C.O.R.N. Newsletter 2009-16

Dates Covered: 
June 2, 2009 - June 9, 2009
Editor: 
Jonah T. Johnson

June 1: Scab Risk Update and Fungicide Efficacy against Scab after Flowering

Authors: Pierce Paul, Dennis Mills

Wheat flowered last week (May 25 – May 31) in some field in the northern third of the state and flowering continues in other fields during this week (June 1 - June 7). Depending on when flowering occurred, some of the fields in northern Ohio that flowered during the week of May 25 would have received rain on at least one day during flowering. In some areas, it rained on Tuesday (May 26), Wednesday (May 27), and Thursday (May 27) of last week and some of the fields that flowered early in the week received rain on all three days. The forecast is for more rain on Monday (June 1), Tuesday (June 2) and Wednesday (June 3) of this week. Again, depending on where the field is located, when flowering occurs, and the accuracy of the weather forecast, some of the fields flowering this week may receive rain on one day while others may receive rain on all three days. Fields that flowered or are flowering that received two to three days of rain during last week and another two to three days of rain this week would be at the greatest risk for scab development and vomitoxin contamination, especially those flowering this week (the week of June 1). Remember, scab develops best when frequent rainfall occurs shortly before, during, and shortly after flowering. For wheat flowering during the week of June 1, rainfall during the week of May 25 would have contributed to the production and spread of spores of the scab fungus (Fusarium graminearum), while rainfall this week would contribute to infection, scab, and vomitoxin contamination.

The three most effective fungicides against scab are Prosaro, Proline, and Caramba, at label-recommended rates. Prosaro is a combination product of Folicur and Proline that has similar efficacy to Caramba, Proline alone, or a tank mix of Proline (3 fl oz/A) + Folicur (3 fl oz/A). All of these fungicides will only suppress scab (efficacy between 45 and 55% reduction) and are most effective when applied to a resistant variety and applications are made at flowering. Some level of scab and vomitoxin suppression can be achieved with applications made up to 5 days after anthesis, but the overall efficacy is greatly reduced. A well timed fungicide application may be the difference between grain being penalized for high vomitoxin (dockage or price discounts) and receiving regular market price at the time of sale. However, it is difficult to tell when fungicide use for scab will be economical. For instance, for a field likely to have vomitoxin contamination between 3 and 4 ppm, an application with a 45 to 50% efficacy may be enough to lower vomitoxin levels below 2 ppm. On the other hand, if conditions are very favorable for scab development, vomitoxin contamination may be well above 8 - 10 ppm, and fungicides alone will not drop vomitoxin levels to 2 ppm. We can predict when the risk of scab will be high or low (http://www.wheatscab.psu.edu), but at this time we have now way of telling beforehand when vomitoxin contamination will be 4 ppm or 10 ppm or 25 ppm.

There is some concern about vomitoxin contamination in the absence of scab symptoms. This is a very valid concern. In some years, vomitoxin may exceed 2 ppm in fields that look healthy. Our research suggests that at least three things may contribute to vomitoxin contamination of healthy-looking wheat: it depends on the variety, the weather conditions, and the amount of spores available. We have observed that if there are a lot of spores around and frequent rains occur between grain fill and harvest, the fungus may still infect the wheat and this late infection can result in vomitoxin contamination as high as 2-4 ppm. Fungicides applied at flowering will generally not protect against these late infections and later fungicide applications are illegal for the products listed above. Click on this link below for information on fungicide efficacy and pre-harvest intervals. http://www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/OFCDwheatfungicides.pdf

Ammonia Volatilization Losses from Weed ‘n’ Feed Applications

Authors: Robert Mullen, Keith Diedrick, David Henry

Ammonia volatilization losses from surface applications can occur during dry weather conditions, and in certain areas this spring has been an area of concern. First, let’s start with what ammonia volatilization is and how it occurs. Ammonia volatilization results from surface applications of a urea-based nitrogen fertilizer (dry urea or liquid 28% urea-ammonium nitrate) that has gone into solution but not incorporated into the soil with adequate moisture. When urea is exposed to water it easily goes into solution, but it requires the urease enzyme to break the molecule into ammonia and eventually ammonium. As the soil surface dries, there may not be enough water to keep the ammonium in solution, and the solution typically has a higher pH. Limited moisture and high solution pH causes ammonium to convert to ammonia that can be lost from the soil as a gas. This issue is exacerbated when plant residue is present on the soil surface for two reasons: 1) the residue serves as a ready source of urease, increasing the breakdown of the urea molecule, and 2) the residue increases surface moisture that causes the urea to quickly go into solution. So should you be concerned about nitrogen loss from your weed ‘n’ feed applications this spring? Based upon our weather pattern (some areas have been dry with lower humidity) there has most likely been nitrogen loss in some areas. Keep in mind, it only requires a couple hundredths of an inch of rain to your urea incorporated into the soil.

