http://www.ag.ohio-state.edu/~corn/

CORN
Crop Observation and Recommendation Network

May 30 - June 4, 2000
C.O.R.N. 2000-14

In This Issue:
A) Flooding and Ponding Damage to Corn
B) Corn Discoloration
C) Problems with Stand, Phytophthora is out There
D) Postemergence Weed Control in Corn
E) Another Windy Spring, Roundup Drift on Corn
F) Drift Can and Should Be Reduced to a Minimum
G) Do "Drift Retardant" Chemicals Work?
H) Cutworm Injury on Soybeans
I) Impact of Alfalfa Weevil on First Cutting Alfalfa
J) Slug Damage in Corn and Soybeans

A) Flooding and Ponding Damage to Corn - Peter Thomison CORN Questions

The recent thunderstorms and heavy downpours that hit parts of Ohio during the past week resulted in localized flooding in some corn fields, and ponding in others. The following are some tips to consider when evaluating possible injury from ponding and flooding.

The extent to which flooding injures corn is determined by several factors including: (1) plant stage of development when flooding occurs, (2) duration of flooding and (3) air/soil temperatures. Prior to the 6-leaf stage (measured by visible leaf collars) or when the growing point is near or below the soil surface, corn can survive only 2 to 4 days of flooded 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 flooding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival. Once the growing point is above the water level the likelihood for survival improves greatly. Corn in southern Ohio that had not yet reached the 6-leaf stage, had great potential for flooding and ponding injury.

Even if flooding 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. Flooding and ponding can also result in losses of nitrogen through denitrification and leaching.

If flooding in corn lasts less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. Also look for new leaf growth 3 to 5 days after water drains from the field. Sometimes the growing point is killed by bacterial infections during and after flooding, but plant growth continues in the form of non-productive tillers (suckers).

Additional disease problems that become greater risks due to flooding and cool temperatures are corn smut and crazy top. 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 is difficult until later in the growing season.

B) Corn Discoloration  - Peter Thomison CORN Questions

I've received several reports of fields with "off-color" corn. With warmer temperatures on the horizon, much of this corn will return to the normal green we are accustomed to. The following is an article by Dr. Bob Nielsen, Agronomy Dept., Purdue Univ., that discusses some of the factors contributing to off-color corn.

Isn't Corn Supposed to be Green? R.L. (Bob) Nielsen - There are many causes of discoloration. Some are important, some not. Pick your colors.... Yellow, Purple, Red, and even White. These are the colors that many corn growers are seeing as they walk their corn fields this spring. Given the growing season to date, multi-colored corn is not unexpected. Here's why.

As Mother Nature often reminds us, corn grows and develops best with sunny days and warm temperatures. Indiana/Ohio have simply not had many days like that so far in the 2000 growing season. Couple that with the fact that a sizeable percentage of this years corn crop was planted on the early side back in early to mid-April when temperatures were even cooler and it is no wonder that many fields are an ugly yellow-green rather than the darker green that we would prefer to see. Warm, sunny days will correct this. Purpling results from the accumulation of a purple pigment called anthocyanin. Whether or not a corn plant produces anthocyanin is determined by the hybrid's genetics. Some hybrids contain more 'purpling' genes than others.

Purple corn is caused by one of two factors. The first factor is simply a genetic response to cool nights following bright, sunny days. Warmer weather will cause the purpling to slowly disappear. The second factor is restricted root development, coupled with an abundance of plant sugars produced by photosynthesis that triggers the purpling.

If the cause of the root restriction is temporary (e.g., cool temperatures), then the purpling should disappear as the plants develop further and yield losses should be minimal, if any. If the cause of the root restriction continues to affect plant growth for some time (e.g., soil compaction, grub feeding), then the purpling may continue for some time and some yield loss may result if the plants become stunted. Remember that the effects of early season damage to the seed or root system can be magnified when corn is already developing slowly due to cool, cloudy weather.

White corn historically was often blamed on the herbicide clomazone (e.g., Command), but can also be caused injury by the herbicide isoxaflutole (e.g., Balance) and by spray drift of glyphosate (e.g., Roundup). Single white plants in a field are usually genetic mutants.

