http://www.ag.ohio-state.edu/~corn/ |
|
For May 24-31, 1999
C.O.R.N. 99-10
In This Issue:
A) Postemergence Weed Control Considerations For Corn
B) Postemergence Weed Control In Soybean
C) Corn "Leafing Out" Underground
D) Field Crop Pest Activity
E) No Major Problems In Wheat Yet
F) Potential N Losses From Preplant N Solutions
1) Now that we have gotten our much needed rain (most of Ohio), consider spraying those escaped weeds in corn fairly quickly. However if choosing any ALS herbicides (examples: Accent, Accent Gold, others) seriously consider not spraying these herbicides until warmer weather returns! The most consistent environmental condition that has caused severe corn injury from ALS herbicides has been a period of cold and cloudy weather for two or more days, which is what we are experiencing right now. When ALS herbicides are sprayed on the last day of the cold spell or the first day that temperatures return to normal, significant corn injury is likely. Therefore, wait for at least two days of normal temperatures to return before spraying. Because of the cold temperatures the weeds will not grow too rapidly, so there is still some time to use these products and be safe to the corn. Also, this cold spell can cause translocating herbicides to be less effective.
Spraying these escaped weeds in corn when they are small will guarantee control of emerged weeds and allow for the opportunity for reduced rates of herbicides. Also corn cannot compete as well with weeds as do soybeans, so early control will maintain maximum corn yields. When weeds are less than 2 inches tall a half rate of a herbicide is usually sufficient to control the weeds present at the time of application. Since postemergence grass control in corn is expensive, remember that grasses less than 1.5 inches tall can be controlled with Atrazine, Basis, Bladex, or Extrazine. If grasses are between 1.5 to 3.0 inches tall then Basis Gold or Accent Gold will control grasses and Basis Gold will give some residual weed control because of the atrazine in the product. When using additional atrazine to control grasses, do not apply more than a total of 2.5 pounds of active ingredient per acre for the season and that atrazine and Princep should not exceed 1.6 pounds of active ingredient per acre to safely rotate to soybeans. If grasses are beyond 3 inches then use Accent, but be sure to apply before grasses reach 4 to 6 inches tall as corn yield may be reduced.
2) For those areas in the state that are still dry, consider using a methylated seed oil (MSO)(examples: Priority MSO, Meth Oil, SunIT, etc), with Accent, Accent Gold, Basis, and Basis Gold to improve control of stressed grasses. Adding MSO's with any ALS herbicide in corn or soybean will improve control of stressed weeds. If adding other herbicides with the above mentioned, read the label, as MSO's may not be used as crop injury can occur.
Regarding application strategies in dry weather, we have come to the following conclusions:
This may result in the need for a later application if late rain stimulates a new flush of weeds, but is a more effective strategy than trying to kill large tough weeds with herbicide mixes that are excessively injurious to the crop.
Scout early planted soybeans as weeds are off to a good start. If the weeds are big and the soybeans are small consider applying a herbicide with residual activity or plan for a second trip across the field. Again small drought stressed weeds are easier to control than large drought stressed weeds.
I have received several reports from northwest Ohio of corn seedlings leafing out below the soil surface. Seeds which exhibit this abnormal emergence may have a twisted appearance because internal leaves start expanding before the seeding has elongated. Several factors (or combination of factors) may be responsible for this abnormal growth. These factors may be characterized as environmental, chemical or mechanical. Environmental conditions associated with underground leafing include light penetration, cold soils, or heavy rains soon after planting. In a Clodd field where soil coverage of seed is poor and irregular, sunlight can reach the germinating seedling and induce leaf emergence beneath the soil surface. Also, 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 occur. Planting the seed too deep, which may cause poor germination and emergence, may also result in premature unfurling of the corn.
Bob Nielsen, Purdue Corn extension agronomist, suggested in one of his recent newsletter articles that underground leaf emergence may be related to large diurnal fluctuations in day and night temperatures. Past research has indicated that abnormal mesocotyl/coleoptile development occurred when temperatures fluctuated from daytime highs of about 80 degrees F to nighttime lows of about 55 degrees F. In Ohio, we have experienced fairly large daily temperature swings and these may have affected growth. Moreover, a lack of soil moisture in past weeks may have aggravated such a problem.
Certain herbicides such as Lasso and Dual, and the various premixes that contain their active ingredient 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 leafing out 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 unless used carefully.
