CORN
Crop Observation and Recommendation Network

June 18 - 24, 2002
C.O.R.N. 2002-18

In This Issue:

A) Optimizing Herbicides for Large Weeds, etc.
B) Staging Vegetative Growth in Corn
C) Soybean Seedling Diseases
D) Plantings Soybeans in Late June
E) Insect Update
F) A Few More Wheat Fields with Freezing Injury
G) Predictions for Wheat Head Scab Low With a Few Exceptions
H) More Corn Seedling Diseases, More Plant Death
I) Drift Can and Should Be Reduced to Minimum

A) Optimizing Herbicides for Large Weeds, etc. - Mark Loux CORN Questions

Herbicide activity has been generally good over the past week, based on our research plots in west central Ohio. We have observed fairly rapid weed death, but also some minor injury to corn from postemergence application of Steadfast and similar materials. Injury symptoms included chlorosis or bleaching of new growth and internode shortening, which gives the plants a somewhat compressed appearance. Based on our past experiences, corn seems to outgrow this type of injury with continued good weather. We also observed some leaning and twisting of corn from application of 2,4-D, and stalk brittleness can accompany this type of injury. While corn often outgrows the injury, the stalk brittleness can make it more susceptible to damage from wind.

Some producers still have corn and soybean fields where, for one reason or another, large weeds will require herbicide treatment. Some tips for maximizing herbicide activity in these situations:

1) Research from OSU and other universities has shown that herbicide activity is affected by time of day. While the nature of this effect varies with herbicide, weed, and environmental conditions, the general trend is for herbicide activity to be reduced in early morning or later evening. In OSU research last year, the activity of glyphosate and Flexstar was reduced at 6 am and 9 pm. Between the hours of 9 am and 6 pm, herbicide activity was not reduced or affected by time. Time of application did not affect the activity of FirstRate.

2) Use the appropriate carrier volume for the herbicide and the weed size and density. Activity of contact herbicides can be greatly affected by volume, and they should be applied in at least 15 to 20 gpa. Although systemic herbicides can be applied in volumes less than 15 gpa, higher volumes may improve control where weeds are large or present in high densities.

3) Nozzle selection also influences spray coverage and herbicide activity. Herbicide activity will generally be maximized with flat-fan type nozzles, especially for contact herbicides or problem situations. Be aware that drift control measures, if taken to extremes, can reduce herbicide activity. The combination of a low-drift nozzle and a drift-reducing agent may reduce coverage to the point that herbicide activity is affected.

4) Use spray adjuvants that maximize herbicide activity. Labels often specify the use of a certain adjuvant in "normal situations", but allow the use of others where weather conditions are less than ideal or for control of specific weeds. However, be cautious about "jungle-mixes" that might cause excessive crop injury. When in doubt about adjuvants, consult a manufacturer's representative for more information.

B) Staging Vegetative Growth in Corn - Peter Thomison CORN Questions

When estimating losses in corn due to defoliation and other types of plant injury, it's often necessary to establish the stage of plant growth at the time when damage occurred. It's also important to know the stage of development in order to use postemergence herbicides effectively with minimum crop damage.

Several systems are commonly used to stage vegetative growth in corn. The "leaf collar" system is probably the method most widely used by Extension 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 National Crop Insurance Service. 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 be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be a 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 usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.

At about the six-leaf stage of the collar method (V6) 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. Degeneration and eventual loss of the leaves results. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process. To determine the leaf stage after this loss of lower leaves, split the lower stalk lengthwise and inspect for internode elongation. You can identify leaf location since leaf. For example, node 5 is usually attached to the top of the first noticeably elongated internode. (The first node above the first elongated stalk internode is generally the fifth leaf node. This internode is about 0.4 inches in length.) Leaf growth stage can then be determined by counting from the fifth leaf to the uppermost collared leaf, or the uppermost leaf that is 40-50% exposed from the whorl (depending on which growth staging system is being used).