C) Problems with Stand, Phytophthora is out There - Anne Dorrance and Jim Beuerlein CORN Questions

The first symptoms of Phytophthora sojae are appearing in soybeans. Susceptible varieties were found with symptoms of Phytophthora root rot last week at field plots at the NW Branch. These plants had failed to emerge from the soil, the cultivar has very low levels of partial resistance and no seed treatment was used.  

Phytophthora Root Rot

Some fields in the state have very low populations or have failed to emerge at all. Dig around to find some seed to determine what may have been the cause: disease or insect. Count the populations. Fields with populations of 75,000 to 100,000, if weeds are managed, will have less than a 10% yield loss (3 to 5 bushel/A). For producers with lower populations faced with soybean replant decisions - several factors need to be taken into consideration.

If you are going to replant:

  1. Be sure to have the seed bought and in the barn ready to go before you do any field preparation.

  2. Run the calculations on the soybean replant worksheet, refer to Ohio line, Bulletin Corn, Soybean, Wheat and Alfalfa Field Guide, Bulletin 827-98. As well as the replant worksheet, there are tables at this web site for yield effects from delayed planting, minimum soybean population at different emergence dates, yield effects from reduced plant populations.

  3. If Phytophthora is the problem - change to a variety with a different Rps gene. Look for varieties with combinations of Rps-1c plus 3a; Rps-1k plus 3a or Rps-1k plus 6. Be sure that the variety has high levels of partial resistance and use a seed treatment for Phytophthora.

For those producers who would like a positive diagnosis for the field, the C. Wayne Ellett Plant & Pest Diagnostic Clinic does have "quick" Phytophthora test kits. The cost is $35.00 for the first sample and $12.00 for subsequent samples for the same producer. It is critical that the plants arrive in good shape, not soggy and wet nor dried out. Gently, wash the seedlings to remove excess soil, pat dry and place in a clean, plastic bag sent overnight mail. These tests will test for the presence of Phytophthora and cannot determine which race is present.

D) Postemergence Weed Control in Corn - Mark Loux CORN Questions

With the fairly abundant soil moisture and warm temperatures coming this week, get ready for weeds to grow very rapidly. These type of conditions are generally ideal for herbicide activity, but weeds can become larger and more difficult to control within a few days. In corn, broadleaf weeds are more easily controlled than grasses, so grass size often determines the timing of postemergence herbicides. Most herbicides should be applied before grass weeds exceed 4 inches in height to avoid yield loss and ensure herbicide effectiveness. Some herbicides should be applied when grasses are smaller than 4 inches. Corn height and growth stage may also start to limit herbicide choices - corn is some areas is approaching kee-high. Keep in mind that corn becomes more sensitive to growth regulators (2,4-D, dicamba) as it reaches about 8 inches or the 5-leaf stage. Sensitivity to ALS inhibitors increases as corn reaches the 6-collar stage or about 20 inches in height. It is important to remember also that corn height can vary considerably for a given growth stage. For example, 6 collar corn could be 18 inches tall or 24 inches tall. Some guidelines for grass control are listed below. Information on maximum corn size and spray additives can be found in Table 4 on pages 58 and 59 of the 2000 OSU Weed Control Guide. See product labels also for more information.

Maximum Grass Size for Postemergence Corn Herbicides:

Canada Thistle Control in Corn - There are a number of effective postemergence herbicides and herbicide combinations for control of Canada thistle in corn. Systemic products are generally more effective than contact products for thistle control. Systemic products often keep the thistle from growing further for the rest of the season, even if the thistle plants never actually die, while thistles are more likely to regrow following treatment with a contact herbicide. Competition from the corn is an important part of a thistle control program, and both types of herbicide can be effective if the corn receives enough moisture to suppress the thistle plants from mid-season until harvest. Results of OSU research lead to the following rough ranking of thistle treatments (control refers to current growing season only).

