C.O.R.N. Newsletter 2006-18

Dates Covered: 
June 19, 2006 - June 26, 2006
Editor: 
Greg LaBarge

Is Soybean Aphid in Ohio?

Authors: Bruce Eisley, Ron Hammond

By this time last spring, we had already found numerous fields with developing populations of aphids, and had a single field in Wood County that had already reached threshold. So what is currently happening in Ohio in 2006?

Last week, we made visits to numerous fields in northwest OH and after much searching, located a single field where we found soybean aphids. This field was in northern Wood County, relatively close to the field mentioned above that needed an early treatment in 2005. However this year, aphid density averaged less than 1.0 aphid per plant. We found a few plants after 10 minutes of searching that each had one or two aphids per plant, although a single plant was located in the same field that had >20 aphids on one leaflet. Finding aphids, albeit extremely few, was to be expected. Other states have also been finding a few aphids, and it was assumed that Ohio would be no exception. But observations in Ohio and these other states suggest very low populations whereas high densities were being found at this time in 2005. These low numbers support our predictions of few if any economic populations in Ohio, although we still will recommend keeping vigilance.

If scouting for aphids on your own, you should expect them to be in low numbers, perhaps only finding one or two aphid per plant. At these low densities, you need to keep from misidentifying them as small potato leafhopper nymphs (http://ohioline.osu.edu/icm-fact/images/113.html), thrips (http://entomology.osu.edu/ag/thrips.htm), mealy bugs (http://entomology.osu.edu/ag/mealy1.htm), or other small insects. You can only do this using a small hand lens. Soybean aphids will be located on the upper most leaves of the plant at this time, those leaves that are just opening up and expanding. They are round or oval shaped with 2 cornicles (or tail pipes) and will usually remain in one spot if touched. Most of these other insects, especially the thrips and leafhoppers, will be very active on the leaf or when disturbed.

We will continue to monitor the soybean aphid situation throughout the summer. There is no reason to panic or assume the worst will happen. There are a lot of beneficial predators in the soybean fields that will help to keep the aphid numbers reduced. We urge vigilance and continued scouting, and use of the 250 aphids per plant threshold with a rising population before any action is taken. And keep reading the CORN newsletter for up to data information.

First Instar European Corn Borer (ECB) Can Be Found

Authors: Ron Hammond, Bruce Eisley

Last week we mentioned that ECB moths had been flying for the past couple of weeks and on Wednesday, we found our first evidence of ECB larval damage in central Ohio. The damage was evidenced by slight “window-pane” feeding on the whorl leaves of knee high corn. A whorl was pulled and carefully unrolled to reveal a newly hatched ECB larva (about 1/8”) long in the whorl. Less than 10% of the plants showed any whorl feeding injury so the field was not close to treatment threshold. However, this does indicate that it is time to begin scouting non-Bt corn borer corn that is knee high or above for possible ECB infestations. Refer to the ECB article in last weeks CORN newsletter (http://corn.osu.edu/story.php?setissueID=138&storyID=822) for more information about ECB.

Sweep for Potato Leafhopper in Alfalfa ­

Authors: Bruce Eisley, Ron Hammond

Potato leafhopper (PLH) adults have been in the state for the past several weeks having migrated from the south and since a lot of first cutting alfalfa has been made, it is time to start sweeping alfalfa for PLH adults & nymphs on this second cutting. We have had reports that some second cutting fields have required treatment for PLH. Also, new seedings should be checked for leafhoppers since new seedings cannot take as much damage as older seedings. Checks should be made with a standard sweep net by taking several 10 sweep samples randomly across the field and counting the number of PLH adults and nymphs found in each 10 sweep sample. Treatment for PLH is based on the number of adults and nymphs found when sweeping versus the height of the alfalfa. For example, treatment is warranted if the number of adults and nymphs in 10 sweeps equals or exceeds the height of the alfalfa in inches (8 adults & nymphs in 10 sweeps in 6 inch alfalfa would justify treatment for PLH). Information about PLH can be found in a fact sheet on the web at: http://ohioline.osu.edu/ent-fact/0033.html and insecticides for PLH control in alfalfa can be found at: http://entomology.osu.edu/ag/545/aiplh.pdf

