Temperatures can influence the extent of damage from flooding or saturated soils. Cool, cloudy weather limits damage from flooding because growth is slowed and because cool water contains more oxygen than does warm water. Warm temperatures, on the other hand, can increase the chances of long-term damage.
Silt deposition in the whorls of vegetative corn plants can inhibit recovery of flooded corn plants. Enough soil can be deposited in the whorl that emergence of later leaves is inhibited. A heavy layer of silt on leaf surfaces can potentially inhibit photosynthesis or damage the waxy surface layer of the leaf (cuticle), making the leaves subject to drying out. New leaves should not be affected if they can emerge normally. In some instances, the soil in the whorl may contain certain soft-rotting bacteria. These bacteria can cause the top of the plant to rot. The whorl can easily be pulled out of a plant infected with these soft-rotting bacteria. In addition, a rather putrid odor will be present. These plants will not recover.
Flooding can increase the incidence of moisture-loving diseases like crazy top downy mildew. Saturation for 24 to 48 hours allows the crazy top fungus spores found in the soil to germinate and infect flooded plants. The fungus grows systemically in the plant, but visual symptoms will not appear for some time. Symptom expression depends on the timing of infection and amount of fungal growth in the plant. Symptoms include excessive tillering, rolling and twisting of upper leaves, and proliferation of the tassel. Eventually the tassel can resemble a disorganized mass of small leaves, hence the name “crazy top.”