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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2007-12

Dates Covered: 
May 8, 2007 - May 14, 2007
Keith Diedrick

Dry Weather and Preemergence Herbicides

Authors: Mark Loux

Residual preemergence herbicides applied at the time of planting are most effective when enough rain occurs within about a week or so after application to move herbicide into the upper few inches of soil where weed seeds are germinating (this assumes a weed-free start through use of tillage or burndown herbicides). Rainfall patterns in Ohio tend to result in frequent enough rains to ensure preemergence herbicide effectiveness, but exceptions to this occur in some years. Many areas of Ohio are currently experiencing one of these exceptions; growers should be aware of the possibility for preemergence herbicide failure.

When rainfall is scarce following preemergence herbicide application, weeds will escape herbicide treatments and start to emerge. These weeds are usually not well controlled by preemergence herbicides even if later rainfall occurs, especially for herbicides that move into plants via shoot uptake (when the shoot is above the soil, herbicide uptake no longer occurs). Most preemergence grass herbicides fall into this category, including all of the acetamides, such as metolachlor and acetochlor. Herbicides that are taken into the plant via roots (e.g. Balance, Callisto, atrazine) may still provide some control of emerged plants if rainfall occurs before plants have much size. Growers may be dealing with either of the following situations:

- The crop has been planted and with preemergence herbicides applied around the time of planting, and it appears that insufficient rain has occurred or will occur within a week or so of herbicide application. This situation results in the probability that preemergence herbicides may provide less than acceptable weed control. Actions to take in this situation are: 1) rotary hoe the field just as weeds are starting to emerge, which will provide some control and essentially “buy some time” for a rain to occur and activate herbicides; and 2) scout fields and apply postemergence herbicides as necessary to control weeds that escape preemergence treatments. In corn fields where preemergence herbicides were fairly ineffective, postemergence herbicides should be applied when weeds are less than 4 inches tall to avoid risk of yield loss.

- The crop has been planted, or will be soon, but preemergence herbicides have not been applied. In this situation, the decision is whether it is a good bet to go ahead and apply herbicides, based on the assumption that it will rain soon, or alter the herbicide use strategy. Where the latter is being considered, two alternatives exist:

1). Most preemergence corn treatments that contain atrazine can be applied after the crop and weeds have emerged, and will adequately control small emerged weeds. The application can be timed after the first flush of weeds has emerged, which can amount to a significant portion of the weeds that will emerge in a season. This is essentially an “early-postemergence” treatment that still provides residual control once it receives enough rain, but it should be applied when weeds are 1 to 2 inches tall (grasses less than one inch) for best results.
2). Switch to a total postemergence herbicide program, typically applied when weeds are around 4 inches tall. This application occurs somewhat later than the early postemergence mentioned above, but including herbicides with at least some residual activity results in more effective control through the rest of the season.

The difference between approaches #1 and #2 – the first uses preemergence herbicides with limited postemergence activity applied to very small weeds, while the second uses postemergence herbicides that are more effective on larger weeds, but have less residual activity. Growers with Roundup Ready corn have the option to apply a mixture of glyphosate and residual herbicides anytime before weeds exceed four inches in height. Residual herbicide rates should be higher in approach #1, which is applied earlier in the season to smaller corn. Either one can work, but herbicide programs need to provide control through mid June, so plan weed management strategies accordingly.

In Roundup Ready soybeans, failure of preemergence herbicides often forces growers to make the initial postemergence glyphosate herbicide application earlier in the season, which can result in greater need for a second postemergence application to control later-emerging weeds. Planning for two postemergence applications is a more effective strategy than delaying the first application until weeds are larger, since weeds should be removed before they exceed about 6 inches in height to avoid soybean yield loss.

Using Herbicides and Tillage to Control Large Weeds

Authors: Mark Loux

As we move later into spring and weeds become larger and deeper rooted, it becomes more difficult to remove weeds with tillage alone. Tillage implements that are designed to prepare seedbeds with minimal soil disturbance do not always effectively remove large weeds. Effective control of weeds with tillage usually involves complete uprooting or at the very least, severing of the stem as close to the roots as possible. Failure to accomplish this level of disruption can result in weeds that are appear bent over and fairly beat up, but these weeds often recover within several weeks. Weeds that survive tillage can be difficult to control with postemergence herbicides, and may persist throughout the entire growing season. Failure to adequately control a dense population of dandelion or thistle can result in poor crop stands, due to the competitive nature of these weeds, especially under dry conditions.

Solutions include more thorough and deeper tillage, or treatment of weeds with herbicide prior to tillage to ensure complete control. When using a combination of herbicides and tillage to control weeds and prepare a seedbed, apply the herbicide at least 24 hours prior to tillage to allow translocation. Glyphosate is probably the most logical choice for an herbicide treatment prior to tillage. Gramoxone could also be used, but may be less effective on large weeds. We would avoid the use of 2,4-D or dicamba, even when planting corn, because tillage can distribute herbicide within the seed zone and increase the risk of crop injury.

