C.O.R.N. Newsletter 2005-15

Dates Covered: 
May 31, 2005 - June 7, 2005
Editor: 
Tammy Dobbels

Wheat Head Scab Risk Prediction For Wheat

Authors: Patrick Lipps, Dennis Mills, Pierce Paul

Head scab occurs in wheat when above normal moisture and temperature occurs as the wheat crop flowers. Wheat has been flowering in southern Ohio over the past week and it is currently flowering in central Ohio, but wheat fields in the more northern counties of the state are still at the heading growth stage. Therefore, it appears that there will be a range of at least two weeks in heading and flowering from the more southerly fields to those in the northern parts of Ohio. Weather has also been quite variable in the state. In many places it has been dry, whereas rain has occurred every other day in other regions of the state. Regardless of the weather or location, the Fusrium Head Blight Risk Prediction Center (http://www.wheatscab.psu.edu/) has indicated that the risk of head scab throughout the state has been very low for wheat planted after soybeans. The reasons for this low prediction is likely due to low numbers of hours of relative humidity above 90% and cool night time temperatures. The risk of head scab is much higher for wheat that was planted into corn residue (at least 10% soil surface area covered by residue) due to the higher amount of Fusarium spores available to infect the wheat in these fields. Areas of the state with high risk of scab in wheat planted after corn are west central and central Ohio.

The difference in scab risk between wheat fields planted after soybeans (low risk) and wheat fields planted into corn residue (moderate to high risk) is quite great this year. It appears that under similar environmental conditions the risk of disease is proportionally greater when there are lots of spores in the field originating from corn residues than when the spores must come into the field from distant sources. This is one of the major reason why we recommend planting wheat after soybeans and not after corn.

Wheat in northern Ohio will be heading and flowering during this next week. Temperatures are predicted to increased as the week progresses, but the forecast for rain remains low into the weekend. Therefore, weather conditions will likely remain unfavorable for scab infection during the flowering period of the crop in the north.

Flowering is the time period when the wheat crop is most susceptible to infection by Fusarium, but under favorable weather conditions the fungus can infect wheat after this time period as well. Any weather change to more warm, humid conditions with lengthy periods of precipitation will impact the overall level of scab in the crop. Flowering is the first possible time predictions can be made, so continue to monitor weather conditions to evaluate the risk of scab. Growers can usually begin to see the symptoms of scab in 14 to 18 days after infection of the heads. Therefore growers in southern Ohio can start to look for affected wheat heads with dead and bleached out florets on heads in about two weeks. Hopefully, the low risk prediction will be accurate and scab will not be important this year.

Staging Early Season Growth In 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 will be particularly important this year given the slow, uneven growth of corn this year due to cool wet conditions.

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 corn field, plants may be taller in those areas characterized by darker soil (with higher organic matter) than in areas with lighter soil, especially 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. The Purdue research indicates that corn leaf developmental rates can 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.

A Few Things to Think About With Regard to Postemergence Herbicide Applications in Corn and Soybeans

Authors: Mark Loux

- Last week we advised delaying postemergence applications if possible due to cold weather and somewhat dubious crop health in some fields . This week’s weather should be more conducive to postemergence activity on weeds, and the warm weather should promote more vigorous crop growth.
- Some parts of Ohio are experiencing dry soil conditions, and we have received questions about postemergence herbicide effectiveness in areas that are dry. Past experience indicates that the best way to minimize the effects of dry weather on herbicide effectiveness is to apply when weeds are relatively small. Crop health may be somewhat reduced under dry conditions, but failure to control weeds when they are fairly small can result in more severe competition with struggling crop and ultimately large weeds that are more difficult to control.
- In recent C.O.R.N. articles, we suggested that corn replanted in mid to late May would grow more rapidly than early-planted corn, resulting in more rapid crop canopy development. However, corn is still growing slowly in many parts of the state, and postemergence treatments applied to corn at this time may also need to have sufficient residual activity to control weeds for several more weeks. Postemergence corn herbicides that provide substantial residual activity on broadleaf weeds include: atrazine, Hornet, Callisto, Exceed, Lightning, Permit, and Yukon. These herbicides will be most effective for residual control of broadleaf weeds, although Lightning and atrazine provide some residual control of grasses.
- In Roundup Ready soybeans, the timing of postemergence glyphosate applications should be based on weed size, not the size of the soybeans. Glyphosate should be applied before the weeds exceed 4 to 8 inches in height. A second application can be made several weeks after the first if necessary to control later-emerging weeds. Producers who wait until just before the soybeans canopy to spray glyphosate risk loss of soybean yield if this results in too long a period of competition between the weeds and crop.