However, even when used carefully, these salvage operations can cause some damage to
seedlings which are emerging normally. 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
Economic levels of black cutworm injury on field corn were detected during the past week in a number of areas in central and western Ohio. At the beginning of the week, most cutworm larvae detected were in the 4th instar stage (ca. 3/4" long) and cutting young plants at the sub-surface level. The feeding of 4th instar larvae on slow growing emerging corn plants can result in significant stand losses if a cutworm infestation on unprotected corn is not detected early and controlled by the application of a timely rescue treatment. However, it should also be noted that as warm temperatures accelerate cutworm development and the recent rains stimulate crop development, both the cutworms and corn crop may rapidly reach a stage of development where application of a rescue treatment may achieve limited results if the potential for additional stand injury is minimal. In brief, it should be recognized that cutworm larvae less than an inch in length will feed on a number of plants especially if the corn plants are small. It also should be noted that as corn plants develop, the number of plants injured by a single cutworm will be reduced. Finally, it is important to note that corn plants cut above the growing point will likely recover, corn plants tunneled below the growing point will not recover, and missing stand often represents the greatest proportion of a stand loss due to cutworm. In summary, assessment of a cutworm infestation requires consideration of the stage of cutworm development, the stage of corn development, the level of cutworm injury currently evident, and the loss of stand that has already occurred.
During the past week we have been evaluating a cutworm infestation of an experiment at the Western Branch plots established to evaluate cutworm management options. The infestation was detected Monday on corn in the 1-2 leaf stage, stand counts were conducted Tuesday, and rescue treatments were applied Wednesday. The stand of the untreated plots prior to treatment exhibited 6.6% cutworm stand injury compared to 2.0% injury in the plots that received at planting granule treatment and 1.6% in plots that had be treated with a pre-emergence broadcast treatment. Stand counts (including injured plants) of the plots with granule treatments were 12% higher than that of the untreated plots. In contrast, stand counts of plots with a pre-emergence treatment were 6% higher than the untreated plots. In general, the at-planting granule treatments were more effective in preventing stand loss due to cutworm than the pre-emergence treatments. During the forthcoming week, we will evaluate the efficacy of the rescue treatments applied. However, it should be noted that stand loss in the post-emergence treatments will likely be 15% or more despite the timely application of the rescue treatments.
A recent study on the possible impact of Bt-corn on Monarch butterflies has recently hit the press and producers of Bt-corn hybrids have responded accordingly. The study speculates on the potential impact pollen from Bt-corn on Monarch butterflies based on the results of Bt treatments of natural host plants such as milkweed. In response, producers of Bt-corn hybrids are emphasizing the reduction of use of pesticide attributed to the use of Bt-corn and challenging the theory that pollen of Bt-corn may be impacting non-target populations such as the Monarch butterfly. The issue that pollen may drift from Bt-corn to another plant and impact a non-target species such as the Monarch butter is nothing new. Over the past decade, both industry and university researchers have conducted studies on this issue, especially the Monarch butterfly.
We can expect more debate on the subject of Bt-corn since the advocates for and against the use of transgenic crops will continue to pursue their objectives. A larger issue currently linked to the production of Bt-corn is the demand for labeling of Bt-corn in market channels - especially in regard to the European market.
In regard to Ohio corn production, we should remind the public that the use of Bt-corn is relatively low in Ohio compared to that of the western corn belt due to the lower level of corn borer activity in the State. As debates continue about the pros and cons of Bt-corn, Ohio growers may benefit in time from producing non-Bt corn for specific markets.
Infestations of wheat by larvae of the cereal leaf beetle have been observed in the eastern region of Ohio. Early larvae of the cereal leaf beetle are shiny black, less than a 1/4 inch in size, and cause foliar scars that appear as fine lines. Infestations generally cause economic injury when two or more larvae can be found per stem. Since the infestations are hitting wheat this year at an earlier stage of development, reduction of the economic threshold to one larvae per stem may be warranted. Normally the cereal leaf beetle is maintained at low levels by beneficial parasites, but variations in cereal leaf beetle activity will occur. Since we just experienced an above normal year of alfalfa weevil activity, we may experience a similar increase of cereal leaf beetle activity.
Weevil injury on 1st cutting alfalfa declined during the past week as late instar weevil larvae spun their pupal cases and entered the pupae stage. Inspection of the pupae cases will often detect the presence of an oval brown pupae case with a white line around the middle instead of the green pupae of the weevil. The brown pupae case within the spun case is the pupae of a common parasitic wasp and indicates that the beneficial parasites are active despite the above normal activity of the weevil this year.