Late Increase of Stagonospora Blotch and Wheat Leaf Rust

Authors: Pierce Paul, Dennis Mills

Reports from several locations across the state suggest that the levels of Stagonospora leaf and glume blotch and leaf rust have increased substantially over the past week or so in wheat. Both of these diseases can potentially affect wheat yield and grain quality. However, the extent to which this occurs depends on the level of damage to the upper leaves and the head (in the case of glume blotch), the level of susceptibility of the variety, and when the upper leaves and head become damaged relative to grain fill. When diseases occur early in the season on susceptible varieties, yield losses can be as high as 25-30%. The upper two leaves (leaf 1 - the flag leaf and leaf 2) and the head contribute most of the sugar for grain fill, so, if they become blighted very early in the season, substantial yield reduction may occur. Most of the sugars produced in these structures are used to fill grain during the head emergence growth stage through grain fill. Between boot and early head emergence, the flag leaf and the leaf below the flag leaf are most important, but as the plant begins to flower, the glumes of the head and the flag leaf are the major contributors to grain fill. During the ripening stages, the glumes of the head are the most important contributors to grain fill.

In most of our fields, only trace levels of leaf rust and Stagonospora blotch were seen before flowering, and in most cases, both diseases were restricted to the lower leaves. However, during the period after flowering (when warm, humid conditions occurred) the level of disease has increased, reaching the flag leaves and glumes in some field. Although this late surge of disease may be less detrimental to the wheat crop than if the diseases had arrived earlier, some yield reduction may still occur in fields planted to highly susceptible varieties, especially those susceptible to glume blotch. Nothing can be done about these diseases at this time. However, for future years, make a note of which varieties were the most susceptible and plant varieties with higher levels of resistance to avoid these losses.

As Predicted, Moderate to Low Levels of Wheat Head Scab in Ohio

Authors: Pierce Paul, Dennis Mills

Based on the information coming in from our counties, is seems like Ohio’s wheat has escaped another severe wheat scab epidemic. Surveys conducted by our county Extension Educators show that the level of scab is moderate to low in most fields. The average incidence (percentage of heads with at least one scabby spikelet out of the total number of heads examined) of wheat scab varies from county to county and from field to field within any give county, and so far, ranges from approximately 1 to 27%. This variation is likely due to differences in variety susceptibility, flowering date, level of Fusarium spores, and weather pattern from one field to another. Fields with slightly higher levels of scab probably flowered during relatively warm, wet periods. If the weather stays relatively dry between now and harvest harvest, we should have a wheat crop with relatively little yield and quality losses due to head scab.

It’s Postemergence Season – Do You Know How Big Your Weeds Are?

Authors: Mark Loux

Every year it seems that even though crop planting can be spread over several weeks or more, the postemergence season happens all at once. Although we usually succeed in making timely postemergence herbicide applications in our research plots, we struggle to stay ahead of the weeds, and many growers and custom applicators face the same challenge. This article offers guidance on maximizing glyphosate effectiveness and preserving maximum yield of Roundup Ready soybeans. The following suggestions are based on research at OSU and other universities, and also on information provided by herbicide manufacturers where appropriate.

- In fields where residual herbicides have not been applied, weed control and Roundup Ready soybean yield is maximized when two postemergence (POST) glyphosate applications are used. Make the first application when weeds are no more than 4 to 8 inches tall, and the second approximately three weeks later. This strategy avoids yield loss from early-season weed competition, improves control of weeds that glyphosate is occasionally not completely effective on, and controls late-emerging weeds. It is extremely difficult to accomplish all of this with one POST application, except in fields with low weed populations and no giant ragweed. Those growers who deliberately delay the first POST glyphosate application until just before soybean canopy closure are allowing weeds to compete with soybeans for too long, and consequently, soybean yield is not maximized.

- Something to think about for next year – if you find yourself in the position of often applying glyphosate to large weeds, make the decision to use residual herbicides next spring. Early-season weed control from residual herbicides will keep weeds from getting too large, too early, and create a wider window for postemergence applications, so that soybean yield is not reduced due to late POST applications.