Rates of Banvel/Clarity in these treatments are around 4 oz product per acre:

E) Another Windy Spring, Roundup Drift on Corn - Jeff Stachler and Mark Loux CORN Questions

With all of the wind that we have had this Spring, questions are arising as to what injury symptoms corn will show with Roundup drift. As with any drift scenario, the area closest to the point source of the drift will show the most injury symptoms and the farther away from the drift point, the fewer the injury symptoms. Corn that has taken up Roundup Ultra, Touchdown, or any other glyphosate product will have a growing point that is most likely white, but may be purple or yellow in color; as well the older leaves possibly turning purple. The plants may also be stunted and deformed. These symptoms may show up in 3-5 days after the drift has occurred depending upon temperatures and sunlight. The warmer the weather the more quickly the symptoms will appear. If it has only been 7-10 days after the drift and white, yellow, or purple growing points are present and purple lower leaves, then it is likely these plants will die, but more time is required to be certain of this. With the current calendar date, decisions need to be made for replanting now, so if it has been 7-10 days and the plants only have a slight yellow color or are completely green, then these plants should recover and produce near normal or normal yields, otherwise consider it dead. If the corn is white, yellow or purple at 12-15 days after the drift has occurred then those corn plants will most certainly die. If the growing point is green or slightly yellow at 12-15 days, then the plants should be fine and yield as expected. Usually only the first few rounds of a field may need to be replanted.

F) Drift Can and Should Be Reduced to a Minimum - Erdal Ozkan CORN Questions

The Ohio Department of Agriculture (ODA) receives more complaints about spray drift in June than at any other times of the year. About 75% of the pesticide non-compliance calls ODA receives annually are related to spray drift. The situation may get even worse as the acreage of genetically modified crops increases.

The bad news is: Spray drift occurs wherever liquid sprays are applied. The good news is: Although complete elimination of spray drift is impossible, problems can be reduced to a minimum if chemicals are applied with the proper equipment under favorable weather conditions.

Regardless of where it occurs and what causes it, drift is undesirable because:

  1. It results in inefficient use of application equipment and applicator time.

  2. It may result in under-application of chemicals and ineffective pest control, this could lead to additional applications, reduced yield or higher production costs.

  3. It may result in over-application if the applicator knowingly over-applies chemicals to compensate for drift losses and to ensure the desired level of control.

  4. Losses and/or costly litigation may result if sensitive crops in adjacent fields are damaged.

  5. Unintentional contamination of foodstuffs from unacceptable pesticide residues can result in mandatory destruction of the crop.

  6. It may contribute to pollution of our shared air and water resources.

  7. It may affect the health and safety of susceptible human and livestock populations.

Drift is influenced by many factors that can usually be grouped into one of the following four categories:

A. Spray characteristics (too many small drift-prone droplets discharged from nozzle, small droplet size, volatility of the chemical applied, etc.)
B. Equipment and application techniques used.
C. Weather (high wind, low relative humidity and high ambient temperature).
D. Operator care and skill.

Operator's knowledge of what causes drift is perhaps the key element in reducing drift potential. Conscientious and experienced operators rarely get into serious trouble with drift damage because they understand drift and take steps to avoid it. Here are some management strategies to reduce spray drift from boom sprayers:

  1. Use nozzles that produce large droplets whenever possible, if biological effectiveness can be maintained (OSU Extension Fact Sheets AEX-523 and AEX-524 contain more about the new nozzles and their effectiveness in reducing drift).

  2. Keep the boom close to the spray target.

  3. Use greater spray volume and use nozzles with larger orifices.

  4. Use lower system pressure and check pressure gauge accuracy.

  5. Use drift retardants if droplet size cannot be controlled with nozzle selection.

  6. Follow label recommendations to avoid drift with highly volatile pesticides.

  7. Avoid spraying on extremely hot, dry days, especially if sensitive vegetation is nearby.

  8. Do not spray when conditions are favorable for an atmospheric inversion.

  9. Although the distance droplets will drift is a function of many other factors such as droplet size, relative humidity, temperature and boom height, it is best not to spray when wind speeds are greater than 5 miles per hour.

  10. Avoid spraying near sensitive areas located downwind. Leave a buffer strip 50 to 100 feet wide and spray this strip later when the wind shifts.

  11. Be sure to keep good records (wind speed and direction, temperature, relative humidity, etc. and evaluate spray results.

  12. New label requirements may use a drift model to determine the required buffer width. Travel slower (and lower boom height) and use nozzles which produce large droplets near sensitive areas. This will reduce the required buffer zone width, then, as distance from the sensitive area increases, nozzles can be changed to produce smaller droplets, if desired, and boom height raised so travel speed can be increased.