Applying Nitrogen to Young Corn

Authors: Robert Mullen, Peter Thomison

For producers that may have missed their weed and feed application prior to emergence due to wet soil conditions, the question arises – can I make my weed and feed application after the corn is up? While we are not completely comfortable recommending foliar nitrogen (N) application, the earlier the corn growth stage the better. This weed and feed is also application rate dependent. We would not recommend more than 10-15 gallons of UAN (approximately 30-45 lbs N). Even at rates lower than 10-15 gallons, burn may still occur. If the application could be scheduled with rainfall (good luck!), low temperatures, and high humidity, foliar burn would be less severe. As applications are delayed passed V4 (four visible leaf collars), we really discourage foliar application of liquid N (research in Minnesota has reported significant yield reduction after V4).

There is an alternative to applying the N solution with a flat fan nozzle (but this is not conducive to herbicide application) and that is using a solid stream nozzle or a dribble bar. Dribbling or streaming on N will decrease the potential for N loss compared to "misting" on the N (especially on no-till ground). This is important when you consider how much of the fertilizer solution will actually come into contact with the plant (around 5% of the total amount applied). This approach will also keep the fertilizer N away from the plant decreasing the likelihood of leaf burn. The thought that foliar application will provide a quick N fix for the corn plant may not be correct because relatively little N is actually taken up through the leaves (around 30%). Streaming nozzles and dribble bars should be available through your local dealer.

Another thing to consider is how vital is it to provide nitrogen at this point in the growing season. Physiologically the corn plant does not have a high N demand at this time. If you are planning on sidedressing in the not too distant future, you can probably delay N application until your sidedressing time.

Scout for Armyworm Now

Authors: Bruce Eisley, Ron Hammond

Field corn planted no-till into grassy habitats should be monitored closely at this point in time for armyworm activity. Fields that may be at risk for significant armyworm infestations include corn planted no-till in rye cover crops and corn planted no-till into old hay fields. A severe infestation of armyworm can reduce stand when an infestation occurs in the pre-whorl stage and cause significant defoliation when corn is hit in the whorl stage. Total destruction of a field of no-till corn can occur if a severe infestation is allowed to develop without application of a rescue treatment. Detection of foliar feeding injury by armyworm on 15 to 20% of a stand should be regarded as an indicator
of a potential problem, and the field should be rechecked within a few days to determine whether the impact of defoliation is increasing and a rescue treatment may be warranted. In general, a severe infestation will impact almost 100% of a stand and defoliation of the plants will exceed 50%, stand height is being reduced, and some plants being eaten
down to ground level. If defoliation remains less than 50% and the new growth exhibits minimal feeding injury, the stand will likely recover with minimal impact on yield. Rescue treatment in corn may be needed if stand infestation is greater than 50% and
larvae are not mature. Since armyworms are foliar feeders, they are relatively easy to control with most foliar treatments. During the day, armyworm larvae will most likely be found seeking shelter in the whorl or possibly in the ground cover. In general, armyworm
larvae will feed first on the lower leaves and then progress to the new growth - especially when corn approaches the early whorl stage. For additional information see the fact sheet at: http://ohioline.osu.edu/ent-fact/0012.html.

Slug Update

Authors: Ron Hammond, Bruce Eisley

Reports of slug feeding are being received from various parts of the state. Our sampling indicates that juvenile gray garden slugs, the species causing most problems in Ohio, have reached sufficient size to cause noticeable feeding in central Ohio (Fairfield County), and are within a few days of feeding in more northern locations (Knox and Wayne Counties). Sampling in northeast Ohio and northwest Pennsylvania indicate that gray garden slugs have just started hatching and are within a one to two weeks of beginning their heavier feeding. For comparison, juvenile slugs in Fairfield County are averaging 30 mg in size, those in Knox County are averaging 21 mg, in Wayne County 10 mg, while those in northeast Ohio are averaging 6 mg. We are still observing numerous eggs that have not yet hatched in all locations. In addition to the eggs and juveniles of gray garden slugs, we are also seeing more banded slugs and dusky slugs than in recent years. These slugs can add to the early defoliation we are seeing.

We expect the incidence of slug problems to increase over the next 1-3 weeks, with greater problems occurring shortly in southern locations following a northward trend to northeast Ohio and northwestern Pennsylvania where the most serious problems are probably a week or so in the future. The increase in problems will be partially attributed to the poor crop growing conditions because of the lower temperatures we are still experiencing, along with the replanting of numerous fields. These replanted crops will be germinating and emerging at the same time the juvenile slugs begin their heavier feeding. Growers with a previous history of slug problems should begin monitoring the situation closely in their fields. They should be aware that although severe feeding might not be occurring now, significant feeding may begin within a short time. When significant feeding is occurring, the only remedy is to apply a molluscicide such as Deadline MPs, or one of the new materials if available, Orcal Snail and Slug Bait or Metarex.

