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For Week of April 27 to May 3, 1998
C.O.R.N. 98-7
In This Issue:
A) Powdery Mildew on Wheat: Scout Fields Now
B) More on Authority and Soybean Tolerance
C) Late Tillage and Large Weeds
D) Cutworm Potential on Corn
E) Armyworm Moths are in Flight
F) Alfalfa Weevil Activity Appears Minimal
G) Optimal Seeding Rates for Corn
Powdery mildew is the most common disease on wheat in the state at this time. The disease is probably more common in the north east and north central sections of the state than in other areas, but it is found in scattered areas in the south as well. Most reports indicate that the level of powdery mildew in most fields is still quiet low, but in some fields the disease is advancing up the plants. The differences in the level of disease from field to field is probably due to the level of resistance to powdery mildew in different varieties. The weather conditions most favorable for powdery mildew development include scattered light showers once or twice a week, moderately warm temperatures (55-75 F) both day and night and high relative humidity. When the temperatures are 68-70, powdery mildew severity can move from 1% leaf area affected to 10% leaf area affected in 5 to 7 days. Thus, when conditions are favorable, the disease can spread rapidly throughout the wheat canopy.
Yield losses to powdery mildew can be as high as 25% on susceptible varieties when the disease is detected on the flag leaf of most tillers in the field by the time the heads begin to emerge. Predicted yield losses are much less if the disease is confined to the lower 1 to 2 leaves on the stems or on more resistant varieties. Varieties with a high degree of resistance to mildew may prevent the development of the disease or may slow down disease spread within the field. Varieties like Hopewell generally prevent disease development, thus these fields may be very disease free at this time. Other moderately resistant varieties, like Freedom, may have some disease development restricted to the lower leaves through flag leaf emergence, but then prevent further disease development at later stages of plant growth.
We suggest that fields be scouted now, especially if the following susceptible varieties were planted: Pontiac, Cardinal, Clark, Dynasty, Grant, Patterson, Podach, Kingston, SR205, 2510, 2540, RS927, SG1550, TS4020 and W9140. If you do not know the relative susceptibility of a variety, it probably should be scouted. Scout fields by randomly selecting 30 or more tillers throughout the field. There are 4 main leaves on each tiller of most modern varieties. Examine each leaf on each tiller and assess disease from the top leaf downward on the stem. A fungicide application is most beneficial when the top two leaves (flag leaf and the one below it) are in danger of becoming infected before flowering.
As the price of wheat continues to drop, the economics of using a fungicide for control becomes more critical. When the potential for disease is high, yield increases from a fungicide application have been 15-20% and when disease potential is low, yield increases are 5-10%. To cover the cost of a fungicide and application with $3.00 wheat price it will take a 5-7 bu yield response. We suggest using fungicides only on high yield potential (70-100 bu/A) fields planted to susceptible varieties. Fungicides are most beneficial when more than one disease is affecting the field.
Below is a list of fungicide and fungicide combinations that have been used most successfully in Ohio for powdery mildew and Septoria (Stagonospora) leaf and glume blotch control with a single application.
1) Tilt 3.6 EC, 4.0 fl. oz/A, apply at or before flag leaf emergence
2) Bayleton 50 DF, 2-4 oz/A for powdery mildew or 4 oz/A. Application should not be made within 35 days of harvest.
3) Bayleton 50 DF, 2 oz/A plus Benlate 50 DF, 4-8 oz/A. Application should not be made within 35 days of harvest.
4) Bayleton 50 DF, 2 oz/A plus a Mancozeb type product, 1.6-2.2 lb/A. Application should not be made within 35 days of harvest.
Double crop soybean planting restrictions following fungicide applications: Tilt: Soybeans may be planted as a double crop following a cereal crop which has been treated at flag leaf emergence growth stage. Bayleton: Soybeans may be planted 35 days after application.
In last week's article on tolerance of soybeans to Authority, we noted that we did not know whether seed companies other than Pioneer had screened their varieties for tolerance. We subsequently received a message from Golden Harvest stating that they have tested 30 varieties since 1995. This screening has been done in the field and plots were taken to yield. In this testing, they observed minimal visual injury and no effect on yield from the use of Authority.
We typically do not expect to have to apply Roundup, Touchdown, or Gramoxone in fields that will be tilled prior to planting. However, in seasons when tillage and planting are delayed due to rain or when warm weather promotes weed growth in early spring, weeds can become fairly large before soil conditions are suitable for tillage. In this case, the tillage may not adequately remove large weeds, such as Canada thistle and giant ragweed. If not adequately removed, these weeds will continue to grow following tillage and planting, and may be too large to be controlled with postemergence herbicides. Consider application of Roundup, Touchdown, or Gramoxone in these fields prior to tillage to aid in control. Application a few days prior to tillage is preferable to application after tillage, since the weeds may be less susceptible to herbicide activity when disrupted. At the very least, check fields before the crop emerges, and make a burndown herbicide application at that time if it is apparent that a significant number of weeds have escaped tillage and will interfere with crop growth and establishment. When using Roundup Ready soybeans, it is not necessary to apply Roundup before the crop emerges, but plan for two postemergence one early to remove weeds that escape tillage, and one later to control the weeds that normally emerge after planting. Do not apply 2,4-D in fields that have yet to be tilled, since tillage will distribute the 2,4-D throughout the seed zone, increasing the risk of crop injury.