  13. Select the time when drift is likely to be low for spraying the parts of the field near sensitive areas.

  14. In the future, label requirements may specify nozzles that produce a certain droplet size spectrum, such as fine, medium, coarse, very coarse, etc. Carefully check the label to determine the optimum droplet size and proper nozzle size for a spray application.

G) Do "Drift Retardant Chemicals Work? - Erdal Ozkan CORN Questions

At Ohio State, we have conducted experiments to determine effect of using drift retardant chemicals on spray pattern, droplet size and spray drift. Results of these tests indicate that if used properly (at appropriate rates), these products indeed reduce spray drift by hindering formation of small, drift-prone droplets.

There are over 30 different "drift retardant" chemicals commercially available to pesticide applicators. These products are normally some type of long chain polymer or gum that increases the viscosity of the spray mixture which results in a coarser spray. Unfortunately, the information related to performance of these products is limited. Results of a study conducted by USDA-ARS Engineers in Texas indicated that the effect of polymer concentration on droplet size is dependent on polymer type. For example, polyvinyl and polyacrylamide polymers were found to be more effective than linear alkyl epoxide or polymide copolymers in increasing volume median diameter and reducing the percentage of spray volume composed of small droplets subject to spray drift.

We at Ohio State, have tested five drift retardant chemicals to determine their effects on droplet size, spray pattern, and spray drift reduction. In comparison to spraying water only, all drift retardants tested reduced volume of portion of small droplets in the spray but at varying magnitudes. For example, the reduction of spray volume contained in droplets smaller than 100 microns ranged from 30% with the least effective product, to 68% with the most effective product.

Some studies have found that some of these polymers tend to be sheared by passing through a sprayer pump, as would occur in normal bypass, hydraulic mixing in common agricultural sprayers. This means that the drift retardant would lose its ability to increase droplet size - its ability to reduce drift - as the spray tank became empty. Gums are not sheared as easily as the long chain polymers, and some types of polymers (poly-ethylene oxide) are sheared in fewer passes through a pump than other types of polymers (polyacrylamides).

Although drift retardant chemicals are effective in reducing the number of drift-prone droplets, in most cases, it is more effective to select the proper size and type of nozzles and operate sprayers at low pressure to produce the desired drop size rather than attempt to increase droplet size with a drift retardant chemical.

H) Cutworm Injury on Soybeans - Hal Willson CORN Questions

An infestation of dark-sided cutworm, Exuoa messoria (Harris), on soybeans planted no-till in corn stubble on a Madison County farm was treated on May 17th to prevent additional stand losses. The infestation was found throughout the soybean field, which was over 400 acres in size, and the rescue treatment was applied by aerial application. Inspections of the cutworm infested soybeans on May 23rd and May 24th enabled collection, identification and photographic documentation of the remaining cutworm larvae and injury to the soybean plants.

At the present time, the dark-sided cutworms are in the late instar stages of development. Where remaining infestations can be found, a larvae will have cut multiple stems at ground surface level. Larvae of the dark-sided cutworm have light brown head capsules and stripes somewhat similar to an armyworm. The ventral side is pale in color. Each side has a dark stripe, although close inspection under a microscope will indicate a rather blotched pattern of dark and colored patterns. This cutworm does not exhibit any chevron patterns on the dorsal side like the dingy cutworm.

The dark-sided cutworm has one generation per year and over-winters in the egg stage. Thus, the eggs were deposited in the corn field last fall. An initial search of the literature located no records of dark-sided cutworm infestation of soybeans. Although, this cutworm is known to infest a wide range of crops including corn, canola, tobacco, onions, and various vegetable crops.

Given the current case of dark-sided cutworm infestation of soybeans, detection of soybean injury where the stems have been clipped off at ground level or below the cotyledon should be followed by a thorough search for cutworm larvae below the soil surface. There have been a number of reports of bean leaf beetle chewing soybean plants down to the stem. If only stubble of stems remain, it is possible that a cutworm problem may exist.

It is unlikely that the current case of dark-sided cutworm injury on soybeans is limited to a single field. Presumably other cases exist since the historical reports of dark-sided cutworm report that such outbreaks are often widespread.

