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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2007-14

Dates Covered: 
May 21, 2007 - May 29, 2007
Keith Diedrick

Purple and Yellow Corn, What is Going On?

Authors: Robert Mullen, Peter Thomison

Corn seedlings often turn yellow (due to low nitrogen uptake and/or limited chlorophyll synthesis) or purple (reduced root development) under cool, wet conditions. Some hybrids are more likely to increase anthocyanin (purple pigment) content when plants are cool. Yellowing or purpling of corn plants at this stage of development generally has little or no effect on later crop performance or yield potential. If it's induced by environmental conditions, the yellow or purple appearance should change to a healthy green after a few sunny days with temperatures above 70 degrees F. If plants remain yellow then closer inspection and assessment is needed to determine if yellowing is caused by nutrient deficiency or some other factor.

Environmental conditions (high rainfall causing saturated soils) can lead to the appearance of yellow corn. The visual appearance may be interpreted as N deficiency, but this is rarely the case. Excessive water leads to poor respiration of the roots inhibiting nutrient uptake. This results in a chlorotic appearance which resembles N deficiency. After soils dry out, the appearance returns back to normal. If the chlorotic condition persists after the soil dries, the problem should be investigated further. This short-term condition should not affect yield potential of the crop.

When you combine recent cool nighttime temperatures, high radiation levels during the day, and wet field conditions, you are likely to start seeing purple plants in some corn fields. The first thing that may come to mind is a phosphorus deficient soil. This is unlikely the case, especially this early in the year. The purple tint is more attributable to the production of anthocyanins which is a plant response to a stress or a combination of stresses. Cool temperatures, high solar intensity, and water stress (drought and water-logged conditions) combine to inhibit root growth. Other factors including soil compaction, herbicide injury, etc. can make the effect even more pronounced. Purple corn can also be the result of what is known as the “fallow syndrome.” If corn follows a fallow season, a root fungus called mycorrhizae reaches a low population. Mycorrhizal infection of corn aids in phosphorus and zinc uptake. Until the fungal growth is stimulated by the corn roots, which exude starches and sugars, the purple color may persist. Fortunately, the purple tint is short-lived and rarely persists beyond the V6 growth stage. It should not have an impact on the yield potential of the field.

"Patching In" Poor Corn Stands

Authors: Peter Thomison

Various emergence problems including poor drainage and soil crusting have reduced stands in some Ohio corn fields. In affected fields, the remaining plants are often unevenly spaced within rows and not developing uniformly. The following are some guidelines to consider in these situations based on findings of Illinois and Wisconsin research. A good source for more information on this subject is the National Corn Handbook Chapter 36, "Effects of Uneven Seedling Emergence in Corn" which is available on-line at

Growers will sometimes attempt to plant over or "patch in" a poor stand rather than kill the existing plants and replant at a full population. However, "patching in" is generally of limited benefit unless the surviving plant population is less than one half that of the original. The success of such an approach is even less likely late in the planting season (i.e. after June 1). Later planted corn cannot compete effectively with the remnants of the original plant population for sunlight, water, and nutrients. In these late planting situations, late emerging plants often function more like weeds, and contribute little to grain yield.

· If you replant within 2 weeks of planting the original, patching-in may be a viable option. Yields will be similar to those from a uniform-emerging replanted stand, if you can get relatively uniform plant spacing within the row between the old and new plants. However, within 2 weeks of planting, it probably will be too early to determine what the final stand will be (and whether patching will be needed).

· If you replant within 3 weeks after the initial planting, yield potential is about 10% greater if you tear up the field and start over with an even emerging stand rather than just patch-in the original stand. Balance this possible yield increase against the additional cost of tillage, seed, and dryer fuel.

· If the delay in emergence is less than 2 weeks, replanting will have a minimal effect on yields, regardless of the pattern of unevenness.
· If one-half or more of the plants in the stand emerge 3 weeks late or later, then replanting may increase yields by up to 10%. To decide whether to replant in this situation, estimate both the expected economic return of the increased yield compared to your replanting costs and the risk of emergence problems with the replanted stand. For more information on replanting corn, check last week’s CORN newsletter at:

· If the delayed plants emerge only 1 1/2 to 2 weeks late, use shields and avoid burying the late-emergers during cultivation.
· Protect plants emerging 3 weeks late if one half or more of the plants in the stand is late-emergers.
· If less than 1/4 of the stand emerges 3 weeks late or later, it probably will not pay to encourage their survival. Yields will be about the same whether or not these delayed plants are buried during cultivation.

