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Ohio State University Extension


C.O.R.N. Newsletter 2005-22

Dates Covered: 
July 18, 2005 - July 26, 2005
Harold Watters

Soybean Aphid Update July 18

Authors: Ron Hammond, Bruce Eisley

We are beginning to get reports from numerous locations, especially in northern Ohio, of treatments going on for soybean aphid, most of which have truly reached the threshold of 250 aphids per plant. We want to remind everyone that this threshold is 250 soybean aphids per PLANT, not per leaf. Also, we are beginning to observe aphids occurring down in the canopy over the entire plant, and starting to coat the middle and lower leaves with honey dew. Thus, we want to remind growers to now begin checking the entire plant for aphids, not just the upper leaves. The appropriate sampling technique is to pull 20-30 random plants and count the aphids over the entire plant, and then make your decision. Treating when the threshold is reached is the best way to ensure maximum yield!

Twospotted Spider Mites

Authors: Ron Hammond, Bruce Eisley

With the persistence of hot and dry weather in parts of Ohio, we have continued to see the beginnings of spider mites in numerous fields, including having some fields being sprayed. For example, a visit was made last week to a field in west-central Ohio that was in a dry area of the state. The field had no visible symptoms from the field, except for looking dry. It was very easy to find mites, not only along the field edge, but also in spots throughout the field. Although the rainfall and high humidity the past week should slow spider mite development and allow the soybeans to outgrow some of the mite injury, a return to hot dry weather over the next few weeks would likely cause the mite problem to worsen. Thus, vigilance is still recommended. A refresher article was felt necessary to help growers understand the problem, and how best to deal with it. The following is information that was presented a number of years ago, including a rating system to use to determine the need for spraying.
Field Symptoms: Spider mites feed on the underside of the foliage with sucking mouth parts and may be very destructive when abundant. Under hot and dry field conditions, spider mites thrive on plants that are under stress. Soybean foliage infested with spider mites initially exhibits a yellowish speckled or stippled appearance. As plants become heavily infested, foliage turns yellow, then bronze, and finally the leaves drop off the plants as the effect of heavy feeding leads to dehydration and death of plant.
During the 1988 spider mite outbreak on soybeans, a scheme was developed for evaluating infested fields base on observable symptoms and conditions as follows:
1. Mites barely detected at field perimeter or other dry locations within the field. Multiple plants need to be inspected before mites are found. Assessment: Clearly non-economic and no action is warranted.
2. Presence of mites easy to detect at field perimeter; however, difficult to detect within field. Foliage is still green but stippled feeding injury with few mites under lower sides of leaves detectable, but not on every plant. Assessment: Clearly non-economic, but warrants monitoring.
3. Many plants in the field exhibit some sign of infestation with stippling and some discoloration of lower leaves. Foliage exhibiting various levels of stippled feeding injury on relatively healthy foliage. Field perimeter and dry spots exhibit severely damaged plants. Assessment: Rescue treatment warranted at this point, especially if immature stages are in abundance or heavy egg laying present.
4. Infestation widespread with discolored and wilting foliage easily detected throughout field. Most plants heavily infested when examined closely. Severe damage evident. Assessment: Effective rescue treatment will save field.
5. Total field discoloration and drying down of foliage. Significant foliage and stand loss is evident. Assessment: Field may be beyond point of recovery if rescue treatment applied. However, new growth may resume if treated.
In making an assessment of a spider mite infested field, it is important that one recognize the early signs of mite feeding, which is the stippling or speckled effect that initially appears on the foliage when foliage is still green ( and . In addition, it is essential that one use a good hand lens to view relative abundance of mites in egg, nymph and adult stages. It is important to note that one field may exhibit a severe spider mite infestation while fields nearby may exhibit minimal or no spider mite activity.