Wheat is advancing through the heading and flowering growth stages throughout the state (GS 10.1 to 10.5.1). Most fields in southern Ohio are ending the flowering period and fields in the far north are just entering head emergence and flowering. Most areas of Ohio have had some rain over the past 5 days. Rain during the grain filling period (next 14 days) will help wheat meet its full yield potential. Everything still looks good for an excellent wheat year. Below are diseases to watch in certain areas of the state.
POWDERY MILDEW continues to develop on wheat in the north central section of the state with the continued cool and recently moist weather. Monitor fields for one week past initial flowering. If mildew begins to develop on the leaf below the flag leaf Tilt fungicide could be applied. Apply fungicide only if the variety is known to be susceptible and the yield potential of the field is high.
STAGONOSPORA LEAF BLOTCH has not developed in any locations that we have visited due to the lack of rain. However, we are seeing a few small chocolate brown spots on plants of susceptible varieties. If rain showers continue to develop this week we may see some leaf blotch development in 10 to 14 days. This would put us past the time when major yield losses would be expected.
HEAD SCAB development will be difficult to predict. For disease to develop three things must happen at one time; the crop must be in a susceptible stage, the weather must favor infection and the fungal pathogen must be producing spores. The current situation indicates that: the wheat crop is now in its most vulnerable stage (flowering) for infection and the rains that developed over the weekend have provided the 48 hours of continuous wetness that usually favors head scab development. The third factor, presence of fungal spores, has been very limited. We have been monitoring the presence of Fusarium spores in the air over the past few weeks and have not found very many. This has probably been due to the very dry weather that has prevented spore development on crop residues. It is entirely possible that we will see only a very limited mount of scab because so little fungal inoculum is in the air. We will not be able to detect symptoms in the fields for about 7 to 14 days. We will continue to monitor the situation and relate any development of disease in future CORN messages.
BARLEY YELLOW DWARF (BYD) virus symptoms can now be detected in many fields, but the number of plants affected appears to be quiet low. Reddish and yellow flag leaf tips that stand erect in the field are common symptoms. A photograph of BYD affected plants, showing typical symptoms, is posted on the C.O.R.N. web site. Our experience this year indicates that relatively few affected plants are generally scattered throughout the stand. We do not expect yield losses from BYD in most fields, but those that were planted extremely early (before the hessian fly safe date) may have yield loss. Barley yellow dwarf is usually more important in early planted fields because the aphids that carry and transmit the virus are most active at that time of year.
 |
Flag leaves of wheat plants showing red and yellow coloration caused by barley yellow dwarf virus infection |
We have received questions about N loss from no-till cornfields that received early surface applications of 28% liquid N fertilizer. At the time of application, conditions were dry for most of the state. This dry condition may have caused N volatilization losses from urea, one of fertilizer compounds commonly used in 28% solutions. How much loss would be determined by the interval between application and rainfall and soil conditions. If no rain fell within 6 days of application, N loss may be >30%; within three days, losses would be less than 10%. Losses may increase in soils with a pH above 6.5 and may increase if rapid drying conditions were present (moist soil surface, warm temperature, and strong winds). Thus it is difficult to say the exact amount (if any) of N that may have been lost under these dry-like conditions. Keep in mind that urea generally provides about half of the N from 28% solution, so depending upon conditions after application, N losses could be between 0 and 50%, but are more likely between 15 and 35% for many of the dry fields this spring. Another tool that could be used to help estimate soil N is the Pre-Sidedress Nitrate Test. Since temperatures have been adequately warm, ammonium-N from ammonium nitrate and urea should have converted to nitrate-N. If the nitrate concentration is greater than 30 ppm, additional N at sidedress may not be necessary.
Readers can subscribe electronically to this newsletter by sending an e-mail message
to: corn-out-on@postoffice.ag.ohio-state.edu.
A successful subscription message will receive by an automatic reply from the listserv.
Contact your local Ohio State University Extension Office or e-mail
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 & Anne
Dorrance (Plant Pathology), Hal Willson (Entomology), Peter Thomison (Corn Specialist),
Jeff Stachler (Weed Science), and Jim Beuerlein (Soybean & Sm Grain Specialist);
DISTRICT SPECIALIST: Ed Lentz (Agronomy, Northwest); EXTENSION AGENTS: Dennis Baker
(Darke), Clark Hutson (Seneca), Dave Jones (Allen), Woody Joslin P.A. (Shelby), Greg
Labarge (Fulton), Howard Siegrist
(Licking), and Barry Ward (Champaign).
Editor: David A. Jones Web Editor: Steve
Lichtensteiger
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.
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.
TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868
| C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |