C.O.R.N. Newsletter 2006-12

Dates Covered: 
May 9, 2006 - May 16, 2006
Editor: 
Tammy Dobbels

Weeds Escaping Preemergence Herbicides?- Some Strategies to Consider

Authors: Mark Loux

Residual herbicides applied at the time of planting are most effective when enough rain occurs within about a week or so after planting 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). 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 – metolachlor, acetochlor, etc. 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.

In parts of Ohio, rainfall in the past several weeks has been insufficient to ensure the initial activity of preemergence herbicides. There appears to a chance of rain later this week, and the occurrence of significant amounts of rain should move herbicide into the soil and cause it to be active. However, we have reports of weeds already escaping herbicides that were applied a week or more ago. This may not be much of a problem in Roundup Ready fields, since glyphosate can be applied as necessary prior to or after crop emergence (although the lack of preemergence activity can result in the need for an earlier postemergence glyphosate application or multiple postemergence glyphosate applications). We offer the following suggestions for controlling preemergence weed escapes in non-Roundup Ready fields, and limiting the additional costs that might be incurred from trying to remedy preemergence herbicide failures.

1) where weeds have emerged but the crop has not, apply glyphosate or Gramoxone to control escaped grass and broadleaf weeds. The weeds should be small, allowing use of low rates of either herbicide.
2) where the crop has already emerged, and the weeds are just starting to emerge, rotary hoe the field. This will control many of the weeds that are starting to emerge, and “buy some time” for the preemergence herbicides to receive rain and become active.
3) where the crop and weeds have emerged, scout fields and apply postemergence herbicides as necessary to control weeds that escape preemergence treatments. Two approaches to postemergence herbicide applications in this situation: a) assuming that preemergence herbicides will at some point become active, postemergence herbicides can be applied soon after the first flush of weeds has emerged, and the small weed size allows the use of lower rates and minimizes costs; or b) postemergence applications can be delayed until weeds are 2 to 4 inches tall, which is the typical application window in corn for total postemergence herbicide programs. The second approach involves higher herbicide costs, but can result in the most effective control if rain continues to be lacking.

Regardless of the approach taken in non Roundup Ready fields, be sure to scout fields frequently enough so that if postemergence herbicides are needed, they can be applied when weeds are small enough to be readily controlled.

Kill dandelions now!

Authors: Mark Loux

While dandelion populations in some fields have been brought under control over the past year or two through use of effective fall and spring burndown treatments, populations are still out of control in other fields. When using spring burndown treatments, the most effective reduction in dandelion populations occurs when herbicides are applied before dandelion plants go to seed and start to senesce. This corresponds roughly to applications by early May in central and southern Ohio, and mid-May in northern Ohio. Applying too late often results in the appearance of good control, but the reality is that herbicides are less likely to control the dandelion root, resulting in extensive regrowth later in the year.

Effective burndown treatments are essential to prevent the dandelions from interfering with crop establishment and early-season growth. Where the dandelions are not effectively controlled and conditions turn dry early in the season, the dandelions can reduce crop stands and suppress crop growth to the point that there is no yield in heavily infested areas. Some observations based on our research on dandelion over the past several years:

- glyphosate activity on dandelion can be slow, especially under cool conditions. A combination of 2,4-D ester plus glyphosate is more effective than glyphosate alone. We suggest using a glyphosate rate of 1.5 lbs acid equivalent (ae)/A for control of dandelions, especially when it is not mixed with 2,4-D or chlorimuron (Canopy, SynchronyXP). Include the full rate of ammonium sulfate with any glyphosate treatment.
- 2,4-D ester should be included in any burndown treatment where dandelion is present, if possible. The single most effective burndown treatment in soybeans for control of dandelion is 2,4-D ester plus glyphosate plus chlorimuron, and this is one of the more effective treatments even when the 2,4-D ester is omitted. The addition of FirstRate to glyphosate treatments can also improve control, but not necessarily to the extent that occurs with chlorimuron.
- The most rapid control of dandelions within the first 14 days or so occurs with combinations of glyphosate plus either Valor or Gangster. However, while the plants show symptoms sooner, there is usually more extensive regrowth with these treatments at about 42 days after treatment, compared to the treatments mentioned previously.
- Lumax and Lexar have been the most effective preemergence treatments in corn for control of dandelion, and also the most rapid. The addition of 2,4-D ester can result in more effective reduction in dandelion populations, but does not generally contribute much to control in the early part of the growing season.
- Where significant dandelion regrowth occurs within the first 4 to 6 weeks after burndown treatment, apply postemergence herbicides to prevent further regrowth and help reduce the dandelion populations.