Foliar Fungicide Use in Corn

Authors: Pierce Paul, Dennis Mills

Over the past few years there have been lots of discussions about the plant-health benefit of fungicide application in corn, and this year is no different. Research at OSU shows that the use of resistant hybrids is still the most effective, economical, and widely recommended control measure for foliar diseases (such as gray leaf spot and northern corn leaf blight) of corn. When a susceptible hybrid is planted under conditions known to favor the development of these diseases, an early application of a fungicide may be warranted. Both the gray leaf spot and northern corn leaf blight fungi survive in corn residue left on the soil surface from the previous growing season. If a susceptible hybrid is planted in a reduced-tillage, continuous corn field and conditions are favorable for disease development (extended periods of high relative humidity and moderate to high temperatures), extensive blighting of leaves may occur prior to grain fill, leading to substantial yield reduction.

To achieve the desired effect, fungicides should be applied at the proper time. The goal of fungicide application is to prevent disease from spreading to the ear leaf and the leaves above the ear, since these leaves contribute a significant portion (about 75%) of the carbohydrates for grain fill. If disease is present on the leaves below the ear leaf, applications should begin prior to tasseling. Fields should be scouted regularly to determine the appropriate time for fungicide application. A highly-susceptible hybrid combined with extremely favorable disease conditions may require two applications to reduce the level of disease; however, more than one fungicide application is generally not economical. Yield increase is often not enough to offset the cost of two applications. In addition, fungicide trials from 2006 showed that there was no economic yield benefit to applying fungicide in the absence of foliar disease.

The following should be taken into consideration when making decisions regarding fungicide application:
1- Susceptibility and yield potential of the hybrid
2- The amount of disease in the field
3- The growth stage of the crop
4- Weather conditions
5- Fungicide and application cost
6- Grain price
7- Directions and restrictions on product label

Pokeweed Control - It’s Not Easy

Authors: Mark Loux

As we approach postemergence herbicide application season, the questions about pokeweed have already started rolling in. We have heard about pokeweed problems with increasing frequency over the past several years, in spite of the fact that glyphosate should be more effective on this weed than most other postemergence herbicides. While some perennial species have become less of a problem due to the use of glyphosate in Roundup Ready crops, reducing pokeweed populations has proven to be more difficult. Pokeweed is inherently difficult to control due to its perennial life cycle and ability to regrow from a well-established root. Successful control during the growing season depends upon use of the appropriate herbicide(s), herbicide rate and timing, and spray boom height. Pokeweed can be fairly large at the time of postemergence herbicide application, and the spray boom should be high enough to ensure coverage of the entire plant with spray.

The most consistently effective strategies for controlling pokeweed are to either cut the root off at least 6 inches below the soil surface, or apply herbicide directly to a freshly cut pokeweed stump. For the latter, cut the pokeweed stems off with a pruner or a sharp corn knife and apply concentrated (undiluted) herbicide to the cut surface until it begins to run off. Glyphosate may be the most appropriate herbicide when using this method in a crop, but other non-crop herbicides are also effective in situations where they can legally be used. This method may not kill every plant, but can be considerable more effective than a single postemergence application. Studies conducted by several OSU county Extension educators in 2006 showed both methods could be effective for control during the growing season. At one site where regrowth this spring was evaluated, both methods prevented regrowth of treated plants. However, some growers have indicated that control into the following season from the cut stump method may be less than complete.

Most effective postemergence control of pokeweed in non-GMO corn results from a combination of the full rate of an ALS-inhibiting herbicide (Spirit, Beacon, Exceed, Permit, or Equip) with Distinct at 4 oz/A or dicamba at 6 to 8 oz/A. Apply when pokeweed plants are at least 8 inches tall, but before 24 inches tall. A second application or row cultivation with wide sweeps may be necessary for complete control. Other herbicides with significant activity include NorthStar, Yukon, Callisto, dicamba (1 pt/A), and Distinct.

