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June 4-10, 2002
C.O.R.N. 2002-16
In This Issue:
A) Corn Replant Considerations - Clarification
B) Corn Diseases: Seedling Blight, Anthracnose and Stewart's
Wilt
C) Insect Update in Corn
D) Postemergence weed control in corn
E) Sidedress Considerations
F) Most Locations in Ohio Have Relatively Low Predictions for
Wheat Head Scab
G) Variety selection is still a key decision to maximize profits
H) Insect Update in Alfalfa
This a follow up to last week's article on replanting corn. With regard to reapplying an insecticide if replanting is necessary - check the label restrictions on the soil insecticide used during the original planting operation. Most soil insecticides cannot be applied more than once per year. Consequently, another product may need to be used when replanting.
For more information on this topic, check out Dr. Kevin Steffey's article "Replanting and Application of Soil Insecticides" in Issue No. 5 of the 2002 University of Illinois Pest and Crop Bulletin available at http://www.ag.uiuc.edu/cespubs/pest/articles/200205g.html
Anthracnose leaf blight is becoming common on the lower leaves of young plants in many fields, especially those in continuous corn fields. The anthracnose fungus survives the winter in old corn residue. In the spring the spores are spread to young seedling leaves by splashing rain. Although the leaf blight phase of the disease is not always yield limiting, it does set the stage for anthracnose stalk rot late in the season. Spores that develop on leaf lesions are rain splashed or washed onto and behind leaf sheaths where infection of the stalks takes place. Those locations that had serious problems with anthracnose last fall are vulnerable to anthracnose again. For a photo of anthracnose on leaves see the Ohio Field Crop Disease web page at: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/corn/anthracnose3.htm
Flea beetles are out in large numbers in a number of corn fields in the state. It is likely that the beetles may be carrying the Stewart's wilt bacterium. When adult beetles feed on corn plants the damage the leaf cells and place the bacterium in the cells where they can begin to grow and multiply. Typical symptoms seen on young corn plants include linear yellow to white stripes on the leaves that are parallel to the leaf veins. These young plants may be killed if the bacterium becomes systemic resulting in wilt of the plants. Frequently, young dent corn plants are not killed, but continue to grow and produce multiple tillers. These plants rarely produce significant ears. Corn hybrids are available with good levels of resistance to Stewart's wilt, but insecticides are available for control of flea beetles if beetle populations are high and a susceptible hybrid was planted. For more information on Stewart's wilt see: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/corn/stewarts.htm
Black Cutworm - Black cutworm injury was observed in 3 to 4 leaf corn in central Ohio last week. The cutworms were cutting both above and below the soil level and the worms were about ¾ to 1 inch in length. Cutting was occurring in corn that had been no-tilled into a field with a lot of winter annuals including chickweed. Rescue treatment for black cutworm may be warranted when 3 percent of the plants in a field are being cut, worms are present and less than 1 inch in length or less.
Common Stalk Borer - Common stalk borer (CSB) was also found in the same field that had the black cutworm damage. CSB over winters as an egg on weed residue. The eggs hatch in the spring and the larva feed on the weeds until the weeds are killed and then the larva move to the corn and start feeding. The larva in this field had entered the corn plants at the soil line and burrowed upward into the plant killing the center of the plant. The worms were small (1/4 to 3/8 inch in length) and will be moving to other corn plants to feed. No-till corn fields that have been planted into fields with a lot of weed cover and areas along fence rows and ditch banks need to be checked for CSB damage. Rescue treatment for common stalk borer may be warranted when 3 percent of the plants in a field are being infested by stalk borer and the worms are small because they will be moving from plant to plant which is a good time to apply controls.
Corn Flea Beetle - Corn flea beetle (CFB) were reported causing damage to cornfields throughout Ohio last week. This insect over winters as an adult and moves into corn at this time of year. The over wintering adults are is the only generation that can cause problems in field corn. Adults are less than an inch in length, black in color and jump when disturbed. The CFB transmits the bacteria that causes Stewart's wilt disease which was discussed in letter 11, May 6. Rescue treatment is warranted if plants begin to wilt and a potential loss in stand appears likely.