Staging Growth in Early Season Corn

Authors: Peter Thomison

When estimating yield losses in corn due to hail, frost, and other types of plant injury, it’s essential to establish the stage of plant growth at the time damage occurred. In recent years, it’s also become increasingly important to know corn stage of development in order to use postemergence herbicides effectively with minimum crop damage. This knowledge could be particularly important this year if the dry conditions we’re experiencing in parts of the state continue resulting in uneven growth of corn.

Several systems are currently used to stage vegetative growth in corn. The "leaf collar" system is probably the method most widely used by university and seed company agronomists in the Corn Belt. With this method, each leaf stage is defined according to the uppermost leaf whose leaf collar is visible. The first part of the collar that is visible is the back, which appears as a discolored line between the leaf blade and the leaf sheath. The oval shaped first leaf is a reference point for counting upward to the top visible leaf collar. This oval shaped leaf is counted as the number 1 leaf when staging. If a plant has 4 visible leaf collars, then it is defined as being at V4. Normally a plant at the V4 stage will have parts of the 5th and 6th leaves visible, but only four leaves with distinct collars. A field is defined as being at a given growth stage when at least 50% of the plants show collars.

Another widely used staging method is the "hail adjustor's horizontal leaf method" developed by the crop insurance industry. Rather than using the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40 to 50% exposed and whose tip points below the horizontal. Typically a given "horizontal leaf" growth stage based on the hail adjustor's method will be 1 to 2 leaf stages greater than the collar method. From growth stage V1 through about V5 there is typically one additional leaf (above that leaf with the last visible collar) whose leaf tip is pointing below the horizontal. Beyond growth stage V5, two or more additional leaves with 'droopy' leaf tips will be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be a 8-leaf plant according to the hail adjustor's horizontal leaf method). One problem with the horizontal leaf method is that it is often difficult to identify the uppermost horizontal leaves in fields that have recently experienced severe leaf damage. Hail adjustors get around this problem because they usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.

Corn leaf stage is a more reliable indicator of corn development than plant height. This is especially true in a cool, wet spring when corn is growing more slowly from a height standpoint. Differences in tillage and soil type often have a pronounced effect on plant height but relatively little effect on the stage of vegetative development. For example, within a field, corn may be taller in those areas characterized by darker soil (with higher organic matter) than in areas with lighter soil, especially the clay knolls, yet plants in both areas of the field may be at nearly the same stage when counting leaf collars.

At about V6 stage, or 8-leaf stage of the hail adjustor's method, increasing stalk and nodal growth combine to tear the smallest lower leaves from the plant. This results in degeneration and eventual loss of lower leaves. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process.There may also be occasions when the lower leaves are hard to identify prior to V6 stage. When extensive early season leaf damage has occurred, identification of the first rounded leaf and subsequent leaf collars may be difficult.

Dr. Bob Nielsen at Purdue has described a method for predicting leaf stage development using accumulated heat unit or growing degree day (GDD) information. Given an understanding of corn leaf stage development and heat unit calculation, a grower can predict what leaf stage of development a particular field is at given its planting date and temperatures since planting. It is useful to know when the crop emerged, but if you do not you can estimate that event also. Corn emergence typically requires 100 to 150 GDDs.
Dr. Nielsen proposes that corn leaf developmental rates may be characterized by two phases. From emergence to V10 (ten visible leaf collars), leaf emergence occurs approximately every 85 GDDs. From V10 to tasseling, leaf emergence occurs more rapidly at approximately one leaf every 50 GDDs. Previously, about 60-65 GDDs were associated with the appearance of new leaf collars during vegetative growth.