Western Corn Rootworm Adults in Corn

Authors: Bruce Eisley, Ron Hammond

There are reports of feeding on leaves in corn fields with large numbers of western corn rootworm beetles and silks not yet out. The beetles scrape off the outer layer of the leaf resulting in a leaf that looks as if it has been burned. We normally are not concerned about this leaf feeding but will be concerned in these fields when the silks begin to emerge because the beetles will move to the silks and begin feeding. Japanese beetles can also be a problem with silk clipping, especially around the edges of the fields. Rescue treatment for rootworm beetle silk clipping is warranted if 5 or more beetles are found per silk mass when 75% of the plants have silked and silk clipping to 1/4 inch or less is observed. Rescue treatment for Japanese beetle silk clipping may be warranted if there are 3 or more beetles per silk mass and pollination has not occurred. The important thing to check for in corn at this time is whether or not the corn has pollinated. If pollination has occurred, then silk clipping by rootworm or Japanese beetles should not affect yield.
Because of uneven silking, there may be only a small portion of the field that has fresh silks at any one time. In these cases, the beetles will congregate on those emerged silks and populations per plant may be larger than if the whole field had silked. As more plants silk, the population may become dispersed throughout the field and reduce the beetle numbers per plant and thus the need for treatment.
Chemicals that are labeled for silk-feeding beetles include:
Asana XL*
Capture 2 EC*
Mustang MAX*
Warrior *
* Use is restricted to certified applicators only.

Assessing the Impact of Hot, Dry Weather on Pollination in Corn

Authors: Peter Thomison

Although many corn fields in Ohio benefited from much needed rain over the weekend, other areas received little or no rain. Reports of short, waist high corn tasselling, as well as uneven flowering within fields, are not uncommon in parts of the state which have received negligible rain since early June. Many corn growers want to know what impact drought stress has had on corn pollination, the stage in corn development most sensitive to such stress conditions. When severe drought stress occurs before and during pollination, a delay in silk emergence can occur. Sometimes the length of this delay is such that little or no pollen is available for fertilization when the silks finally appear. When such delays in silking are lengthy, varying degrees of barrenness will result. This year it's very likely that silk emergence will be delayed in many drought-stressed corn fields unless we get some significant rain very soon.

There are two techniques commonly used to assess the success or failure of pollination. One involves simply waiting until the developing ovules (kernels) appear as watery blisters (the "blister" stage of kernel development). This usually occurs about 1 1/2 weeks after fertilization of the ovules. However, there is a more rapid means to determine pollination success.

Each potential kernel on the ear has a silk attached to it. Once a pollen grain "lands" on an individual silk, it quickly germinates and produces a pollen tube that grows the length of the silk to fertilize the ovule in 12 to 28 hours. Within 1 to 3 days after a silk is pollinated and fertilization of the ovule is successful, the silk will detach from the developing kernel. Unfertilized ovules will still have attached silks.

Silks turn brown and dry up after the fertilization process occurs. By carefully unwrapping the husk leaves from an ear and then gently shaking the ear, the silks from the fertilized ovules will readily drop off. Keep in mind that silks can remain receptive to pollen up to 10 days after emergence. The proportion of fertilized ovules (future kernels) on an ear can be deduced by the proportion of silks dropping off the ear. Sampling several ears at random throughout a field will provide an indication of the progress of pollination.

Unusually long silks that are still "fresh" are a symptom that pollination has not been successful. Unpollinated silks continue to elongate for about 10 days after they emerge from the ear husks before they finally deteriorate rapidly. During this period, silks become less receptive to pollen germination as they age and the rate of kernel set success decreases. If you observe unusually long silks in drought stressed field it may be an indication of pollination failure.

Dr. Bob Nielsen, the corn extension specialist at Purdue University, wrote a good article, ("Some Droughty Fields Experiencing Silk Emergence Delay"), addressing this topic in 2002, which includes images showing this problem; it’s available online at

Potassium Deficient Corn and Soybeans Due to Dry Soils

Authors: Robert Mullen, Peter Thomison

Several locations across the state have crops that may be showing signs of potassium deficiency (lower leaves are becoming chlorotic along the leaf margins which progresses along the main vein toward the leaf base). So the question is, what is causing the deficiency? Depending upon your specific situation it may be due to low soil test potassium (K) or dry weather conditions that are inhibiting the plants ability to take up nutrients. Limited, shallow root development due to cool, wet early season conditions and low soil pH are likely contributing factors. Extremely dry soil conditions typically result in lower soil pH levels. For a soil with an initial pH level that is marginal, soil pH levels can get low enough to inhibit root development and decrease nutrient uptake (especially potassium and magnesium). Compacted soils can also induce potassium deficiency (which can explain tire track type symptoms).