With the abundant soil moisture and warm temperatures (finally) we are experiencing, expect rapid weed and corn growth and some challenges in effectively implementing postemergence herbicide programs in corn. Effective postemergence control can be even more difficult to achieve when the corn is still small but the weeds have reached a size that requires treatment. Some of the things we know about weed interference in corn:
- When the weeds and corn emerge together, weeds need to be controlled before they exceed 4 to 6 inches in height to avoid yield loss. This is based on studies where giant foxtail height was the determiner of application timing. On average, this weed height corresponds to approximately 25 to 30 days after planting (but this varies considerably due to planting date differences, weather, etc). Corn will be more competitive with the weeds when it has a head start, so weeds that emerge several weeks after corn emergence will have less affect on corn yield. Conversely, when the weeds emerge several weeks prior to corn emergence, they can have a greater affect on yield, and may have to be removed sooner after corn planting to avoid yield loss.
- Weeds that emerge after a properly timed postemergence treatment can reduce yield if present in high enough numbers. For this reason, we encourage the use of postemergence programs that provide at least some residual weed control. However, the risk of yield loss from these later-emerging weeds is much less than the loss that can occur when the postemergence application is too late. The number of weeds emerging after the postemergence application will be reduced for an early June corn planting, compared to an early May corn planting, and the need for residual weed control is therefore not as important in late-planted corn.
- As with any crop, we expect that the corn leaf canopy will be the primary weed control within 6 to 8 weeks after planting, or within several weeks after a postemergence application. When the leaf canopy development is delayed, or large weeds require a very early postemergence application, the crop may not provide adequate weed suppression without the help of residual herbicide activity.
There are three basic approaches to postemergence weed control in corn, and the best approach for a given field depends upon weed pressure, size of corn and weeds, and the date of planting. One of the important considerations is the degree of residual herbicide activity required. These approaches are:
- Early postemergence application of preemergence herbicides, when weeds are less than 2 inches tall. This can almost be considered a delayed premeergence application, except that the first flush of weeds should have at least emerged. Application of atrazine premix products such as Bicep II Magnum, Degree Xtra, Guardsman Max, Fultime, etc at this time can control emerged weeds and provide a full measure of residual weed control. When weeds are more than 2 inches tall, additional herbicide(s) may need to be included.
- Postemergence application when grasses are 3 to 4 inches tall, using herbicides that provide some residual control. Accent and Option do not have significant residual activity, but products containing rimsulfuron (Basis Gold, Accent Gold, Steadfast) can provide several weeks of control of annual grasses and small-seeded broadleaf weeds. Lightning also provides residual grass and broadleaf control. Atrazine can be added to most postemergence treatments to control weeds (primarily broadleaf) that emerge after postemergence application. Hornet, Permit, Yukon, and Callisto have residual activity on broadleaf weeds.
- Postemergence application when grass weeds are 3 to 4 inches tall, using
herbicides that have no residual. This is recommended only in fields with low
weed pressure, and includes treatments such as:
1) Mixtures of Accent or Option with any dicamba or bromoxynil product
2) Liberty on Liberty Link corn, and 3) glyphosate on RR corn.
In corn, broadleaf weeds are more easily controlled than grasses, so grass size often determines the timing of postemergence herbicides. Postemergence grass control in conventional corn hybrids (not herbicide-resistant) usually involves the use of Accent, Option, Basis Gold, or a similar product. Where a herbicide-resistant corn hybrid is planted, other choices are available - Liberty (Liberty Link hybrids), glyphosate (Roundup Ready hybrids), and Lightning (Clearfield hybrids).
Accent and Basis products contain one or both of the following two sulfonylurea
herbicides with activity on grasses - nicosulfuron and/or rimsulfuron. Other
active ingredients are included in some premix products to control broadleaf
weeds as well. By product, active ingredients are as follows:
Nicosulfuron can control larger grasses compared to rimsulfuron, but rimsulfuron provides a short period of residual activity that nicosulfuron lacks. In products containing both rimsulfuron and nicosulfuron, the nicosulfuron rate is reduced compared to Accent. As a result, Accent Gold and Basis Gold must be applied to smaller grasses then Accent (a difference of 1 to 2 inches generally), but they do provide some residual activity to make up for the need for earlier application. Products with the full rate of nicosulfuron, Accent and Celebrity Plus, may be more effective on perennial grasses. Basis is probably the least effective grass herbicide of all mentioned here, but will control small grasses (2 inches or less) at a relatively low cost. Although safe if used properly, rimsulfuron can be more injurious to corn than nicosulfuron, and the label for Accent Gold and Basis products is thus more restrictive with regard to corn size than Accent and Celebrity Plus. Maximum size guidelines for broadcast application are shown in Table 4 of the current OSU Weed Control Guide.