- Adjust glyphosate rates for weed size. General guidelines for glyphosate rates with regard to weed size (most labels are similar): 4 to 8 inches – 0.75 lb ae/A; 8 to 18 inches – 1.1 lbs ae/A; larger than 18 inches – 1.5 lbs. However, our research shows that more consistent control of many giant ragweed and lambsquarters populations would most likely be achieved by using a rate of 1.1 to 1.5 lbs ae/A in the first POST application, which should be made when lambsquarters is less than 6 inches tall and giant ragweed is no more than 8 to 12 inches tall. A second application is often necessary to ensure the consistent control of these weeds. Using the a rate of at least 1.1 lb ae/A in the first POST application is a more effective strategy than using 0.75 lbs, especially where the population’s sensitivity to glyphosate appears to be declining. In soybeans fields that were not treated with a preplant burndown, use a rate of 1.5 lbs ae/A. The maximum amount of glyphosate that can be applied to Roundup Ready soybeans (total of all POST applications) is 2.25 lbs ae/A.

- There are many soybean herbicides that could be mixed with glyphosate in POST treatments. However, our research shows that just increasing the glyphosate rate may be the most effective strategy to improve control in problem situations, along with making a second POST glyphosate application. Some herbicides antagonize the activity of glyphosate, and others add little or nothing to control while increasing the soybean injury. We are not suggesting that use of only glyphosate is an effective strategy in Roundup Ready crops, because this approach is likely to hasten the onset of glyphosate resistance. What we are suggesting is that money spent on herbicides other than glyphosate in Roundup Ready soybeans is best spent on residual herbicides that are applied prior to soybean emergence. One exception is the addition of FirstRate or chlorimuron to glyphosate to improve POST control of glyphosate-resistant marestail, but this will not improve control of populations with resistance to glyphosate and ALS inhibitors.

- One of the consequences of allowing weeds to grow to a large size prior to glyphosate application is that it promotes infestation of the plants by stalk-boring insects, which may reduce herbicide effectiveness. University research has shown that the best prevention for this is to apply herbicide when weeds are small, less than about 6 inches tall. This is a more effective strategy than just increasing glyphosate rate, which may not overcome the negative effect of the stalk borers on herbicide activity.

- Research at OSU and other universities has shown that the activity of glyphosate is affected by the time of day of application. We conducted a two-year study on this, making glyphosate applications at three-hour intervals from 6 am to midnight. Glyphosate was effective when applied between 9 am and 6 pm, but activity was always reduced at 6 am and midnight, and usually at 9 pm as well. All weed species present in the study except lambsquarters exhibited this effect, and control of ragweed was especially sensitive to time of day. Control of giant ragweed was reduced by 50% or more when applied in the early morning or late evening. Increasing the glyphosate rate did not overcome the effect of time of day. We saw this effect with other herbicides as well. So – what’s our recommendation with regard to time of day? Those of you who are taking advantage of light bars or similar equipment to apply at night or early morning are at risk of poor weed control. Our research suggests that: 1) at this time of the year, we should be applying between the hours of about 8 am and 7 pm to ensure herbicide effectiveness, and 2) using drift-reducing nozzles or adjuvants that allow us to spray at these times is a better strategy to deal with wind than applying at night.

- However, don’t overdo drift reduction measures. Simultaneous use of the most effective drift-reducing nozzles and drift-reducing agents can result in too few droplets of large size, which is not ideal for herbicide effectiveness.

- A comment on other adjuvants that can be used in POST treatments – don’t mess up the glyphosate’s activity with a bunch of stuff that is not contributing anything. For example, our research shows that it is possible to get a slight increase in glyphosate effectiveness by adding some surfactant (most likely in higher spray volumes which result in lower surfactant concentration). However, most glyphosate products already contain surfactant, and increasing the glyphosate product rate in problem situations results in increased rates of glyphosate and surfactant. It has become fairly standard to add ammonium sulfate (AMS) or an ammonium sulfate substitute product to glyphosate treatments, which serves the purpose of negating the effect of hard water on glyphosate activity and improving control of certain weeds such as velvetleaf. We are not sure that AMS helps for control of small annual weeds at this time of the year when weather is conducive for herbicide activity, but it may help for control of the large weeds that many growers seem to like to spray. AMS can burn soybean leaves in hot weather. This burn can be minimized by using lower rates of AMS – 8.5 lbs per 100 gallons instead of 17 lbs. While the AMS substitute products may be more convenient than adding spray-grade AMS, it can be difficult to assess how effective they are compared to true AMS. The AMS rate is too low in some of these products. Resist the sales pitch on acidifiers and other products that claim to improve glyphosate effectiveness.