Potassium deficiencies are especially detrimental under drought conditions. The primary role of potassium in the plant is to maintain osmotic potential (ensure that the roots are taking up water from the soil). Poor potassium nutrition can result in poor water use efficiency (not good when water is scarce to begin with). This is why it is important to collect soil samples and monitor soil potassium status over time.

Can we overcome potassium deficiency with foliar applications? There is some research that suggests application of foliar K (as potassium sulfate) can result in significant yield responses (upwards of 6 bu/acre) for soybeans especially under dry conditions (I think we qualify for that one). Other K formulations may not be as successful (that work is currently on-going), and the benefits for corn are unknown. At this point in the season, for corn specifically, little can be done to address potassium deficient crops. By silking, the corn plant has taken up all the potassium from the soil it is going to use. The one thing to be considered is the potential for leaf burning. The rates reported in the research with soybeans were around 30 lb K/acre. Based on the carrier volume of water necessary to ensure that potassium sulfate does not burn the application is not likely to be economical.

Pay close attention to this year’s soil test information. Potassium is much more efficient for both soybeans and corn when applied to the soil.

Manganese Deficient Soybeans

Authors: Robert Mullen

Many counties across the state are reporting what appear to be manganese (Mn) deficient soybeans. The reason for such rampant Mn deficiency this year is primarily due to dry conditions. Manganese deficiency typically appears on high pH soils and muck soils, but excessively dry soil conditions can contribute to the deficiency as well (this year qualifies). The symptoms of the deficiency are interveinal chlorosis (yellowing) of the new leaves.

So what can be done to correct the deficiency? Foliar application of various inorganic manganese compounds can be made to address the problem, but multiple applications of these materials may be necessary to overcome the deficiency. This is obviously not encouraging considering the cost of application. Manganese sulfate (26-28% Mn), manganese chloride (41-68% Mn), or manganese chloride (17% Mn) can be foliarly applied at rates between 1 to 4 pounds of Mn/acre to correct the deficiency. Manganese chelates can also be applied, but the rates of application should be significantly lower (around 0.15 lb of Mn/acre). Application of higher rates can result in phytotoxicity (plant injury).

If we could just get some rain, manganese deficiencies should be lessened.

Late Summer Seeding of Forage Crops

Authors: Mark Sulc

Late summer can be an excellent time to establish forage crops, provided there is sufficient moisture for germination and good seedling growth. It is also a good time to seed in bare or thin spots in forage stands established this spring. The following steps will improve the chances for successful forage stand establishment in late summer.

1. Apply lime and fertilizer according to soil test and control problem perennial weeds ahead of seeding. Be careful with herbicide selection because some have residual soil activity and will harm new forage seedings in proper waiting periods are not observed. Read the labels for details.

2. Prepare a firm seedbed if using tillage. Loose seedbeds dry out very quickly. Deep tillage is not ideal for late summer seedings, but if done should be completed several weeks ahead of seeding so rains can settle the soil before final seedbed preparation. A cultipacker or cultimulcher is an excellent last-pass tillage tool. The soil should be firm enough for a footprint to sink no deeper than 3/8 to 0.5 inch.

3. No-till seedings conserve moisture, and can be very successful provided weeds are controlled prior to seeding. Remove all straw after small grains. Any remaining stubble should either be left standing, or clipped and removed. Do not leave clipped stubble in fields as it forms a dense mat that prevents good emergence.
CAUTION: No-till or reduced-till summer seedings of legumes are at risk of infection by Sclerotinia crown and stem rot, especially in fields where clover or alfalfa were present recently. Mid- to late-August plantings dramatically increase the risk of damage by this disease compared with planting earlier.

4. Don’t plant alfalfa immediately after older established alfalfa. Autotoxic compounds are released by old alfalfa plants, which inhibit growth and productivity of new alfalfa seedlings. It is best to rotate to another crop for a year before going back to alfalfa; however, thickening up spring seedings is fine because autotoxicity is only an issue with older, established alfalfa plants.

