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For Week of May 18-24, 1998
C.O.R.N. 98-10
In This Issue:
A) Early Season Pests of Corn
B) Managing Postemergence Herbicides
C) Did You Lose Spring Applied Nitrogen?
D) Will Wheat Diseases Dry Up?
E) Physiological Leaf Spots on Freedom and Other Wheat Varieties
F) Cloddy Seedbeds and Abnormal Corn Emergence
Wet and warm spring conditions in parts of southwestern Ohio have delayed planting and favored weed growth, while in the north fields of corn are currently beginning to emerge. Many Ohio corn fields will likely be emerging near the end of May or early June. Where wet spring conditions delay corn planting and increase unwanted weeds, the potential for cutworm infestations may be high. Thus, emerging corn should be monitored very closely for cutworm infestations, which may warrant a rescue treatment to reduce stand losses where significant infestations occur.
When cutworm injury is found on 3% or more of corn that is in the early stages of development, and cutworm larvae are predominantly in the middle stages of development (generally less than 1.5 inches in length), then a rescue treatment will be warranted. In general, a timely rescue treatment will arrest cutworm activity and prevent additional stand losses. However, during a field assessment of cutworm injury or losses, it is also important to determine whether significant stand losses have occurred and whether preventive treatments in the future might be warranted.
In conducting an assessment of cutworm injury, it should be recognized that there are three basic types of cutworm injury, namely: (1) missing stand that represents an absolute loss, (2) visible below ground plant injury resulting in wilted plants that will not recover, and (3) visible above ground cutting injury, which will likely recover. Determination of missing stand requires a stand count per unit area (such as 50 row ft) in both infested and non-infested sites of a field. Evaluation of visible injury above or below ground may be based on determining the proportion of remaining stand exhibiting injury, but counts per unit area would be preferable to compare visible injury to missing stand.
In general, three to four plants may be missing for every plant exhibiting visible cutworm injury. If an infestation is detected early, the ratio between missing stand and visible injury will be low and a timely rescue treatment should prevent significant levels of additional stand loss. However, if the ratio of missing plants to plants exhibiting visible injury is high, then one may assume that a significant loss has already occurred and that application of a rescue treatment may or may not prevent additional stand loss depending on the relative stages of development of the plant host and cutworms detected.
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Black cutworm larva in base of corn plant. |
Last week we emphasized the importance of using a burndown herbicide rate appropriate for the large weeds in some fields. The addition of ammonium sulfate to Roundup and Touchdown can also help when going for maximum herbicide activity, and is more important than using additional surfactant. We have seen another possible problem developing in our research trials. In some fields that were planted three weeks ago, the crop is small but some of the weeds that emerged after planting are already 3-6 inches tall. It is frequently difficult to manage total postemergence programs in early planted crops. Applying herbicide too early may result in poor control of later emerging weeds. Applying too late can result in poor control because the weeds are large, and can also allow some competition between the crop and weeds to occur, possibly reducing yield. The Roundup Ready program helps out somewhat in this regard, because it is more effective on large weeds than other postemergence programs. Another approach in this situation is to use a sequential postemergence program, where one-half of the herbicide is applied when weeds reach 2-3 inches in height, and the second half is applied in two to three weeks. This stretches out the effectiveness of a postemergence program, so that early application does not result in poor control of later emerging weeds. In OSU research, we have had outstanding results with this approach under a variety of conditions and weed populations. For more information on sequential postemergence programs, see pages 128-135 of the OSU Weed Control Guide.
Any nitrogen applied before May 1 may have had significant losses due to Denitrification on water logged soils. If N-Serve was included in anhydrous ammonia, I would not expect much nitrogen loss. For all other nitrogen programs farmers should observe the corn and make an individual field evaluation. The best time to make that evaluation is between June 1 and June 20. If the field has been wet for extended periods in April and Early May and the corn is off color (Yellow) when it is 12-14 inches in height, an additional 40-60 lbs. of nitrogen would be recommended.
The use of the Post Sidedress Nitrogen Test (PSNT) test for nitrate could help the farmer make this decision. If the test is in excess of 25 ppm then little nitrogen was lost. If it is less than 15 ppm then the nitrogen has been lost or leached below the one foot sampling zone or not converted to the nitrate form. With the warm weather of mid May nearly 100 percent of the nitrogen should be in the nitrate form by June 1.
Adding N-Serve in sidedress nitrogen after June 1 has been researched for many years. It was thought to improve yields of corn by letting the plant take up more ammonium nitrogen and less nitrate nitrogen. After several years of research, it appears under some undefined conditions this is true for some hybrids. About one time in five you could expect a 5-10 bu increase from this program for many high yielding hybrids. Based on the fact that we do not know the conditions where this yield increase occurs and the low return on investment, it is not a program we recommend.