Remember that prior to the V6 stage (collar method), the growing point is below or at the soil surface. As long as the growing point remains healthy and intact, severe defoliation during early vegetative stages seldom translates into significant yield losses.

C) Soybean Seedling Diseases - Anne Dorrance CORN Questions

There are a number of soilborne pathogens that can attack soybeans, especially under the favorable environmental conditions that have occurred over the past two weeks. The areas of the state that received heavy rains at the end of the week of June 10, will be begin to see symptoms this coming week, June 17. Soybeans which are affected will turn yellow to tan and begin to wilt and die. The classic symptoms of Phytophthora post-emergence damping-off and Rhizoctonia damping off can be seen at the following web site: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/soybeans1.htm. Diseases caused by these two pathogens can be separated by the symptoms. Phytophthora seedling blight will be a brown discoloration that will move up the stem (Figure 1 and 2 on website) and Rhizoctonia will have a brick-red lesion at the base of the stem (figure 3).

Phytophthora can be managed by planting varieties with resistance genes, however, many of our fields in Ohio have populations of Phytophthora sojae that now cause disease on plants with these resistance genes, a normal biological process. A new fact sheet on Phytophthora can be found at: http://ohioline.osu.edu/ac fact/0017.html. The North Central Soybean Research Program also has information on Phytophthora sojae, at: http://www.planthealth.info/prr.htm.

If replanting is necessary, Rhizoctonia, can be managed with seed treatments, not all seed treatments are effective against all pathogens. For more information on Rhizoctonia see: http://Ohioline.osu.edu/ac-fact/0025.html.

For more information on which seed treatments are effective for which soybean seed and seedling pathogens go to: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/soybeanseedtreatments.htm

D) Plantings Soybeans in Late June - Ed Lentz CORN Questions

Fields have still been too wet for some growers to plant soybeans, or patch in previous lost stands. Success at this time depends upon late July and August rainfall. Producers may need to adjust their varieties selected (earlier maturity groups), seeding rates, and/or row widths. Most of the state may still follow their planed program with medium to full season varieties. The northern one-third of the state should review their varieties for maturity, and consider varieties 0.5 a maturity group earlier than the adapted full season varieties for their area.

For the past three years (1999-2001) OARDC research at the Northwest Branch near Custar has investigated the relationship between maturity and planting date for Roundup Ready soybeans. The overall average showed a 60% yield reduction when planted late-June compared to early May. The group 3-variety had 19% and 10% larger yields than the group 2.1 and 2.5, respectively, for a late June planting. The full season variety (group 3.6) had 2% larger yields than the group 3 in a late June planting.

This data would suggest that for Northern Ohio, producers should stay with group 2.8 to 3.2 soybean varieties for late June plantings. All of these plots had a seeding rate of 200,000 seeds per acre. Only one variety was used for each maturity group. The very northern counties should consider varieties between 2.1 to 2.6 maturity rating. For the rest of the state, selecting varieties between 3.5 to 3.8 (south central region) and 3.8 to 4.0 (south region) should provide the most vegetative growth and mature before frost. More vegetative growth generally leads to larger yields for late planting dates. Populations should be at least 200,000 plants per acre and row widths less than 15 inches.

E) Insect Update - Bruce Eisley and Ron Hammond CORN Questions

1) Armyworms in Wheat - We began receiving calls last week about problems with armyworms in wheat and fields being sprayed in west central Ohio for armyworms. The problem does not seem to be as widespread or as severe as last year. At this time the most important thing is whether head clipping is occurring or not. If head clipping is occurring then treatment may be warranted. In addition to head clipping, we need to determine the size and condition of the armyworm larva. We had reports last year of fields with dead armyworms due to either predators and/or a fungus disease.