- Applying glyphosate with manganese has the potential to reduce glyphosate activity. Be sure to use full rates of AMS (preferably the spray grade formulation, not AMS replacements) and only chelated forms of manganese. Add the AMS to the spray tank first, followed by the manganese, followed by the glyphosate. Large weeds (greater than 12 inches) are likely to survive when tank-mixing the manganese with glyphosate, especially velvetleaf. When tank-mixing the two, use glyphosate at 1.1 to 1.5 lbs ae/A to minimize the loss in control due to the manganese.

- Glyphosate activity is maximized in low spray volumes – 10 gpa or lower – which should be fine for most small annual weeds. However, spray coverage is an issue when glyphosate is applied to large weeds. Spray volumes of 15 to 20 gpa may provide more effective coverage on large weeds or dense populations. Also – the spray boom should be high enough to ensure that even the taller parts of plants receive spray.

- In an effort to have an edge in the marketing battle between glyphosate products, which are usually similar in activity, the rainfree interval has been reduced to as little as 30 minutes on some product labels. Our greenhouse research with lambsquarters indicates that a rainfree interval of 4 hours may be more appropriate.

- In fields planted with Roundup Ready corn in 2005, any volunteer corn showing up will also be resistant to glyphosate. This can also occur if your corn field was adjacent to a neighbor’s Roundup Ready corn field due to pollen movement. When applied with just crop oil concentrate, any of the postemergence grass herbicides (sethoxydim is the weakest) for soybeans will control volunteer Roundup Ready corn. When mixed with glyphosate, use only Assure II or Select Max to ensure effective control.

- Many perennial species are only now at their optimal size for maximum long-term glyphosate activity. We noticed common pokeweed beginning to show flower buds for the first time last week. For maximum perennial control with glyphosate, apply when the weeds are in the bud to early flowering stage. The most effective glyphosate rate is between 1.1 and 1.5 lb ae/A. A second application is needed for maximum control of many perennial species. Since perennial weed species are usually taller than annual weed species at the time of application, be sure the boom height is set properly to maximize perennial weed control.

OSU Weed Science Field Day

Authors: Mark Loux

The OSU Weed Science field research tour will be held at the OARDC Western Agricultural Research Station on Thursday, July 6. The tour, which is self-directed in nature, starts at 9 am and runs until noon. OARDC Western Agricultural Station is north of South Charleston on State Route 41, approximately 5 miles south of Interstate 70. There is a cost of $5 per person for the tour.

Uneven Plant Height in Corn: Effects on Yield

Authors: Peter Thomison

Uneven corn stands are a hot topic this week and receiving considerable attention across the state. In many corn fields, it’s not unusual to see differences in plant growth stage and height within and between corn rows. Although much of the variability in plant height can be related to uneven emergence, it’s important to recognize that plant height is not a reliable indicator of plant growth stage in corn. In some fields that show variability in plant height, tall and short plants may actually be at fairly similar stages of growth based on leaf collars.

The primary causes of delayed emergence and plant heights are probably soil moisture and temperature variability within the seeding depth zone. Other factors contributing to the problem include poor seed to soil contact due to cloddy soils, soil crusting prior to emergence, seeding depth, residue distribution, etc.

What impact will variability in plant height have on crop yields? It’s been well documented that uneven emergence affects crop performance because competition from larger, early emerging plants decreases the yield from smaller, later emerging plants. According to one popular rule of thumb, if two neighboring plants differ by two or more leaves, the younger plant will almost always be barren or produce a nubbin ear at maturity. In a recent article in the Iowa State University Integrated Crop Management Newsletter (on-line at http://www.ipm.iastate.edu/ipm/icm/2006/6-12/cornheight.html), Roger Elmore and Lori Abendorf reviewed research from Europe, Canada, Argentina, and the U.S. to determine how later emerging plants performed within a field of normal emerging corn. The studies they described usually involved delaying the planting of a certain percentage of corn plants with a field to simulate variable emergence.