It is an early spring, weed growth has been substantial in untilled fields, cutworm adults have been flying, and wet weather has delayed planting. In brief, conditions appear favorable for cutworm problems this spring. Last year, cutworm activity was minimal despite reports of significant collections of cutworm adults in pheromone traps. We did have one significant infestation in a block of late tillage continuous corn at the Western Branch Station, but infestations in our no-tillage trials were very low. In contrast, in 1996, we observed a 11% increase in stand in no-till plots having a preventive treatment compared to untreated plots. In 1995, cutworm activity was minimal. In 1994, cutworm infestations were widespread and significant.
Stand losses due to cutworm can be effectively prevented by the application of a soil insecticide (that is effective on cutworm) at planting or by application of a pre-emergence broadcast treatment. In contrast, a post-emergence rescue treatment will arrest a cutworm infestation, but a significant loss in stand may occur if a rescue treatment is applied too late.
During the past week, we have initiated weekly sampling of our plots at the Western Branch for cutworm activity by flushing half meters of soil with soapy water. A few large dingy cutworms have been flushed out, but no black cutworm larvae to date. Last year, the block in which we detected early black cutworm activity using the flushing method was also the block having severe cutworm injury following corn emergence.
Predicting which fields have a high risk for cutworm and would benefit from preventive treatments is difficult. In general, the potential for cutworm infestations increases with a reduction in tillage and an increase in early weed growth. In addition, some geographical areas have a history of cutworm infestations while other areas do not. In summary, the justification of applying preventive treatments for cutworm should be based on an assessment of stand losses observed over time in treated and untreated fields.
Alfalfa stands are growing rapidly and weevil activity appears to be minimal. Given the warm spring conditions, tip injury due to weevil feeding should be visible at this time. However, the fields in central Ohio that I have looked at to date exhibit no visible signs of weevil activity.
When corn is produced for grain in Ohio, recommended plant populations at harvest range from 20,000 to 30,000+ plants/A, depending on the hybrid and production environment. Populations for corn silage may exceed those for grain by 2,000 to 4,000 plants/A. Most seed companies specify a range in seeding rates for the various corn hybrids they market and these seeding rate guidelines should be followed closely. The final stands recommended for typical mid-season hybrids in Ohio may vary by 2,000 to 4,000 plants/A depending on the production environment and likely growing conditions. Seeding rate adjustments should be made on a field by field basis using the average yield potential of a site over a 3 to 5 year period as the major criterion for determining the appropriate plant population.
Other factors such as hybrid ear type (flex vs. fixed), plant height (tall vs. short), etc., influence corn response to plant populations but the yield potential of the production environment (i.e. soil productivity, etc.) is the overriding factor determining hybrid yield response to increasing plant population. Higher seeding rates are recommended for sites with high yield potential which have high soil fertility levels and water holding capacity. On very productive soils which typically average yields of 160 bu/A or more (such as a drained Kokomo or Brookston silt loam), final stands of 30,000 plants/A or more may be required to maximize yields. High plant populations are also warranted when irrigation is used to maximize crop performance. On soils averaging 120-150 bu/A final stands of 26,000 plants/A may be needed to optimize yield.
Lower seeding rates are preferable when droughty soils or late planting (after June 1) limit yield potential. On soils that average 120 bu/A or less, final stands of 20,000 - 22,000 plants/A may be adequate for optimal yields. Recent Ohio studies show that under less than favorable weather conditions high plant populations do not cause significant yield grain reduction. Planting a hybrid at suboptimal seeding rates is usually more likely to cause yield loss than planting at above recommended rates, unless lodging becomes more severe at higher population levels.
Hybrid response to high population can be limited by stalk lodging which often increases at higher plant density. Some hybrids which have shown positive yield response to higher populations cannot be grown at high plant densities because of the increased risk of lodging at harvest. Lodging reduces yields and slows the harvest operation. Therefore it is essential that hybrids planted at high seeding rates possess superior stalk quality characteristics for standability. Hybrids should also have resistance (or the best levels of tolerance available) to fungal leaf diseases (such as gray leaf spot and northern corn leaf blight) which contribute to stalk lodging problems caused by stalk rots (such as anthracnose and gibberella).
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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.
Contributors to C.O.R.N. this week include: Pat Lipps (Extension Specialist, Plant Pathology); Ann Dorrance (Extension Specialist, Plant Pathology); Mark Loux (Extension Specialist, Weed Science); Hal Willson (Extension Specialist, Entomolgy); Peter Thomison (Extension Corn Specialist); Ed Lentz (Northwest District Agronomist); Jim Jasinski (Extension Associate, Integrated Pest Management); Extension Agents - Larry Lotz (Fayette), Ray Wells (Ross), Roger Bender (Shelby), Greg LaBarge (Fulton), Dennis Baker (Darke), Max Mohler (Lima Campus), Tom Krill (Van Wert), Gary Wilson (Hancock), Barry Ward (Marion), Steve Bartels (Butler), Mike Estadt (Pickaway) and Steve Prochaska (Crawford).Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
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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