Consideration should also be given to the chemical that is being used because some have a longer waiting time to harvest than others. Chemicals that are labeled on wheat for armyworms, rate/acre and the waiting time in days to harvest are:

Corn fields next to wheat will need to be watched in the next couple of weeks for armyworm movement from the wheat into corn. The armyworms will normally only move 10 or 12 rows into the corn and a border treatment can be used to obtain control.

2) European Corn Borer - European corn borer (ECB) moths continue to fly as evidenced by pheromone traps in central Ohio. The catches were down last week compared with the prior week. Early window-pane and shot-hole feeding was observed on a few V6 stage plants last week in central Ohio. Smaller plants (V2-3) that common throughout much of the state did not show any ECB symptoms. Plants approaching the V5 to V6 stage or taller need to be checked to ECB.

3) Potato Leafhopper on Alfalfa - Alfalfa needs to checked, especially new seedings, for potato leafhopper (PLH). Populations greater than 3 PLH per sweep have been found in central Ohio. New seedings cannot stand as much damage as an established stand. Treatment is justified for PLH in a new seeding anytime sampling reaches one PLH per sweep regardless of height.

F) A Few More Wheat Fields with Freezing Injury - Pat Lipps and Dennis Mills CORN Questions

We have received reports of some freezing injury in wheat in some central Ohio wheat fields. Earlier reports were from south central counties, but the potential for freezing injury apparently has occurred in more northern counties as well. Wheat producers in counties along and just north of I-70 may wish to check low-lying wheat fields with poor air drainage for freezing injury. According to our records wheat was just beginning to head in these counties when low temperatures occurred during the period of May 19 - May 22. We suspect that there will be a few isolated fields with damage, but the extent of damage cannot be predicted. The visual symptoms of frost injury of bleached out glumes on the heads are not evident. You cannot see the damage unless you inspect the heads closely. The grain in unaffected heads is now in the watery ripe to milky ripe growth stage and are about full size. Grain in freeze-damaged heads did not develop or only a few florets on the head were able to develop grain normally. As the grain continues to increase in size the glumes will spread wider, whereas glumes on the freeze affected florets will remain narrow giving the damaged heads an odd appearance. Additionally, freeze damaged florets appeared to be lighter green in color than unaffected florets on the heads, but this was not evident in all fields we examined. You will need to walk the field and inspect individual heads to see the damage at this time.

Wheat Heads Exhibiting Freezing Injury

G) Predictions for Wheat Head Scab Low With a Few Exceptions - Pat Lipps and Dennis Mills CORN Questions

Wheat is beginning to ripen in southern Ohio with predicted harvest of some fields by the end of June. Growers in central and north central Ohio can now begin to assess their fields for head scab. Reports of head scab in south central and south west Ohio have been relatively low with most fields reporting 0 to 3 percent incidence of diseased heads. 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. Predictions for head scab have been low for most other areas of the state as well. Although rain has occurred during flowering the scattered showers have not provided the hours of head wetness required for severe disease development. Additionally, the cooler temperatures have limited scab development. We are continuing to monitor the conditions in these areas and will have a more complete picture of the scab risk over time.

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.

H) More Corn Seedling Diseases, More Plant Death - Pat Lipps CORN Questions

Last week C.O.R.N. newsletter had an article on corn seedling diseases. Recent wet and cooler weather have not helped the situation at all. Many growers are reporting seedling death in replanted corn. The continued wet soil conditions are promoting continued development of soil-borne pathogens that are attacking seedling roots. These young plants are trying to grow leaves and the diseased roots are not supporting this growth causing discolored leaves and plant death. Lack of growth, reddened leaf tips and yellow leaves are all symptoms of root rot. To evaluate root rot wash off the roots with water and look for darkened areas on the seminal root system, the mesocotyl (stem between seed and crown) and nodal roots emerged from the crown. The wet soil conditions and cool soil temperatures have provided the conditions for serious seedling root diseases caused by a number of different organisms. Fusarium is the most frequent fungus isolated from seedling roots and the mesocotyl, earlier Pythium was more frequent on seedling roots.