Based upon what we know, should we manage weed ‘n’ feed applications differently? As with any question asked of an Extension professional, it depends. First, should we be applying nitrogen with a weed ‘n’ feed application? It depends upon your capabilities. If you have starter equipment on your planter, then we would certainly recommend planter applied nitrogen to get you through the early part of the growing season because that material is not at risk for volatilization loss. If your planter does not have starter attachments, and the bulk of your nitrogen is supplied as a sidedress at V3-V5, then weed ‘n’ feed applications can be used for what we call a pseudo-starter. This will supplement some nitrogen and allow you some time to get your sidedressing done. This nitrogen is at risk for loss (especially when you have some surface residue), but it is really your only option for early-season nitrogen supplementation, even more so should sidedressing be delayed by poor weather. Another consideration is the choice of herbicide for the weed ‘n feed application; do read all of the herbicide labels for directions, especially in emerged corn.

Early season soybean and corn diseases.

Authors: Anne Dorrance, Pierce Paul

Production fields are finally planted and with the predicted rains we will hopefully end up with great stands in 2009. There are already early replanting reports of both soybeans and corn in some parts of the state due to flooding and heavy crusting. Ohio has a large proportion of acres with very high clay content which makes them prone to crusting, flooding as well as seedling blights. The repeated drying and wetting is actually perfect for the most common group of soil borne pathogens – the water molds. These fungus-like organisms produce swimming spores when the soil is saturated; these spores are attracted to plant roots. The damage by these pathogens are most severe prior to when soybeans and corn emerge from the soil and when the plants are still seedlings. For soybeans, the hypocotyls and cotyledons are infected prior to emergence and the same is true for the corn mesocotyl. In addition, roots of both plants can be infected as necrotic root lesions develop throughout the season. Without a strong healthy root system – plants have a tough time making grain.

Management of these water molds is a bit of a challenge for us on the eastern edge of the soybean/corn belt. The soils in Ohio are perfect for these pathogens but there are some things we can do.

1. Manage the drainage. Fields that are poorly drained are especially prone to these and most likely many of you know which fields require replanting more often.

2. Use a seed treatment. For both corn and soybeans, choose seed treatments that are comprised of several active ingredients. We have shown that one active ingredient (i.e. seed treatment coating) does not control all pathogens, so choose one with at least 2 fungicide active ingredients. An insecticide will not work on these water molds.

3. Use a seed treatment when you replant. This is probably more important the second time you plant than the first. To get disease in the greenhouse, we repeatedly flood and dry the soil, the more often we do this – the greater the disease levels. So for a replant situation, be sure that the seed is treated because the pathogen population in that field is primed and ready to go.

4. Choose resistant varieties/hybrids. For soybeans this is especially true for Phytophthora sojae; be sure to pick a variety with a gene combined with high levels of partial resistance. For Pythium spp. which attack soybean and corn, right now resistance to seedling blights is a bit of an unknown. Some inferences could be stretched for ability to resist these early season pathogens from early stand ratings but this one is going to take some time.

5. Don’t mud the seed in. This is probably the best management plan. For problem fields it is critical to only plant when conditions are best for the plant – not the pathogen.

Armyworm in Corn

Authors: Ron Hammond, Andy Michel, Bruce Eisley

Although there are no indications that the true armyworm will be a widespread problem this year in Ohio, we are receiving reports of larvae beginning to feed on corn that had been planted into a rye cover crop. Adult armyworms that migrate from southern states often prefer to lay their eggs in rye compared with wheat. When the rye is killed with a herbicide application, the corn then becomes the primary feeding site for the larvae. This feeding can become economic if the population of armyworm is high. It is recommended that growers who have corn planted into rye covers scout those fields for possible problems. See http://entomology.osu.edu/ag/images/cicaw.pdf for information on when to treat and a list of insecticides labeled for armyworm in corn. Remember that transgenic corn should also be scouted for potential true armyworm problems.

