C.O.R.N. Newsletter: 2014-14

This week, wheat is heading or will begin to head in fields across Ohio, particularly in southern and early-planted fields. In fact, in some southern fields, the crop was at full head emergence (Feekes 10.5) at the end of last week (May 15-17) and will likely reach the flowering growth stage this week. This is the time to begin thinking about head scab and its management with fungicide. Heads are most susceptible to the scab fungus during flowering and infection is favored by warm, wet or humid conditions. Although, we have had frequent rainfall over the last few weeks and conditions have been fairly humid, most of the scab-favorable moisture occurred before flowering. In addition, temperatures have been cool across most of the state. This will likely reduce the risk of head scab. As indicated by the scab risk tool (www.wheatscab.psu.edu), for wheat flowering today (May 19), the risk of scab is low, and will likely continue to be low during most of this week. However, most of our crop will reach anthesis (flowering) within the next two weeks, during which time temperatures are likely to increase. This could increase the risk for scab, particularly if it continues to rain. Continue to use the scab prediction tool to monitor disease risk as the crop approaches this critical growth stage.    

Persistent rains last week further delayed corn planting. According to the USDA/NASS (http://www.nass.usda.gov/) rainfall was highly variable across Ohio for the week ending May 18 with precipitation ranging between 0.81 and 4.37 inches, with the state averaging 2.26 inches. Excessive rainfall in some areas resulted in localized ponding and flooding of corn. If the ponding and flooding was of a limited duration, i.e. the water drained off quickly within a few hours, the injury resulting from the saturated soil conditions should be minimal.

The extent to which ponding injures corn is determined by several factors including: (1) plant stage of development when ponding occurs, (2) duration of ponding and (3) air/soil temperatures. Prior to the 6-leaf collar stage (as measured by visible leaf collars) or when the growing point is at or below the soil surface, corn can usually survive only 2 to 4 days of flooded conditions. Since most of the corn that’s been planted so far is not beyond the V2-3 stage, it’s especially vulnerable to damage from ponding and saturated soil 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 ponding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival so the lower temperatures we have experienced recently should be beneficial. Once the growing point is above the water level the likelihood for survival improves greatly.

Even if ponding doesn't kill plants outright, it may have a long term negative impact on crop performance. Excess moisture during the early vegetative stages retards corn 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. Ponding can also result in losses of nitrogen through denitrification and leaching. Even if water drains quickly, there is the possibility of surface crusts forming as the soil dries that can impact the emergence of recently planted crops. Growers should be prepared to rotary hoe to break up the crust to promote emergence.

For corn that’s emerged, check the color of the growing point to assess plant survival after ponding. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. For corn not yet emerged, evaluate the appearance and integrity of seeds or seedlings that have yet to emerge (likely rotting if discolored and softening). Look for new leaf growth 3 to 5 days after water drains from the field.

Disease problems that become greater risks due to ponding and cool temperatures include pythium, corn smut, and crazy top (http://oardc.osu.edu/ohiofieldcropdisease/t01_pageview2/Home.htm).

Fungicide seed treatments will help reduce stand loss, but the duration of protection is limited to about two weeks. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage from corn smut and crazy top is difficult until later in the growing season. However the economic impact of these latter two diseases is usually negligible.

Reference:

Nielsen, R.L. 2011. Effects of Flooding or Ponding on Young Corn. Corny News Network, Purdue Univ. [On-Line]. Available at:http://www.kingcorn.org/news/timeless/PondingYoungCorn.html