So, which product is best to use? It depends upon the situation and the weed population. In a total postemergence situation or following a reduced rate of a preemergence herbicide, any of the products except Basis would probably be acceptable. Basis Gold may be the most economical choice among Steadfast, Accent Gold, Accent, Basis Gold and Celebrity Plus. Steadfast and Basis Gold are similar in price, but the differences between them are: 1) Basis Gold includes atrazine while Steadfast does not, and 2) Steadfast has a higher nicosulfuron rate compared to Basis Gold and may therefore be more effective on larger grasses. In OSU research, Basis Gold has been most effective on broadleaf weeds when mixed with additional dicamba (the same goes for Lightning). Accent Gold also needs some additional dicamba to control lambsquarters, nightshade, or waterhemp. Accent, Steadfast, and Option lack significant activity on broadleaf weeds, and should be mixed with the labeled rate of broadleaf herbicide(s). Basis Gold and Accent Gold provide residual control of many broadleaf weeds, while Accent, Option, and Celebrity Plus have essentially no residual activity. With regard to corn size, you will be forced to use Accent, Steadfast, Option, or Celebrity Plus if corn is more than 12 inches tall.
Maximum Grass Size for Postemergence Corn Herbicides:
The delay in corn planting this year may be a reason to make adjustments to your N program at sidedress time. The biggest adjustment should be your yield expectation. Corn yields tend to decline after May 10 planting dates. Even though some areas may have been planted the end of April, below normal temperatures in early May limited growth enough to behave more like a May 15 planting. Yield potentials drop an estimated 15 to 30 % between May 15 and June 5 planting dates, respectively. Thus, a yield goal of 200 bu/A would now become 170 for the early-planted corn, and recent plantings would now become 140. Based on the university recommendation, an expected 170 bu corn crop following soybeans would need 175 lb N/A, and a recent planting would require 135 lb/A. The amount of N applied before sidedress should be deducted from these amounts. Since the recommendations are based upon preplant applications, a more aggressive approach may reduce this amount another 10%, but research has not been completed to confirm that amount. However, the chance for N loss generally becomes less after June 1 because of warmer and dryer conditions and faster growing plants demanding N. When we have losses it is generally caused by denitrification (N gas escaping to the atmosphere). For denitrification, warm soils need to be water-saturated for a least three days and N in the nitrate form.
Historically, Ohio State University has not recommended stabilizers at sidedress time because N losses are minimal at the later application times. Thus, the additional cost may not be recovered in larger yields or other agronomic benefits. Several companies have challenged this theory with new products and differing opinions. Several on-farm studies have been established across the state to investigate these possibilities. Some of these studies may be observed at Farm Focus in Van Wert County and at the Southwest Corn Growers' Field Day in Fayette County.
The presidedress nitrate test may be useful on fields that have had manure applications or possibility of having large amounts of soil N. This test estimates the level of nitrate-nitrogen in a field and is adapted to a sidedress program. Samples should be collected 5 to 10 days prior to time of sidedress from 15 to 20 sites across a uniform area no larger than 20 acres. The depth of core samples should be 12 inches. Samples should be dried, or at least, not sent to laboratories in plastic bags. Lab selection should be based on accuracy and timeliness (receiving samples and reporting results). If the results are in excess of 30 ppm, adequate nitrogen should be available for this year's corn crop. If it is less than 15 ppm, the normal nitrogen rate should be applied. Between 15 and 30 ppm, other factors should be considered before reduction of the normal nitrogen rate.
