C.O.R.N. Newsletter: 2015-15
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Damping-off is Not Always Caused by Water Molds and Fungi; Insects Can Play a Role Too
Editor's note: Meredith Eyre, Graduate Research Assistant, was an author on this article.
Though soil borne pathogens are usually the cause of damping off in Ohio’s poorly drained soils, seedcorn maggot infestation may cause similar symptoms. This maggot is easy to diagnose in the field because it causes very characteristic tunnels as it burrows through any plant material below the soil line (see Figure A). The maggots are small, yellowish-white, and legless (Figure B). If scouting your fields this week, you may find the larvae or perhaps the pupae, which look like small grains of brown rice (Figure C)
These maggots may also cause mild to severe damage to the cotyledons before the plant emerges (Figure D). If tunneling and feeding is severe, the plant will not emerge resulting in a reduced stand count. If mild, the plant may recover just fine, but will have large scars on the cotyledons. Though the scarring alone does not seem to affect the viability of the plant, it indicates the seedcorn maggot is present in your field
If the tunneling occurs in the taproot or lower stem, the seedling may attempt to send new roots out above the damaged tissue. This may or may not be successful, allowing the occasional plant to recover. Unfortunately, the more likely outcome is damping off as the plant wilts and dies. The diameter of the tunnel may range in size from the tip of a pen to the width of a piece of rice and vary in length.
The maggots are most likely to be found in fields with high organic matter, especially fields that have had cover crops or weeds recently incorporated into the soil. In order to minimize the chances of seedcorn maggot infestation, it’s best to wait at least a week between tilling and planting, especially when soil is cool and damp.
No economic threshold exists and no rescue treatment is available. Growers experiencing lack of emergence, reduced stand counts, or damping off caused by this maggot may need to replant.
Studies suggest seed treatments may reduce or prevent damage from this maggot and are very effective for seedcorn maggot control. While clothianidin (in Poncho) and thiamethoxam (in Cruiser) work well, our data, along with others, suggests imidacloprid (Gaucho) does not.
For a complete list of labeled insecticides, see http://entomology.osu.edu/ag/
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Ponding Effects on Corn
Rainfall was mixed across Ohio over the past weekend. Although some areas of SW Ohio missed appreciable rainfall, many fields in NE and NW Ohio received up to 3 to 4 inches of rain resulting in localized ponding. If 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 corn is not beyond the V5-V6 stage, it’s 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 of corn plants, so the lower temperatures forecasted this week 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 Corn Prior to Tasseling. Corny News Network, Purdue Univ. [On-Line]. Available at:http://www.agry.purdue.edu/ext/corn/news/timeless/pondingyoungcorn.html (URL accessed June 1, 2015)
Crop Observation and Recommendation Network
C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.
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