Example: A field was planted on April 28, but you do not know exactly when it emerged. Since planting, approximately 785 GDDs have accumulated. If you assume that the crop emerged in about 125 GDDs, then the estimated leaf stage for the crop would be between V7 and V8. We arrived at this estimate by first subtracting 125 from 785 to account for emergence, then dividing the result (660) by 85 to equal 7.8.

Dr. Nielsen warns that these predictions of leaf stage development are only estimates. One of the factors that most influences the accuracy of these estimates is the existence of other growth-limiting stresses and conditions (nutrient deficiencies, compaction, etc.). Despite these potential drawbacks, this heat unit method may be useful in timing when plants will reach an approximate stage of growth.

Bean Leaf Beetle

Authors: Ron Hammond, Bruce Eisley

With soybean planting going full speed now, growers should plan on monitoring their emerging fields for bean leaf beetles during May through early June. Although populations were not especially high during 2005, we did see some fields with significant numbers. If early defoliation reaches 50% or cotyledons are being destroyed, and plants appear stunted, an insecticide treatment would be recommended. Keep in mind that bean leaf beetles will tend to be most numerous in early-planted fields. Our hope is that with the number of acres already planted to soybean, the overwintered adult population will tend to be spread over a large number of acres, thus hopefully reducing potential problems. However, we still recommend scouting your fields. If populations are deemed sufficient to warrant treatment, a list of labeled insecticides is available at http://entomology.osu.edu/ag/545/siblb.pdf.

As in past years, there is still concern among some growers with the beetle's ability to vector bean pod mottle virus. The bean leaf beetle transmits this virus, especially in early season during feeding by the over-wintering beetle. This concern tends to be greatest on food grade soybeans and those being grown for seed where seed quality is an important issue. If growers choose to treat the bean leaf beetle for virus control, they should spray the bean leaf beetle during the VC-V1 stage after the soybeans emerge from the soil and when beetles begin to appear in the field. Recommendations also suggest that a second spray be made in July at the beginning of the first beetle generation.

Black Cutworm

Authors: Ron Hammond, Bruce Eisley

Black cutworm (BCW) moths continue to be captured in pheromone and light traps in central Ohio. We have been capturing BCW moths in pheromone traps for the past several weeks and eggs have been laid in some of fields throughout 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.

Our suggestion is to scout fields as soon as the corn begins to emerge from the soil and especially those fields that have already emerged. Check for leaf feeding and early cutting. If cutting is found, dig around in the soil to find the worms. Rescue treatments can then be applied if necessary and a list of insecticides labeled for cutworm rescue treatment can be found at: http://entomology.osu.edu/ag/545/cicw.pdf. For additional information about BCW and other early season pests of corn, see OSU FactSheet FC-ENT-12 at http://ohioline.osu.edu/ent-fact/0012.html.

Slug Update

Authors: Ron Hammond, Bruce Eisley

We are now seeing newly hatched gray garden juvenile slugs in east-central OH in Fairfield County, and expect to see them in fields in Licking County this week. The egg hatch we are seeing in this area of Ohio is at the same time as last year. We expect this egg hatch to move further north over the next few weeks. We expect to see slug injury on both corn and soybeans at these more southern locations within the next few weeks, which will also proceed northward throughout the month of May. Thus, no-till growers who have experienced slug problems in past years should be prepared to monitor their slug situation over the next few weeks. Hopefully with all the early planting we have already had in parts of OH, the impact of slug feeding will be reduced.