Miscellaneous weed management issues and questions

Authors: Mark Loux

1. Is it possible to control marestail in emerged soybeans? In numerous no-till soybean fields, omission of 2,4-D ester in burndown treatments resulted in a failure to control marestail, and soybeans have now emerged. As many growers know from previous experience, it can be almost impossible to adequately control marestail plants that have recovered from earlier treatments with herbicide. The best case here is that the marestail plants are not herbicide-resistant, and the soybeans are Roundup Ready, which allows use of a high rate of glyphosate or a combination of glyphosate plus FirstRate or Classic. Resistance to glyphosate and/or ALS inhibitors will limit the effectiveness of these treatments. Control is more difficult in non-GMO soybeans, since FirstRate and Classic are the only options. These two herbicides are not that good on large marestail anyway, and populations in many non-GMO fields are ALS-resistant. All (yes – all) other postemergence soybean herbicides have essentially no activity on marestail, and usually cannot be counted on to even slow the growth of marestail.

2. Will preemergence herbicides still have activity after remaining on the soil surface for several weeks? The answer to this is generally “yes”, and you can expect some activity from herbicide once rain finally moves it down into the soil. Herbicide can be lost from the soil surface via degradation, but dry soil conditions will limit the rate at which this occurs. Minor losses from photodecomposition or volatility are possible with certain herbicides, but most are not especially susceptible to this. The bigger issue here is the emergence of weeds between application and rainfall, which will generally require some type of postemergence herbicide treatment even if the preemergence herbicides are finally “activated”.

3. Dry weather has limited weed emergence – can we expect a major late flush of weeds when we finally we get substantial rain? This question obviously applies only to certain areas within the state, since other areas have received an abundance of rain. The second part to this question was whether postemergence herbicide applications should be delayed in corn to wait for later-emerging weeds, even if it meant switching herbicides. One way to look at this is that in any spring, a certain portion of the weed seedbank has broken dormancy and is “primed” to germinate once environmental conditions (primarily moisture and temperature) are suitable. So, it is certainly possible that forth-coming rain will result in substantial weed emergence in areas that have been dry up to now.
Where preemergence herbicides were applied at planting and are providing substantial control still, it may be possible to delay postemergence treatments to allow time for any late weed emergence. However, this delay can result in the need to switch to a product that is safer on corn that is more advanced in growth stage. Where preemergence herbicides were not applied, or where more than a few weeds are escaping preemergence herbicides, the postemergence treatment should be applied when weeds are small to maximize effectiveness and reduce weed-crop competition. Keep in mind also that weeds emerging after the corn reaches a height of about 20 inches have little potential to reduce yield anyway, even if they are present at crop harvest. Our general recommendation is to apply postemergence herbicides by the time corn reaches this height, and let the crop do the work of suppressing later-emerging weeds. One exception to this is burcucumber, which can require application of herbicides when the corn is upwards of 30 inches tall.

4. Giant ragweed in corn survived the first postemergence application of glyphosate – what should I spray now? The answer to this depends upon how they survived treatment with glyphosate. If most were controlled and the survivors were still substantially affected, then it may be possible to apply glyphosate again and obtain adequate control. The addition of another product with activity on giant ragweed could certainly improve control, however, especially where plants are large. The question here is whether the survival is indicative of a low level of resistance, and an alternative to glyphosate should be used if there are doubts that a second glyphosate application will work.
Where many plants survived the first glyphosate application and showed little response to glyphosate, which is indicative of resistance, it will be necessary to apply a postemergence alternative to glyphosate that is effective on giant ragweed. Possibilities here include Status, Hornet, Impact, Callisto, Laudis, and NorthStar, among others. In Liberty Link corn, Ignite would be an effective option. Impact, Callisto, and Laudis are most effective on giant ragweed when mixed with atrazine. Labeled rates and recommended adjuvants should be used to maximize effectiveness of alternatives to glyphosate. It is possible to add some glyphosate to improve control of other weeds. However, be sure to add crop oil concentrate or MSO if recommended on the label of the alternative herbicide, instead of relying on the surfactant that is in most glyphosate products.

5. Is it safe to eat the produce from vegetable gardens affected by drift of corn and soybean herbicides? The answer to this “no”, except where the herbicide is labeled for the garden plant of concern (such as Callisto on sweet corn). Since most corn and soybean herbicides are not labeled for use on vegetables, there is no established tolerance for the allowable herbicide residue in the produce, and therefore no guidelines for “safe” use have been established.

Staging vegetative 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 apply post-emergence chemicals effectively with minimum crop damage.

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 usually be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be an 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 generally assess hail damage 5 to 10 days after the storm, by which time one or more leaves have emerged from the whorl.

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

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

Predicting leaf stage development in corn.