Glyphosate Formulations

Authors: Mark Loux

Glyphosate products are available from a number of manufacturers, and these products vary with regard to the percent activ e ingredient, type of glyphosate salt, and the type and amount of surfactant. In OSU field research, we are usually unable to discern differences in the effectiveness of glyphosate products when applied at the same rate of glyphosate acid per acre. From our standpoint, the primary differences among glyphosate products in most situations are the price, whether additional surfactant is required (an added cost), and the warranty on performance. The least expensive glyphosate products may carry no performance warranty, while the more expensive products may tie in to a complete package that includes some type of warranty on performance, rebates on techonology fees in replant situations, etc.

Glyphosate product rates vary due to differences in the amount of glyphosate acid per gallon of product. Glyphosate is formulated as various salts, but the amount of glyphosate acid per gallon is what’s important. The amount of glyphosate acid in products ranges from 3 to 4.5 lbs per gallon, so the product rate can vary considerably for a given rate of acid. For example, the often-used rate of 0.75 lbs of glyphosate acid per acre corresponds to 32 oz of Clearout 41Plus and many other generic products, 22 oz of Roundup WeatherMax and Original Max, and 24 oz of Touchdown Total or Glyphomax XRT. Table 22 on page 173 of the current Weed Control Guide for Ohio and Indiana (OSU Extension Publication 789) lists glyphosate products along with their use rates and surfactant requirements. This table is also available on the web at https://agcrops.osu.edu/weeds.

Ammonium Sulfate with Glyphosate – Why and How Much?

Authors: Mark Loux

For many producers and applicators, the addition of ammonium sulfate (AMS) or an ammonium sulfate replacement product to glyphosate has become a common practice. The addition of AMS to glyphosate is not always necessary, but it tends to improve glyphosate effectiveness on certain weeds and in certain situations. The addition of AMS to glyphosate accomplishes the following: 1) it prevents the cations in hard water from binding to glyphosate, which reduces its effectiveness; 2) it improves control of certain weeds, such as velvetleaf and some perennials; 3) it helps maintain glyphosate effectiveness under cold or other adverse conditions; 4) it helps overcome the reduction in glyphosate effectives when mixed with other herbicides that cause antagonism; and 5) it helps maintain glyphosate effectiveness when mixed with manganese for foliar feeding of soybeans.

Although many of us tend to always add AMS at the rate of 17 lbs of dry material per 100 gallons of spray solution when applying glyphosate, this rate may be higher than is needed for some situations. The amount of AMS can be adjusted based on water hardness if it is known, and a rate of 8 lbs per 100 gallons may be adequate for many sources of water. We believe that, aside from concerns on hard water, velvetleaf, or mixing with manganese, the use of AMS is less critical for postemergence glyphosate applications in warm weather, compared to early-season burndown applications.

AMS is available as a dry material, and also as a liquid. Problems with mixing the dry materials with water can be minimized by using spray-grade AMS, which is formulated specifically for use in herbicide applications. Other dry AMS materials may be available, but those not specifically formulated for use in herbicide applications may present more problems in mixing or clogging of sprayer parts. We use a commercial ammonium sulfate solution in our research at the rate of 5% v/v (5 gallons per 100 gallons of spray solution), because we know how much AMS we are applying, we can measure it by volume, and it mixes easily. Various liquid AMS-replacement products are also available, and these are frequently used by many applicators due to the convenience. It can be difficult if not impossible to know the rate of a replacement product required to provide the same benefit of 8 or 17 lbs of AMS. Some manufacturers provide this information, but it may be found only in technical or promotional literature, and not on the product label. This does not mean that these products are less effective than dry or liquid AMS, just that information is lacking to know how they compare to true AMS in their capacity to negate hard water or overcome antagonism from manganese.

Soybean Rust Update

Authors: Anne Dorrance

Soybean rust has not expanded beyond what was reported earlier this month. Models from Iowa State University indicate that the environment in the southeastern US has not been favorable for this disease to really get rolling. This indicates that the risk for rust to occur in Ohio near flowering is low. There is just not enough inoculum to get this rolling over the millions of acres of soybeans anytime soon. There have been more developments on the USDA website, http://www.sbrusa.net/. You will see that I can write a state commentary 2 to 3 times a week on the status of our sentinel plots. Thirty-seven of Ohio’s 48 sentinel plots have emerged and we received reports from 25 sites by the end of the week. Bean leaf beetle feeding was found in 7 seven of these locations, no aphids and more importantly no rust.