Research in Ontario indicated that when one of six (17%) plants was delayed in emergence by two leave overall yield was reduced 4 percent; when delayed by four leaves, 8 percent yield losses were observed. Plants neighboring late emerging plants only partially offset yield losses.

Illinois and Wisconsin research considered the response of corn when 25, 50, or 75 percent of the plants were planted either 10 or 21 days after the original planting date. Overall, grain yields were reduced 6 to 7 percent by a delayed planting of 10 days regardless of the percentage of plants delayed. However, when planting was delayed 21 days, yields were reduced 10 percent when 25 percent of the plants were delayed, 20 percent when 50 percent were delayed, and 23 percent when 75 percent of the plants were delayed.

Do I Need More Nitrogen this Year Because of Denitrification/Leaching?

Authors: Edwin Lentz, Robert Mullen

Two years ago we published a sidedress decision question sheet in CORN to help producers determine whether or not additional nitrogen application was warranted due to an excessively wet spring. Unfortunately, this spring was a little damp and the same questions have popped up again. If extensive denitrification (clay soils) or leaching (sandy soils) has occurred, sidedressing can make a large economic impact on the operation. Determining how much N was lost and what the sidedress rate should be is complex and difficult. Soil temperature, nitrogen form, length of saturation, and organic matter all play critical roles in determining microbial activity and resultant denitrification. As an aid to help make sidedressing decisions, University of Minnesota scientists have developed a simple question and answer point system. We have adapted that point system to Ohio and propose its use (see below).

1) What N source was utilized?
1 point - Anhydrous ammonia with nitrification inhibitor
2 points - Anhydrous ammonia
3 points - Other fertilizer banded
4 points - Other fertilizer broadcast

2) When was the N applied?
2 points - After April 20
5 points - Before April 20

3) How much N has been applied?
1 point - >200 lbs/A
2 points - 150-200 lbs/A
3 points - 100-150 lbs/A
6 points - <100 lbs/A

4) What has been the predominant soil moisture status in the field this spring?
1 point - Normal
2 points - Wet
4 points - Excessively wet (saturated – standing water)

5) What is crop’s condition?
1 point - Green plants > 12” tall
2 points - Green plants < 12” tall
3 points - Chlorotic plants < 12” tall
5 points - Chlorotic plants > 12” tall

Total the score and use the following guidelines:

Less than 13 - Additional fertilizer not recommended
13-16 - Evaluate again in 4-7 days
17 or greater - Add an additional 40-70 lbs N/A


Some producers may consider the use of the presidedress soil nitrate test (PSNT) to determine if additional N fertilizer is warranted. To attain a representative soil sample, collect 15, 1-ft deep random cores from a field and mix them thoroughly. Submit a grab sample from the composite to a reputable lab (a list of labs is available at the following web address: http://www.ag.ohio-state.edu:8000/%7Ecorn/library/testlabs.pdf). You may want to contact the lab and find out the turn-around time (some may be able to complete analysis in a couple of days). If the nitrate level in the soil is between 25-30 ppm then additional N is probably not warranted. Nitrate levels lower than 25 ppm have an increased likelihood of response, but the rates should not be greater than 70 lbs N/A. Work out of Illinois reveals that application of only 50 lbs N/A results in maximum yield over a wide variety of growing conditions.

Another consideration is what to do about replanted corn or late planted corn? Should I invest in more nitrogen if I applied preplant? If I did not apply preplant, should I consider cutting back nitrogen rates? With regards to replanted corn, you can use the above questions to help you decide whether you should invest in more nitrogen or not. For late planted corn where nitrogen has not been applied, nitrogen rates should be adjusted downward. The adjustment downward is not solely based upon lower yield potential, but warmer soil temperatures and resultant increased nitrogen mineralization as well.

Archive Issue Contributors: 

State Specialists: Ann 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)and Robert Mullen (Soil Fertility). Extension Educators: Howard Siegrist (Licking), Harold Watters (Champaign), Glen Arnold (Putnam), Roger Bender (Shelby), Steve Bartels (Butler), Steve Prochaska (Crawford), Greg LaBarge (Fulton), Todd Mangen (Mercer), Mark Koenig (Sandusky), Jim Skeeles (Lorain), Gary Wilson (Hancock) 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.