The warmer, drier weather over the past week has been a welcome change, which should help reduce the spread of most wheat diseases. This increase in temperatures will have the greatest on powdery mildew. The fungus causing powdery mildew does not grow well at temperatures above 77 F. Hot days cause the fungus to enter its more resistant stage in which it develops small black fruiting bodies within the old mildew lesions. The fungus will survive the heat and UV radiation of the summer in the black fruiting bodies until weather becomes more fitting for its growth next fall. Thus, you can expect powdery mildew to decrease in importance over the next week or so. Symptoms of Wheat Spindle Streak Mosaic will also begin to disappear with the warmer days. You can expect the light yellow streaks in the upper leaves to turn green, but the more pronounced yellow or brown areas will not recover. Severely affected fields will have definite yield loss due to this viral disease.
The lack of rain will reduce the impact of Stagonospora (Septoria) nodorum leaf blotch. This disease is already well started in many fields, but the recent dry weather will slow the development of the disease. Any time we get a rain shower, or a wet period, you can expect the disease to spread upward on the plants. The number of rain showers that occur from now until plant maturity will determine the overall severity of the disease. So we still need to keep an eye on this disease!
The dry weather will also limit the number of infections from Head Scab. Wheat in the southern half of Ohio has been flowering over the past week and wheat in Northern Ohio will be in flower over the next week. The dry weather during this time period will limit the amount of head scab in the state this year. We are hopeful that our wheat crop will escape this devastating disease if the dry weather persists for another week or so.
Septoria tritici Blotch and Septoria nodorum Blotch, AC-2-96
We are getting reports and samples of Freedom wheat with distinct leaf spots on the upper leaves of the plants. These are the same type of spots we saw on Freedom last year, but were unable to find any fungal pathogens associated with the spots. The spots are yellow, mostly with dark brown centers. They are relatively small 1/16 inch x 1/8 inch, but tend to coalesce. The most distinctive spots are on the flag leaves. The lesions look like Stagonospora nodorum leaf blotch in that they have brown centers and yellow halos. Freedom is moderately resistant to Stagonospora nodorum, so we would not expect to see this disease on Freedom on the upper leaves, especially this early in the season.
There are other varieties that also show these physiological leaf spots, but in most cases the spots appear as flecks or very small spots speckled over the leaf surface. A very good photograph of physiological leaf flecking can be found on page 46 of OSUE Bulletin 811, Profitable Wheat Management. We believe physiological leaf spot is due to UV radiation damage that occurs when we have an extended period of overcast rainy weather followed by a bright sunny day. This occurred over the past weekend in many areas of the state. The bad news is that these spots may affect yield to some extent, but in most cases it has a minor effect. The good news is that last year the intensity of the spots decreases once we were able to get some sunny, drying weather. We did not detect any yield depression in Freedom wheat in research plots that had these symptoms in 1997.
Corn planted in dry, cloddy fields is often associated with abnormal emergence (in particular, seedlings unfurling leaves below the soil surface). Corn seedlings develop a twisted appearance under cloddy seedbed conditions because they may start unfurling leaves below ground. In a cloddy field where soil coverage of seed is poor and irregular, sunlight can reach the germinating seedling and induce leaf emergence beneath the soil surface.
Other factors (or combinations of factors) can also result in abnormal unfurling symptoms. Heavy rains after planting can cause a hard crust which makes emergence of small seedlings very difficult. As a result, bending and twisting of the seedling below the crusted layer often occurs. Planting the seed too deep, which may cause poor germination and emergence, may also result in premature unfurling of the corn. Certain herbicides such as Lasso and Dual, and the premixes that contain their active ingredient (Lariat, Bullet, Bicep) can show similar symptoms (i.e. twisting, abnormal growth) when excessive rates are applied preemergence. Besides excessive rates, improperly closed seed furrows can allow the preemergence herbicide to come in direct contact with the seed. Excessive soil insecticide dosage or in-furrow placement of insecticide phytotoxic to seed can also cause twisting sprouts and abnormal leaf expansion underground. In addition, anhydrous and aqua ammonia fertilizer injury has been associated with these symptoms. Certain corn hybrids are also more prone to this problem during emergence when unfavorable environmental conditions occur.
Corn seedlings that exhibit abnormal unfurling symptoms during emergence will be unable to penetrate any but the loosest soil even if the crust is broken mechanically or softened by rain. Prompt treatment with a rotary hoe, weeder, spiketooth harrow or cultipacker may help break the crust and improve emergence. To minimize poor seedling emergence due to unfurling below the soil surface, watch for cloddy seedbeds, open seed furrows, and crusting surface soils after rains. Also check planting depth periodically and adjust accordingly during the planting operation and monitor herbicide and soil insecticide rates.
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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: State Specialists: Pat Lipps (Plant Pathology), Hal Willson (Entomology), Peter Thomison (Corn Production), Jay Johnson (Fertility), Mark Loux and Jeff Stachler (Weed Science), District Specialists: Curtis Young (IPM), Ed Lentz (Agronomy), Extension Agents: Steve Bartels (Butler), Steve Prochaska (Crawford), Dennis Baker (Darke), Larry Lotz (Fayette), Greg La Barge (Fulton), Roger Bender (Shelby), Ray Wells (Ross), Clark Hutson (Seneca).
Editor: Clark Hutson Web Editor: Nathan Watermeier
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.
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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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