Brown spot, caused by Septoria glycines, was found in two of the sentinel plots. Brown spot is a brown spot, often surrounded by yellow. This fungal disease appears to be most common on soybeans planted in no-till fields where there is a lot of crop residue. This fungus overwinters on this residue and when conditions are favorable it can infect leaves. There is absolutely no evidence that fungicide sprays are needed to manage this disease at this time. For more information on brown spot you can refer to our website at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/septoria.htm

Insect Update in Corn

Authors: Bruce Eisley, Ron Hammond

Common Stalk Borer - Common stalk borer (CSB) larvae were found feeding on corn in central Ohio at the end of last week. The larvae were small (< 3/8 inch in length) and they were feeding in the whorls of 3 leaf stage corn. CSB larvae can be difficult to control especially if they are deep in the whorls of corn. However, since these larvae were still small and the plants were small, the worms may be moving to additional plants to feed and a rescue treatment may be beneficial. Check areas in no-till fields that have weeds residues from last year or along field margins for CSB feeding. Treatment is justified if 3 to 5% of the plants show feeding injury from CSB worms are small and not deep in the corn whorls.

European Corn Borers - We began operating a pheromone traps for European corn borer (ECB) moths last week. We did not captured any moths in central Ohio last week. Since most of the corn is still very small, ECB may not be attracted to the corn at this time.
However, as the corn grows and moths begin to emerge early feeding in the whorl from ECB may become evident in some fields. We will continue to operate the ECB pheromone traps and the moth counts will be reported in the newsletter and on the web at: http://entomology.osu.edu/ag/05traps.htm.

Potato Leafhopper on Alfalfa

Authors: Ron Hammond, Bruce Eisley

Potato leafhoppers have arrived in the Midwest, including Ohio. Now that many alfalfa fields have been or are being harvested, growers should plan on initiating scouting for the leafhopper as the regrowth reaches sufficient height for sweep-net sampling. Thus, growers in southern to central Ohio should probably begin sampling now, while those in the north should be thinking about this within a week or so. Sampling is done using a sweep net, and taking 15 samples throughout a field; each sample should consist of 10 sweeps with the net. Search through the net counting all potato leafhoppers, adults and nymphs (although mostly adults will be seen). When the average number of leafhoppers in a single sample (10 sweeps) equals or is greater than the height of the alfalfa, treatment should be considered. For example, if the alfalfa is 6 inches tall and the average number of leafhoppers per sample is 6 or higher, treatment is warranted. If the average is 5 or lower, the grower should come back within a few days to see if the population is higher or lower.

Pictures of PLH adults and nymphs can be found on the WEB at: http://ohioline.osu.edu/icm-fact/images/112.htmland http://ohioline.osu.edu/icm-fact/images/113.html. Chemicals labeled for PLH on alfalfa include Ambush, Baythroid, Furadan, Imidan, Lannate, Lorsban, Mustang MAX, Pounce, Sevin, and Warrior. Check label for rates and restrictions.

Let’s Get Ready Sprayer Clinic & Wheat Production Field Day

Authors: Mark Koenig

On June 21, 2005 from 1:00 to 4:00 pm, a sprayer clinic will be held in conjunction with the Production Field Day at Northwest Agricultural Research Station 4240 Range Line Road Custar, Ohio. With all the challenges you faced this spring, how about a refresher on sprayer set-up? It’s time to take a break from the daily routine. Come for the Wheat Production Field Day and stay for the Sprayer Clinic. Lunch will be available at a nominal fee helping to support the McComb FFA. We will have hands-on activities and field demonstrations. CCA CEU’s and Pesticide Re-certification credits will be available.
For directions to the Northwest Agricultural Research Station click on:
http://www.oardc.ohio-state.edu/branches/Northwest.htm#Directions

Archive Issue Contributors: 

State Specialists: Pat Lipps, Anne Dorrance and Dennis Mills (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science), Roberty Mullen (Soil Fertility), Peter Thomison (Crop Science-Corn), and Bruce Eisley and Ron Hammond (Entomology) Extension Educators: Roger Bender (Shelby), Steve Foster (Darke), Gary Wilson (Hancock), Dusty Sonnenberg (Henry), Harold Watters (Champaign), Tammy Dobbels (Logan), Glenn Arnold (Putnam), Mark Koenig (Sandusky), Alan Sundermeier (Wood), Steve Bartels (Hamilton), Bruce Clevenger (Defiance), Ed Lentz (Seneca) , Steve Prochaska (Crawford), Greg LaBarge (Fulton) and Howard Siegrist (Licking)

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.