5. Seed when soil moisture is adequate or a good rain system is in the forecast. This is especially critical this summer after the dry weather we’ve experienced. It is risky to place seeds into dry soil, because there may be just enough moisture to germinate the seed but not enough for establishment.

6. Seed as early as possible. Seedlings require 6 to 8 weeks of growth after emergence to have adequate vigor to survive the winter. Seed by August 15-20 in northern Ohio and by September 1 in southern Ohio. Slow establishing species like birdsfoot trefoil or reed canarygrass should be planted in early August. Fast establishing species like red clover, alfalfa, and orchardgrass can be seeded up to the dates listed above if moisture is present. Kentucky bluegrass and timothy can actually be seeded 15 days or more later than the dates listed above. Keep in mind that the above dates assume sufficient moisture to establish the crop. Planting later than the dates mentioned above is sometimes successful depending on fall and winter weather patterns, but there is increased risk of failure and reduced yield potential for the stand as planting is delayed. A good rule of thumb for alfalfa is to have 6-8” of growth before a killing frost.

7. Plant seed shallow and in firm contact with the soil. Carefully check seeding depth, especially when no-tilling. Drills with press wheels usually provide the greatest success in the summer. Broadcasting seed on the surface without good soil coverage and without firm packing is usually a recipe for failure in the summer.

8. Use high quality seed of known varieties. Cheap seed often results in lower yield and shorter stand life. Make sure legume seed has fresh inoculum of the proper rhizobium.

9. Do not harvest new summer seedings this fall. The only exception to this rule is perennial ryegrass. If perennial ryegrass has tillered and has more than six inches of growth in late fall, clip it back to 3 to 4 inches before snowfall.

10. Scout new seedings for winter annual weeds in October to November, and apply herbicides as needed. Winter annual weeds are much easier to control in late fall than in the spring.

Time to Set Traps for Western Corn Rootworm Activity in Soybeans

Authors: Curtis Young

The variant of the western corn rootworm (WCR) that lay their eggs in soybeans, also known as the first year corn rootworm, continue to be a threat to corn production in Ohio. Trap results from 2004 continued to show increases in the number of western corn rootworm beetles captured in a number of fields sampled. One method to sample for WCR in soybeans is by using Pherocon? AM unbaited yellow sticky traps. The traps are attached to fence posts slightly above the soybean canopy (6-8”).

Traps should be placed in the soybean fields this week. Place 6 Pherocon? AM unbaited sticky traps at least 100 feet from the field edge and evenly distributed in the field. Also be careful not to get too close to an adjacent corn field. Maintain at least a 100 foot distance or more into the soybean field from any edge of the field. Twist tie wires are included with the traps, however I find it more convenient to use cable zip ties. The zip ties are thinner and faster to connect the traps to the posts. The traps should be changed each week for six weeks and a new trap put in its place just above the soybean canopy.

After changing the traps, count and record the number of rootworm beetles on each trap and the number of the days the traps were in the field. The number of beetles/trap/day is determined by adding the number of beetles found on the six traps and dividing that number by 6 and then dividing by the number of days the traps were in the field. Traps should be changed every 7 days if possible but if not be sure to record the exact number of days the traps were in the field.

Research from Purdue indicates that Pherocon® AM trap catches in soybean of 5 or more beetles/trap/day during any trapping week indicates a potential problem with rootworm in the field the following year. The management in the field the following year can be to rotate to some crop other than corn, to use a soil insecticide or plant one of the new Bt transgenic rootworm hybrids.

Soybean Rust Update – July 18th

Authors: Anne Dorrance

No soybean rust has been identified in Ohio. In addition, the closest fields with active lesions are in Baldwin County, ALABAMA, where ONE field was found. In this field in Alabama, there are a few pustules here and a few pustules there. Very low inoculum. In addition, fields in Kentucky where rust-like spores were found in a trap were scouted (Friday, July 15th) and no rust was found. This is great news. The next question will be to see if Dennis brought us any spores. Inoculum levels are very low, a few kudzu plots in Florida and the sentinel plot and one commercial field in Alabama. Any inoculum that was picked up from Hurricane Dennis would be very dilute. The next step will be to monitor sentinel plots – twice a week during the last week of July and the first two weeks of August. If no plots are positive in Ohio and Kentucky, for our normal season beans we could be out of the woods. If a few sentinel plots are positive scattered around where Dennis hit, this will indicate localized areas – scouting fields and making decisions for each field based on growth stages will be the next phase. Many of the triazoles can not be applied after R5 or R6 due to preharvest intervals. Also yield losses at these later growth stages are minimal.