Herbicide Treatments to Control Failed Stands of Wheat

Authors: Mark Loux

We have received a number of questions about the control of failed wheat stands in field where corn will be planted. The two options to kill the wheat appear to be glyphosate or a combination of Gramoxone plus atrazine. Follow these guidelines to help ensure adequate control:

- Glyphosate will be most effective when applied in water with ammonium sulfate (not an ammonium sulfate substitute). Avoid use of 28% UAN as the spray carrier. Where possible, avoid mixing glyphosate with other herbicides. When applying glyphosate alone, use a rate of at least 0.75 lbs acid equivalent (ae)/A. When applying with atrazine or an atrazine premix product, use at least 1.1 lbs ae/A. Higher rates of glyphosate may improve control in either situation. Use a spray volume of at least 15 gpa to ensure adequate coverage.

- Gramoxone Max will be most effective when applied at a rate of at least 2 pints/A with atrazine (0.5 lb ai/A or higher). Apply with nonionic surfactant in a spray volume of at least 20 gpa. Use of 28% as the spray carrier can improve control.

Armyworms, Wheat, and Proper Management

Authors: Bruce Eisley, Ron Hammond

Armyworm moths are being collected in larger than normal numbers in Ohio and surrounding states. These larger moth numbers mean that we will need to closely monitor wheat and then adjacent corn fields, along with no-till corn fields, over the next month for possible economic larval populations. IF, and only IF, larval numbers reach accepted thresholds (see below) should fields be treated. This is basic IPM philosophy. When and if wheat fields need treatment based on thresholds, growers should be aware of the various pre-harvest intervals for the various insecticides because insecticides labeled for armyworm on wheat have pre-harvest intervals ranging from 7 to 30 days.

Our biggest concern at this time is that we have become aware of recommendations being made by others suggesting wheat fields be sprayed with a pyrethroid insecticide, not to control the larvae, but to prevent adult armyworms from entering the field and laying their eggs or perhaps to kill the adults. Although some pyrethroids have been shown to be repellant towards certain adult beetles (for example bean leaf beetle and Mexican bean beetle adults), there is no data we are aware of that indicate repellency towards armyworm adults which are moths. This recommendation is based only on larger than normal adult numbers currently being collected. Because there is absolutely no way of knowing which wheat fields might develop an economic population of armyworms, the chances of wasting money is probably great. This approach would be totally preventive in nature, without any data that we are aware of to suggest it would even work.

We would point out that this is NOT a proper approach in terms of Integrated Pest Management, or IPM. We, and all growers, should always take an IPM approach to the management of insects. When we have the ability to scout a field for insect injury and determine population levels, and when we have good treatment thresholds available, there is NO reason not to use an IPM approach.
Over the coming weeks, wheat fields WILL have to be checked for armyworms. When scouting, we will recommend checking several places in the field to determine the number and size of the worms present. Rescue treatments will be recommended if worm counts average 6 or more per row foot and larva are in the early stages of development or if head cutting is occurring. Because treatment of armyworm larvae reaching maturity will achieve limited results because the period of maximum feeding will have passed, we will be recommending that growers be scouting for larvae earlier than later. But, as always, we will recommend using IPM!

Aphids on Wheat

Authors: Ron Hammond, Bruce Eisley

We have received a couple of reports of aphids being detected in wheat during the past week. There are several species of aphids that may be found in wheat including English grain aphid, bird cherry-oat aphid, corn leaf aphid, and greenbug. Aphids are capable of causing problems on wheat either by the movement of a plant virus, barley yellow dwarf virus (BYDV), or by the aphids feeding on the plants. The virus for barley yellow dwarf is generally transmitted to the wheat in the fall or early spring before growth stage 4.
Aphid infestations great enough to cause economic damage are rare in Ohio. However, aphids can under certain conditions continue to build in number and damage wheat either by feeding on the plant during seedling to boot stage or by feeding on the wheat heads later in the season. At this time, the number of aphids being found in most fields is well below the treatment threshold of 100 aphids per linear foot of row. As aphid numbers build, we also begin to see a number of predators and parasites in the wheat. The predators and parasites usually help to keep the aphid numbers below economic levels. If an insecticide is deemed necessary, see this web site for a list of recommended materials on field crops including small grains: http://ohioline.osu.edu/b545/pdf/b545.pdf.