All N fertilizers may be used at sidedress when applied to minimize N losses. All urea type products (including 28 and 32% solutions) may have volatilization losses if not injected or incorporated. A urease inhibitor may be useful for surface applications of urea. Expect a 5 to 10 day response delay with anhydrous ammonia applications (time for the ammonia to convert to ammonium-N). Ammonium sulfate may be surfaced applied, but its relatively low N analysis often makes it a more expensive product than other N sources.
The development of wheat pushed ahead rapidly over the past week due to the warmer temperatures. Many fields in central Ohio were in flower over the weekend and in the more northern parts of the state the wheat will be in flower this week. This means that significant wet weather this week will influence the amount of head scab in northern Ohio
If you have been reading our reports in this newsletter you probably know we are attempting to make risk predictions for head scab. You can get updated risk predictions from the Ohio Field Crop Disease web site at http://www.oardc.ohio-state.edu/ohiofieldcropdisease by clicking on Forecasting Fusarium Head Scab of Wheat in Ohio. Fortunately, to date most locations have not had conditions highly conducive for scab development prior to flowering (See table below). Scab risk predictions will be made other locations in the state as the crop advances through flowering and early grain filling periods.
If head scab develops in the southern part of the state symptoms will likely begin to show up this week. Heads affected by scab have dead, bleached out florets. Frequently only a few florets are killed on a head, but sometimes the entire head may be killed. Our Scab risk predictions indicate only low to moderate risk for scab in most areas so far, so hopefully little disease will be detected.
Head Scab Risk Predictions Based on Pre-flower Weather Conditions
(Model 1)
Late planted soybean fields - may run into some additional problems down the road. A secondary factor of late season planting is that plants will flower later in the season, possibly when nights are cooler and dew is heavier. These environmental conditions are favorable for Sclerotinia stem rot or white mold. Sclerotinia stem rot can be found in a number of fields in Ohio, concentrated through the east to central side of the state and across the northern counties. There are also reports of a few fields south of rt. 70 that had Sclerotinia. In Ohio, we have had very little white mold in the past three seasons, mainly due to unfavorable environmental conditions during flowering. But the fields with Sclerotinia in the past are still prone to this disease. Host resistance is the best means to manage this disease. Variety selection is a key strategy. For those fields where white mold is known to have occurred in the past, choose varieties with the best resistance ratings to Sclerotinia stem rot. In addition, there is some data coming from some of the North Central States regional study on management of this soybean disease, that plant population may have an effect on the incidence of Sclerotinia stem rot. Fields planted at high populations with a susceptible variety, which promote thick tall, lanky growth, rows close early have higher incidence (more plants will have stem rot) than those fields planted at more moderate population densities (120,000 to 180,000 plants per acre). Ohio is participating in this study and we hope to have more data at the end of this year. More information on how to manage Sclerotinia stem rot can be found at: http://www.ncsrp.com/whitemold/
Potato leafhopper - Alfalfa harvest is underway throughout the state and so it is time to think about scouting for potato leafhoppers (PLH). We would suggest that after harvest you wait until the regrowth reaches about 4 to 8 inches (normally a week to 10 days) and then go into the fields with a sweep net to sample for PLH. A rescue treatment is warranted on alfalfa for PLH when the number of adults and nymphs exceeds the height of the alfalfa in inches. For example, if more than 6 leafhoppers were collected per 10 sweep sample on 6 inch alfalfa, a rescue treatment should be considered.
Chemicals labeled for PLH on alfalfa, rate per acre and pre-harvest
waiting time in days are:
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Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
State Specialists: Pat Lipps, Anne Dorrance & Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Mark Loux & Jeff Stachler (Weed Science), Ron Hammond (Entomology), Bruce Eisley (IPM), and Ed Lentz (Northwest District Specialist) Extension Agents: Dave Jones (Allen), Barry Ward (Champaign), Steve Prochaska (Crawford), Bruce Clevenger (Defiance), Greg LaBarge (Fulton), Gary Wilson (Hancock), Alan Sundermeier (Henry), John Barker (Knox), Howard Siegrist (Licking), Glen Arnold (Putnam), Ray Wells (Ross), Mark Koenig (Sandusky), Clark Hutson (Seneca) and Roger Bender (Shelby)Editor: Clark Hutson Web Editor: Tom Rosati
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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