Authors: Peter Thomison

Dr. Bob Nielsen at Purdue University has described a method for predicting leaf stage development using accumulated heat unit or growing degree day (GDD) information. Given an understanding of corn leaf stage development and heat unit calculation, a grower can predict what leaf stage of development a particular field is at given its planting date and temperatures since planting. It is useful to know when the crop emerged, but if you do not, you can estimate that event also. Corn emergence typically requires 100 to 150 GDDs.
Dr. Nielsen proposes that corn leaf developmental rates may be characterized by two phases. From emergence to V10 (ten visible leaf collars), leaf emergence occurs approximately every 82 GDDs. From V10 to tasseling, leaf collar emergence occurs more rapidly at approximately one leaf every 50 GDDs. Previously, about 60 to 65 GDDs were associated with the appearance of new leaf collars during vegetative growth.
Example (from reference noted below): 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 120 GDDs, then the estimated leaf stage for the crop would be about V8. This estimate is calculated by first subtracting 120 from 785 to account for the estimated thermal time to emergence, then dividing the result (665) by 82 (equal to 8.1).
Dr. Nielsen cautions 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 method may be useful in timing when plants will reach an approximate stage of growth.

For more on Dr. Nielsen’s prediction system, check out the following –

Reference
Nielsen, R.L.. 2008. Use Thermal Time to Predict Leaf Stage Development in Corn
Corny News Articles, Purdue Univ. [On-Line]. Available at http://www.agry.purdue.edu/ext/corn/news/timeless/vstageprediction.html (URL verified 6/1/09)

Estimating Soybean Stands

Authors: Dennis Mills

Soybeans have emerged now and evaluation of the stand maybe needed. Poor stands may occur at one time or another and producers need to be aware of the potential yield loss from those deficit stands and the cost of replanting. Soybeans have a high capacity to compensate for low plant populations and gaps in the row.
If a poor stand does occur producers should investigate. Was the poor stand the result of poor seed quality, cold wet soils, hot dry soils, planting too deep or shallow, soil crusting, herbicide injury, insect feeding, poor soil to seed contact, or disease infection? Determine if the cause can be corrected to avoid a similar situation.
One popular method to determine plant population is by using a hula hoop. This involves placing a circular measuring device such as a hula hoop on the ground and counting the plants contained within. Use the table below, listing various sizes of hoops, to determine the number of plants per acre.

Hula hoop method for determining drilled soybean populations.

Inside Diameter of Hula Hoop
No. of plants 30 in 32 in 34 in 36 in 38 in
Plants (1000's)/acre
6 53 47 41 37 33
10 89 78 69 62 55
14 124 109 97 86 77
18 160 140 124 111 100
22 196 172 152 136 122
26 231 203 179 160 144

Weather Update

Authors: Jim Noel

The main thing this spring was to get agronomic crops planted. Once planted, conditions have been fairly pleasant. After some drier weather lately, rain chances will go up again in the next week. In the past week, we saw 1-2+ inches in the far north, 0.50 inches in the south, while the center portion of the state experienced average rainfall. As with late spring weather, the variability of rainfall seen was with isolated totals higher and lower.

Expect some showers and storms through Wednesday of this week followed by drier weather later this week and into much of the coming weekend. More rain return Sunday into early next week with drier weather again later next week and into the first half of the following week. Temperatures and rainfall are average to near normal for this time of the year, especially over the next 2-3 weeks. There is a 60-90% chance of two (2) inch rain events statewide over the next 2-3 weeks.

Overall, things look good.

The 2009 summer season should be close to normal temperatures and rainfall.

Overall, I expect fairly ideal conditions with limited heatwaves this summer. This may be partly related to sunspots and no El Nino or La Nina.

See NASA article.

http://science.nasa.gov/headlines/y2009/29may_noaaprediction.htm

An El Nino may be in the forecast for the 2009 fall and winter seasons. If so, fall would be drier than normal which could result in good harvest conditions. We will keep you posted on the upcoming weather patterns!

Archive Issue Contributors: 

Pierce Paul, Anne Dorrance, and Dennis Mills (Plant Pathology), Ron Hammond, Andy Michel and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Robert Mullen (Soil Fertility), Mark Loux (Weed Science) and Jim Noel (NOAA). Extension Educators & Associates: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Greg LaBarge (Fulton), Harold Watters (Champaign), Mike Gastier (Huron), Mark Koenig (Sandusky), Wes Haun (Logan), Jonah Johnson (Clark), Ed Lentz (Seneca), Bruce Clevenger (Defiance), Steve Prochaska (Crawford), Alan Sundermeier (Wood), Suzanne Mills-Wasniak (Montgomery) and Tim Fine (Miami).

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.