Frost Damage on Wheat

Authors: Alan Sundermeier

The OSU Northwest Ag Research farm near Hoytville in Wood County had a minimum temperature on April 26 of 26.1 degrees. Temperatures that night were below freezing from 1 am to 8 am. Wheat was in joint (stage 6 to 7 ) with the growing point several inches above ground in the whorl of the plant.

Scattered areas of frost damage on wheat are being reported in Northwest Ohio with symptoms including dead, limp upper leaves, and dead flag leaf emerging from green lower wheat sheath. Kansas State University research has shown that 2 hours of 24 degree temperatures will kill the growing point and cause leaf chlorosis at jointing stage of wheat. Figure 5 and 6 on the following link show these symptoms well - http://wheat.colostate.edu/freeze.pdf.

Fungicides for Powdery Mildew Control on Susceptible Wheat Varieties

Authors: Dennis Mills, Pierce Paul

Powdery mildew seems to be spreading in some parts of the state. Reports from north western Ohio suggest that the disease can now be found on the upper leaves in some fields. Fungicides are available for powdery mildew control in Ohio; however the decision to use these fungicides should be based on the susceptibility of the variety planted, the level of disease in the field, weather conditions, and the yield potential of the field. When the level of disease in the field is high on the top two leaves, wheat growers could benefit from applying fungicides, but in low disease years, fungicide applications would not be economical. Under favorable weather conditions, on susceptible varieties, powdery mildew may cause substantial reduction in wheat yield. Yield response to fungicide application is directly dependent on the amount of disease in the field and the susceptibility of the variety to that disease. Resistant varieties rarely benefit from application of fungicides.

Prior to applying fungicides, growers are advised to determine the susceptibility of the variety in their fields and the level of the disease (to view the reaction of various wheat varieties to powdery mildew and other diseases in Ohio visit the 2005 Ohio Wheat Performance table http://www.oardc.ohio-state.edu/wheat2005/table6.asp?year=2005) Randomly collect 30 to 50 tillers from throughout the field and look for the small white pustules on the leaves and leaf sheaths. If 2 to 3 pustules are detected on the leaf two (counting from the top down) – the leaf below the flag leaf (the top-most leaf) anytime between growth stage 8 (flag leaf emergence) and 10 (boot), fungicides should be applied for powdery mildew control.

fungicides winter wheat disease table


Efficacyof Fungicides for Wheat Disease Control in Ohio Based on Application atThreshhold
Fungicide
Rate/A
Powdery Mildew*
Stagonospora
Leaf Blotch*
Leaf Rust**
Headline
9 fl oz
Fair
Good
Excellent
PropiMax
4 lf oz
Excellent
Good
Excellent
Quadris
6.2-10.8 fl oz
Fair
Good
Excellent
Quilt
14 fl oz
Excellent
Good
Excellent
Stratego
10. fl oz
Fair
Good
Excellent
Tilt
4 fl oz
Excellent
Good
Excellent
*PowderyMildew efficacy based on application at flag leaf stage (growth stage 8)
**Stagonospora and Leaf Rust control based on application at head emergence (growthstage 10.1)
Archive Issue Contributors: 

State Specialists: Anne Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science), Peter Thomison, (Corn Production), Ron Hammond and Bruce Eisley (Entomology). Extension Educators: Howard Siegrist (Licking), Harold Watters (Champaign), Glen Arnold (Putnam), Roger Bender (Shelby), Steve Foster (Darke), Steve Bartels (Butler), Bruce Clevenger (Defiance), Gary Wilson (Hancock), Ed Lense (Seneca), Alan Sundermeier (Wood), Mark Koenig (Sandusky), Jim Lopshire (Paulding), Jim Skeeles (Lorain) and Keith Diedrick (Wayne)

About the C.O.R.N. Newsletter

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.