In Roundup Ready II corn, apply glyphosate at 1.1 lb ae/A when pokeweed are in the bud to flower stage for a single application. Where two postemergence glyphosate treatments are planned, apply 1.1 lb ae/A when the pokeweed plants are 12 to 24 inches tall and make a second application after 12 inches of regrowth. The same recommendation should be effective in Roundup Ready soybeans. However, if using the single application strategy in soybean, the 1.5 lb ae/A rate will provide more effective control. The effectiveness of pokeweed control with glyphosate is tied very closely to rate and timing, so multiple applications of at least 1.1 lb ae/A will typically provide more effective control than a single application.

In non-GMO and non-STS soybeans, the best control option is Synchrony XP at 0.38 oz/A plus Harmony GT at 1/24 oz/A plus MSO at 1.0 %v/v. This can cause significant soybean injury but will provide greater activity than Synchrony XP alone. In STS soybeans, use Synchrony XP at 0.75 oz/A with MSO, and consider adding additional Harmony GT to improve control.

Nutrient Value of Wheat Straw

Authors: Robert Mullen, Edwin Lentz

Due to the decreased wheat acreage across Ohio this year, some producers may be approached about baling straw. This often leads to the question ­ what is the nutrient value of the straw being removed and should removal lead to increased fertilizer applications in subsequent years?

From a pure fertilizer value, wheat straw contains very little in the way of phosphorus (P2O5) but moderate amounts of nitrogen (N) and potassium (K2O). The actual amounts of N, P2O5, and K2O contained in a ton of wheat straw are 11, 3, and 15 pounds, respectively. A sixty bushel per acre wheat crop will produce 2.7 tons of straw per acre removing 30 pounds of N, 9 pounds of P2O5, and 41 pounds of K2O. Thus straw does have some fertilizer value especially with regard to potassium and may require some additional fertilizer input in subsequent years, but soil testing should be conducted to validate the need for additional nutrients.

Wheat straw residue also contains organic matter that when returned to the soil does have value, but it is difficult to put a dollar value on it. Continued removal of the above ground biomass may have negative repercussions in the long-run in the form of decreased organic matter, especially if some organic residue is not returned to the soil.

Barley Yellow Dwarf (BYDV) Virus in Wheat

Authors: Dennis Mills, Pierce Paul

High incidences of barley yellow dwarf virus (BYDV) are being reported from some fields in southern and south central Ohio. Some of these fields reportedly have as much as 20% of the plants showing symptoms typical of this disease: erect leaves with yellowish to reddish-purple tips. These symptoms may also be confused with nutrient deficiency, cold temperature injury, root rots, or herbicide injury. Though characteristic of BYDV, leaf discoloration may be absent in some cases, with infected plants showing reduced growth and normal-looking leaves. Severe stunting and extensive leaf discoloration are more common when infections occur early in the fall, whereas discolored leaf tips are more typical of a spring infection.

This disease is caused by several related viruses transmitted by more than 20 different species of aphids. BYDV tends to be most severe in fields planted before the fly-free date when aphid populations can be high, but some of the fields with high levels of BYDV were planted well after the fly-free date this past autumn, due to warm temperatures. Once infections occur, there is very little that can be done; no fungicide is effective, and though insecticides applied after infection will reduce the aphid population, it will not prevent the disease from developing. The residual effect of the insecticide may not last long enough to protect against aphid population buildup. During active feeding, a few aphids will be enough to transmit the virus from one plant to another.