If cutworms are detected in soybeans, such infestations should be reported. In addition, larvae should be submitted for identification to confirm the identity of the infestation, since this is a unique case of pest injury on soybeans. When cutworm larvae are found, the larvae should be initially killed in boiling water, placed in a vial of alcohol, and submitted for identification via the local OSU Extension office. The hot water treatment prior to alcohol preservation is important to prevent discoloration of the larvae.

Unusual cutworm infestations have occurred this season. During the past week, we received samples of bronze cutworms that had infested a forage field in Jefferson County. A few weeks ago, an outbreak of climbing cutworms impacted a number of apple orchards in Licking County.

I) Impact of Alfalfa Weevil on First Cutting Alfalfa - Hal Willson CORN Questions

The alfalfa weevil represents one of our best examples of classical biological control where beneficial parasites have been released and established to maintain the pest at sub-economic levels of infestations during most growing seasons. In addition, the sampling and economic thresholds applied to management of alfalfa weevil represent one of our best examples of integrated pest management. However, about every five years the biological controls fail to maintain the weevil at sub-economic levels of economic activity and pesticide applications are needed to prevent economic losses. During the current season, weevil damage to alfalfa has been widespread and treatment of many alfalfa stand has been warranted. Weevil infestations have been so severe that we have received requests on how to treat alfalfa fields designated as organic habitats.

We have recently harvested our alfalfa weevil trials and the benefits of the insecticide treatments warrant recognition since some parties may question whether insecticide treatment may or may not be justified when weevil infestations occur on first cutting alfalfa.

On April 26th, we conducted a pre-treatment sampling of our alfalfa plots. At the time of pre-treatment sampling, an average of 3.1 weevil larvae per stem were found on alfalfa with an average stem height of 9.2 inches. Seven treatments of insecticide were applied on April 27th, post-treatment samples of larvae density and stem height were taken 7 and 14 days after treatment, and the plots were harvested on May 17th. At harvest, average yield of treated plots was 1.92 tons per acre and the yield of the untreated check plots was 1.23 tons per acre. Thus, a benefit of 0.69 tons per acre was achieved by applying a timely rescue treatment. If a market value ranging from $80 to $100 per ton is assumed, the value of yield saved by a timely insecticide treatment ranged from $55 to $69 per acre.

J) Slug Damage in Corn and Soybeans - Ron Hammond CORN Questions

Slug damage has been reported from both corn and soybean fields in many parts of the state. Growers should be checking their fields for slug problems, especially in those fields with a past history of problems. If injury is severe and the plants are not outgrowing the damage, treatment might be necessary. The most common molluscicide for growers in Ohio is Deadline MPs. If an application is made with this bait, it should be broadcast over the field at 10 lbs per acre. Even coverage is essential to get good control. At 10 lbs per acre, you should get about 4-5 pieces of bait per square foot. Deadline MPs will hold up very well through rainfall. Our tests indicated that MPs will not breakdown, even with 3 plus inches of rainfall. Reports have been received that Deadline Bullets are still available, this is an older formulation of Deadline. There are a few differences between the MPs and the Bullets which might affect performance. The Bullets are bigger pieces of bait compared with the MPs. At a similar rate (10 lbs per acre), you will get fewer pieces of Bullets per square foot compared with the MPs. If a grower can buy the Bullets at a reduced price, the rate of application should be pushed to 15-20 lbs per acre to ensure an adequate number of pieces per square foot. Also, Bullets will not hold up as well as MPs when significant rainfall occurs. Deadline MPs is the preferred formulation; if the Bullet formulation is used, greater care is needed to get the best control.

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Contributors to C.O.R.N. this week include: State Specialists: Peter Thomison (Corn Production), Pat Lipps (Plant Pathology), Anne Dorrance (Plant Pathology),Erik DeWolfe (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Hal Willson (Entomology) and Ron Hammond (Entomology); District Specialists: Ed Lentz (Agronomy); Extension Agents: Dave Jones ( Allen), Steve Bartels (Butler), Barry Ward (Champaign), Greg LaBarge (Fulton), Gary Wilson (Hancock), Mike Estadt (Pickaway), Glen Arnold (Putnam), Clark Hutson (Seneca) and Roger Bender (Shelby).

Editor: Clark Hutson        Web Editor: Nathan Watermeier

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

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