Fungicide shelf life? Interesting questions this week. During the winter at all of the fungicide talks and rust meetings, all companies repeatedly stated that these materials have long (2 to 3 year or more) shelf life. However, they will not withstand repeated freezes. If you have made purchase, you will be able to hold this over. More information on proper storage conditions is in development.

Did Dennis bring more spores than what Ivan did? The general consensus among pathologists is - probably not. Dennis did not overlap much of South America and there are no reports of widespread epidemics in northern South America. The other big difference between Ivan and Dennis is that Ivan sat in the Caribbean for several days – picking up spores for what is believed to be Columbia. The U.S. had identified a widespread outbreak of soybean rust in a soybean region in Columbia. Reports from USDA colleagues indicate that this is negative this year. Remember with Ivan it took 6 weeks to identify soybean rust in the U.S. and those fields would not need spraying. Dennis hit Ohio – rather wimpley this past weekend – 6 weeks from now – we will be working on the combines.

Northwest Agriculture Research Station Field Crops Day

Authors: Dusty Sonnenberg

On Thursday, July 28th the OARDC Northwest Agricultural Research Station near Hoytville will host its annual Field Crops Day from 6:00 – 9:00 p.m. Featured speakers and topics for the evening include: Dr. Anne Dorrance speaking on fungicide basics for soybean rust: detection, timing and management; Dr. Mark Loux speaking on evolving weed problems and their management; Dr. Robert Mullen speaking on understanding lime recommendations and nitrogen management; and Mr. Alan Sundermeier speaking on nitrogen and tillage interaction on corn production.

OARDC Northwest Agriculture Research Station is located at 4240 Range Line Road, Custar, OH which is one mile east of State Route 235 between Hammansburg and Oil Center Roads in Wood County.

Nitrogen Management for Corn Production Field Day

Authors: Harold Watters

With fertilizer cost and environmental management issues becoming a priority in corn production, we have planned a field day to address corn fertilizer management. State Fertility Specialist, Robert Mullen is an excellent resource and will discuss high fertilizer costs, predicting N use in corn, remote sensing and economics in corn nitrogen management.

The field day is scheduled for August 1st
9AM registration and check in
9:30 program start
- Dr. Robert Mullen, OSU Extension Soil Fertility Specialist, will discuss Nitrogen management in a new era.
- Nitrogen Reduction program on the Mad
- Water Quality in the Mad River and the Great Miami River overview
12 Noon – lunch served

Jerry Bailey, our host, is a participant in the Mad River Nitrogen Reduction program. We will be able to look at the reduced N rate corn plot at the site. We will also discuss the past history of the program. The program will cost $5, payable at the field. The field site is located on SR 296, just west of US 68 and north of Urbana.

Please call the Champaign County OSU Extension office to register - 937 484-1526. If you have any questions about the program you may contact Harold Watters ( or Jennifer Ganson ( at the Champaign County Extension office.

Sponsored by the Mad River Steering Committee & OSU Extension Champaign County.

Archive Issue Contributors: 

Anne Dorrance and Pierce Paul (Plant Pathology), Peter Thomison (Corn Production), Mark Loux (Weed Science), Mark Sulc (Forages), Robert Mullen (Soil Fertility) and Ron Hammond & Bruce Eisley (Entomology). Extension Agents: Steve Foster (Darke), Roger Bender (Shelby), Greg La Barge (Fulton), Dusty Sonnenberg (Henry), Keith Diedrick (Wayne), Gary Wilson (Hancock), Jim Skeeles (Lorain), Curtis Young (Allen), Harold Watters (Champaign), Glen Arnold (Putnam), Steve Bartels (Butler), Bruce Clevenger (Defiance) and Steve Prochaska (Crawford).

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.