Yield reduction due to BYDV is generally greater when infections occur in the fall than in the spring. However, spring infections may still lead to yield reduction with smaller heads and kernels and fewer spikelets per head. It is difficult to estimate the level of damage caused by BYDV at this time. Since grain fill is not yet complete, there is no way of knowing whether kernels on sick plants will be fewer and smaller in size. For more on BYDV, visit the field crops disease website at and for information on estimating yield reduction due to BYDV refer to an article by Shaner and Conley

The Risk of Head Scab is Low

Authors: Pierce Paul, Dennis Mills

The wheat crop is beginning to flower in some fields and flowering is expected to continue over the next few weeks. Head scab develops best when warm and wet weather conditions coincide with flowering. However, in spite of the rains in some parts of the state during last week, the scab forecasting model indicates that the risk of scab is low throughout the state. This is mostly because conditions were very dry during the first half of the month and relatively cold temperatures occurred over the past 7 to 10 days. Once moisture is available, spores will be produced under a very wide range of temperatures (from 55 to 78 F); however, infection of the wheat heads by the fungus requires warmer temperatures. The weather forecast indicates warmer conditions near the end of the week, and if rain or high relative humidity occurs as the crop continues to flower, the risk of scab will increase. Now is the time to watch the scab prediction center at for frequent updates. As the weather changes, the scab prediction center will be updated regularly with comments specific for Ohio.


Authors: Bruce Eisley, Ron Hammond

Armyworm moths have been collected in larger numbers in states to our south. These numbers mean that Ohio growers will need to monitor wheat, adjacent corn fields, along with corn being grown with rye cover crops, during the last week in May and into early June for possible economic larval populations. When larval numbers reach accepted thresholds (see below), fields should be treated. When scouting wheat, we recommend checking several places in the field to determine the number and size of the worms present. Rescue treatments are recommended if worm counts average 6 or more per linear foot of row and if larva are in the early stages of development or if head-cutting is occurring. If wheat fields need treatment, growers should be aware of the various pre-harvest intervals for the labeled insecticides, ranging from 7 to 30 days. When armyworm larvae move from wheat to corn, growers should be reminded that the infestation usually occurs along the edge of the corn field nearest the wheat.

For corn planted into a grass cover crop, foliar feeding injury on 15 to 20% of the stand is regarded as an indicator of a potential problem, and that the field should be re-checked within a few days to determine if the impact of defoliation is increasing and if a rescue treatment is warranted. In general, a severe infestation impacts almost 100% of a stand and defoliation of the plants will exceed 50%. Stand height will be reduced, and some plants will be eaten down to ground level. Rescue treatments in corn may be needed if stand infestation is greater than 50% and larvae are not yet mature. If defoliation remains less than 50% and the new growth shows minimal feeding injury, the stand will likely recover with minimal impact on yield.

Because treatment of armyworm larvae nearing maturity will achieve limited results because the period of maximum feeding will have passed, we recommend that growers scout for larvae earlier rather than later. A list of labeled insecticides for armyworm on both crops is available at During the day, armyworm larvae will most likely be found seeking shelter in the ground cover or possibly in corn whorls.

The Sentinel Plot Report

Authors: Anne Dorrance

Last week, 7 of 11 locations reported the presence of bean leaf beetle; two of these locations required treatment. Scouting will continue this week to detect the presence of bean leaf beetle in these plots. Please see Ron Hammond and Bruce Eisley’s recommendations from last week’s C.O.R.N. newsletter:

Nationwide, drought conditions continue to delay any soybean rust development in the southeast. As of this morning, there have been no additional finds in Louisiana. Inoculum levels remain low in the southern US and the overall risk of soybean rust for Ohio is LOW.

Other Insect Concerns on Field Crops

Authors: Bruce Eisley, Ron Hammond

Over the past week we have begun receiving reports of other insect pests that can cause concern to growers on field crops. These early season pests include black cutworm on corn, bean leaf beetle on soybeans, slugs on both of these crops, and cereal leaf beetle on wheat. This is the time when growers should be scouting all their crop fields to identify potential economic problems beginning to occur, and to take curative action if needed. Refer to for threshold and treatment recommendations.

Archive Issue Contributors: 

State Specialists: Anne Dorrance, Pierce Paul, and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Robert Mullen and Ed Lentz (Soil Fertility), 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), Todd Mangen (Mercer), Mike Gastier (Huron), Steve Bartels (Butler), Wes Haun (Logan), Bruce Clevenger (Defiance), Mark Koenig (Sandusky), Harold Watters (Champaign), Greg LaBarge (Fulton), Steve Foster (Darke), 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.