I would like to repeat what I said last week. There is little the grower can do to help these plants. Warmer soil temperatures and less wet soil conditions are needed for the plants to produce new nodal roots. New roots are needed so adequate nutrients can be taken up to initiate vigorous growth. In most cases soils are too wet to do any cultivation, but cultivation may help get some air to the developing nodal (crown) roots. Expect poor root systems for the remainder of the year, especially on plants in poorly drained soils.

I) Drift Can and Should Be Reduced to Minimum - Erdal Ozkan CORN Questions

This time of the year is the time when Ohio Department of Agriculture (ODA) receives more complaints about spray drift than other times of the year because this is the season for spraying. About 75% of the pesticide non-compliance calls ODA receives annually are related to spray drift. The situation may get even worse as the acreage of genetically modified crops increases.
The bad news is: Spray drift occurs wherever liquid sprays are applied. The good news is: Although complete elimination of spray drift is impossible, problems can be reduced to a minimum if chemicals are applied with the proper equipment under favorable weather conditions.

Drift is more influenced by two factors more than others: wind speed and droplet size. Other factors are: spray characteristics (viscosity and volatility of the chemical applied), boom height, and other weather related things such as relative humidity and temperature.

Operator's knowledge of what causes drift is perhaps the key element in reducing drift potential. Conscientious and experienced operators rarely get into serious trouble with drift damage because they understand drift and take steps to avoid it. Here are some management strategies to reduce spray drift from boom sprayers:

1. Use nozzles that produce large droplets whenever possible, if biological effectiveness can be maintained (obtain copies OSU Extension Fact Sheets AEX-523 and AEX-524 to learn more about the new nozzles and their effectiveness in reducing drift).
   
2. Keep the boom close to the spray target.
   
3. Use greater spray volume and use nozzles with larger orifices.
   
4. Use lower system pressure and check pressure gauge accuracy.
   
5. Use drift retardants if droplet size cannot be controlled with nozzle selection.
   
6. Follow label recommendations to avoid drift with highly volatile pesticides.
   
7. Avoid spraying on extremely hot, dry days, especially if sensitive vegetation is nearby.
   
8. Do not spray when conditions are favorable for an atmospheric inversion.
   
9. Although the distance droplets will drift is a function of many other factors such as droplet size, relative humidity, temperature and boom height, it is best not to spray when wind speeds are greater than 5 miles per hour.
   
10. Avoid spraying near sensitive areas located downwind. Leave a buffer strip 50 to 100 feet wide and spray this strip later when the wind shifts.
    
11. Be sure to keep good records (wind speed and direction, temperature, relative humidity, etc. and evaluate spray results.
    
12. New label requirements may use a drift model to determine the required buffer width. Travel slower (and lower boom height) and use nozzles to produce large droplets near sensitive areas. This will reduce the required buffer zone width, then, as distance from the sensitive area increases, nozzles can be changed to produce smaller droplets, if desired, and boom height raised so travel speed can be increased.
    
13. Select the time when drift is likely to be low for spraying the parts of the field near sensitive areas.
    
14. In the future, label requirements may specify nozzles that produce a certain droplet size spectrum, such as fine, medium, coarse, very coarse, etc. Carefully check the label to determine the optimum droplet size and proper nozzle size for a spray application.

There are several OSU Extension publications (Bulletin 816; and Fact Sheets AEX-523, AEX-524 and AEX 525) on Drift that can be obtained by either contacting County Extension Offices in Ohio, or from OSUE web site, Ohioline.

http://www.ag.ohio-state.edu/~ohioline/b816/index.html
http://www.ag.ohio-state.edu/~ohioline/aex-fact/0523.html
http://www.ag.ohio-state.edu/~ohioline/aex-fact/0524.html
http://www.ag.ohio-state.edu/~ohioline/aex-fact/0525.html

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 labarge.1@osu.edu if you have problems subscribing.

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.

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.

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 |