For fields where tillage failed to adequately control dandelions, marestail, ragweed, or other tough weeds, there are several strategies depending upon the type of crop planted:

- in Roundup Ready soybeans, allow the weeds several weeks to recover from tillage and resume growth, and apply glyphosate at the rate of 1.5 lbs ae/A. The addition of Classic or Synchrony XP may improve control of dandelion.
- in non-Roundup Ready soybeans, apply glyphosate at 1.5 lbs ae/A before the soybeans emerge. Where the soybeans have emerged and this is not possible, we suggest using an aggressive postemergence herbicide program.
- in corn, make an early postemergence application of a product containing 2,4-D or dicamba, or glyphosate in Roundup Ready corn.

Hail Damage Assessment for Wheat

Authors: Jim Beuerlein, Alan Sundermeier

Some parts of the state received hail during the May 1 storms. Hail damage to wheat included shredded leaves, bent, and broken stems. Shredded leaves can be replaced by new leaf growth and should not effect yield significantly. However, damage to wheat stems may cause significant yield losses, as the growing point which contains the wheat head is located above the first node. Stems broken below the node will not form a head, but stems broken above the node can produce a head if the head can get out through the damaged stem tissue above it. Regardless, there will be tillers to produce heads and grain, but the yield potential is reduced. The main tiller produces from 40 to 70 percent of the yield depending on plant population. If half of the main tillers do not develop, then yield will be reduced by 15 to 30 percent. Growers are advised to monitor their wheat development to observe if the main tiller head grows through the damaged stem area before predicting losses.

Managing Wheat Diseases with Fungicides

Authors: Dennis Mills, Pierce Paul

Scout wheat fields to determine which disease is present and at what level before making a decision to apply a fungicide. Although several fungicides are available to control foliar diseases in Ohio, the decision to use these products should be based on the susceptibility of the variety planted, the level of disease in the field, weather conditions, the yield potential of the field, fungicide cost, and the market price of wheat. The first disease to be detected is usually powdery mildew. This disease is important during the month of May and early June in mild seasons with high relative humidity. Randomly collect 30 to 50 tillers from throughout the field and look for the small white pustules on the lower leaves and leaf sheaths. If powdery mildew is present in a field planted to a susceptible variety you should watch its development over the next week or so and decide whether fungicides should be applied. Fungicides should be applied for powdery mildew control (on susceptive varieties) when 2 to 3 pustules are detected on the leaf (leaf two, counting from the top) below the flag leaf (the top-most leaf) anytime between growth stage 8 (flag leaf emergence) and 10 (boot). Stagonospora leaf and glume blotch is most severe when frequent rains occur during the months of May and June. Scout fields between growth stages 8 (flag leaf emergence) and 10.5 (full head emergence) and if 1 to 2 lesions are detected on the leaf below the flag leaf on a susceptible variety, fungicide should be applied.

Choosing a Fungicide: This season wheat growers will have another fungicide to help in the fight against wheat diseases. Proline from Bayer CropScience, with the active ingredient prothioconazole, recently received a federal label (Section 3) for use on wheat. This new addition brings the list of fungicides currently labeled for use by Ohio wheat growers to seven. The others are Tilt (Syngenta) and PropiMax (Dow AgroSciences) containing propiconazole (triazole chemistry); Headline (BASF) and Quadris (Syngenta) containing strobilurin-type chemistry (the active ingredient in Headline is pyraclostrobin and azoxystrobin is the active ingredient in Quadris); and Stratego (Bayer) and Quilt both containing a combination of a triazole and a strobilurin (propiconazole and trifloxystrobin in Stratego and propiconazole and azoxystrobin in Quilt). Check product labels for application timing restrictions. Tilt and Stratego have 24(c) special labeling in Ohio for application to Feekes growth stage 10.5.

All seven of these fungicides are very good materials, but they have slightly different activity against the various foliar wheat diseases in Ohio. Tilt and PropiMax at 4.0 fl oz/A and Quilt at 14 fl. oz/A have excellent activity against powdery mildew, Stagonospora blotch and leaf rust. Headline (9 fl. oz/A), Quadris (6.2 to 10.8 fl. oz/A) and Stratego (10 fl. oz/A) are relatively less effective against powdery mildew, but more effective against leaf rust. The new fungicide, Proline (5 to 5.7 fl. oz/A), is very similar to the others in terms of efficacy against Stagonospora and leaf rust, and current trials will determine how effective it is against powdery mildew. Therefore, if you are targeting powdery mildew then choose Tilt, PropiMax or Quilt and if you are targeting leaf rust choose Headline or Quadris. Any of these fungicides would be adequate for the other diseases or disease combinations when applied at the appropriate time. For instance, if a strobilurin is applied alone after infection has occurred, efficacy may be greatly reduced.

Scab Suppression: Compared to the other fungicides, Proline at the higher rate of 5.7 fl. oz/A is more effective at suppressing head scab. In general, the best results are achieved when this product is applied at flowering, forward and backward mounted nozzles are used to achieve maximum coverage of the heads, and when the product is applied to moderately resistant wheat varieties. However, it should be noted that complete scab control is unlikely with a Proline application, especially if prolonged periods of wet conditions occur during and after flowering. The term here is suppression. What growers can expect is a reduction of head scab but not 100% control. Since applications at flowering are made well before visual symptoms are seen on the heads the scab prediction website, should be used as a guide to assess the risk of scab occurring and to help determine whether or not Proline should be applied. As the wheat crop approaches flowering, the prediction tool will be updated frequently with commentary regarding the risk of head scab occurring to help growers make decisions regarding fungicide use.

Aphids on Wheat

Authors: Pierce Paul, Bruce Eisley, Ron Hammond

We are receiving reports of aphids in wheat, and several species of aphids may be found, including bird cherry-oat aphid, English grain aphid, corn leaf aphid, and greenbugs. Aphids are capable of causing problems on wheat by transmission of barley yellow dwarf virus (BYDV) or by direct feeding. The virus for barley yellow dwarf is generally transmitted to the wheat in the fall or early spring before Feekes stage 4.

Infestations great enough to cause economic damage are rare in Ohio, but under certain conditions, aphids can grow in numbers and damage wheat by feeding on the plant or feeding on the heads later in the season. At this time, the numbers of aphids found in most fields range from 4 to 7 aphids per linear foot of row, well below the treatment threshold of 50-100 aphids. As aphid numbers increase, we also see aphid predators and parasites in the wheat, helping to hold the aphid population below economic levels. If an insecticide is necessary, see the following web site for a list of recommended materials on field crops including small grains:

Alfalfa Recovery from the Spring Frost

Authors: Mark Sulc

Alfalfa recovery from the late frost has become very apparent this past week. Differences in stand vigor are showing dramatic results. Alfalfa stands with good vigor and productivity last year are recovering beautifully, and are well on their way to producing an excellent first harvest.

In contrast, the alfalfa stands that were weaker and less productive last year showed a much slower recovery. Adding insult to injury is that chickweed and other winter annuals taking hold in those weaker stands after the frost. The chickweed I’ve seen is already flowering. At this stage, our herbicide options probably are not cost effective. The chickweed should be dying fairly quickly now. I hate to say this, but it’s probably a “grin and bear it” situation. Aim for higher quality alfalfa in the summer harvests.

It is advisable to delay first harvest of alfalfa this spring, allowing the crop to develop into the flowering stage to build energy reserves for strong regrowth. This is especially true for the weakest stands, but will also help the stands that are now showing good recovery.

Finally, continue to monitor alfalfa stands for alfalfa weevil feeding. I saw very little evidence of weevil feeding near Springfield, but that may not be true everywhere.

Black Cutworm on Corn

Authors: Bruce Eisley, Ron Hammond

Black cutworm (BCW) moths continue to be captured in higher numbers in pheromone and light traps in central Ohio. However, there isn't any way to determine whether BCW is going to be a problem in Ohio in corn this year or in what fields it might be a problem.

Growers should begin scouting fields as soon as the corn emerges from the soil. Check for early cutting and leaf feeding. If cutting is found, dig around in the soil to find the worms. If a rescue treatment is necessary, a list of insecticides labeled for cutworm rescue treatment can be found at: For additional information about BCW and other early season pests of corn, see the OSU FactSheet at

Soybean Rust Identified in Florida

Authors: Anne Dorrance

The first positive find of overwintering soybean rust was reported in Florida on kudzu. Since this area of Florida experienced rain over the weekend, we should be able to monitor disease progress from this find in the next two to three weeks. So far, Georgia, Alabama, Mississippi and Louisiana are all negative for rust. Sentinel plots in Ohio were planted, and as soon as they emerge we will begin scouting for the season.

Archive Issue Contributors: 

State Specialists: Anne Dorrance, Pierce Paul, and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Mark Sulc (Forages), Robert Mullen (Soil Fertility), Jim Beuerlein (Soybean Production), Peter Thomison (Corn Production), Mark Loux and Jeff Stachler (Weed Science). Extension Educators: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Keith Diedrick (Wayne), Steve Prochaska (Crawford), Mark Koenig (Sandusky), Gary Wilson (Hancock), Harold Watters (Champaign), Greg Labarge (Fulton), Steve Foster (Darke), Ed Lentz (Seneca), Alan Sundermeier (Wood), and Jim Lopshire (Paulding).

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.