CORN
Crop Observation and Recommendation Network

May 22-29, 2000
C.O.R.N. 2000-14

In This Issue:

A) Wheat in Flower, Powdery Mildew Still Spreading
B) Wheat Head Scab Predictions: Cool Weather Is Helping Us!
C) Factors to Consider in Corn Replant Decisions
D) Spray Adjuvants Review
E) Postemergence Reminders
F) Armyworms on No-Till Corn
G) Billbugs Feeding on Corn
H) Alfalfa Weevil and Re-growth Following Harvest of 1st Cutting

A) Wheat in Flower, Powdery Mildew Still Spreading - Pat Lipps CORN Questions

Wheat growth rate was slow due to cooler weather over the past week.  Most wheat fields in the state are flowering (growth stages 10.5.1 to 10.5.3) growth stage. The crop is heading (growth stage 10.3) and beginning to flower (growth stage 10.5.1) in the northernmost counties of Ohio.  The ten day forecast is for continued cool temperatures (lows in mid 50s and highs in low 70s) and high humidity with a high chance of scattered showers and thunderstorms this next week.

Powdery Mildew - is still spreading due to the cool temperatures and high relative humidity. The weather predicted for the next week will continue to favor the spread of powdery mildew. Growers that have sprayed for powdery mildew earlier should see the benefits from their spray . It generally takes several days for the powdery mildew to begin to be affected by the spray. The typically white mildew lesions will begin to turn tan to brown as the fungus dies. The greatest effect of the fungicide will be on the upper most leaves of the plants. Keeping the upper two leaves disease free is most important.

Stagonospora Leaf Blotch Is Spreading - in some fields due to the recent rains. The cooler temperatures last week and predicted this week will slow disease development, but rain will help splash spores up the plant to cause new infections.

B) Wheat Head Scab Predictions: Cool Weather is Helping Us!  - Pat Lipps CORN Questions

The wheat crop is now in flower throughout most of the state which is the most vulnerable time for infection by the scab fungus. The Gibberella zeae fungus is producing spores on corn residues.  The occurrence of head scab is dependent on an extended period (24-72 hours) of precipitation or high relative humidity and moderately warm temperatures (above 60) at the time of flowering to initiate the infection process. The southwest, west central, northwest and north central parts of the state have received significant scattered showers and thunderstorms plus high humidity over the past week and the extended weather forecast is for continued showers over these areas. The cold night temperatures (mid 40 F) and cool days (mid 60 F) we have experienced over the last week will likely reduce the amount of disease that may occur. Our long term data indicates that major epidemics occur when temperatures are in the 70-85 F range and temperatures below 60 F limit the amount of scab that develops.

Our prediction for now is that we will probably see some scab in wheat in southwest and west central Ohio, but the level of disease will probably not be at epidemic levels over a wide area. It is still too early to predict what will happen in northwest and north central Ohio because the wheat has just entered the flowering period. The forecast of scattered showers and cool weather indicates that some scab will likely occur in these areas too, but damage will likely be limited.  Symptoms of head scab usually begin to appear about 10 days after infection. Growers in southwest and west central Ohio can begin to look for disease symptoms (bleached spikelets on heads) at the end of this week or over the weekend. We will have more scab updates in the next CORN newsletter. For more information, visit the Fact Sheet on Head Blight or Scab of Small Grains.

C) Factors to Consider in Corn Replant Decisions - Peter Thomison CORN Questions

Replant decisions should be based on strong evidence that the returns to replanting will not only cover replant costs but also net enough to make it worth the effort. Presented here are some guidelines to consider when making a replant decision. Much of this information is available in Agronomy Fact Sheet (AGF-124-95) "Guidelines For Corn Replant Decisions".

Don't make a final assessment on the extent of damage and stand loss too quickly. A corn plant's growing point remains protected below the soil surface until six to seven leaves have emerged. Thus, early damage to above-ground foliage does not necessarily kill the plant. Generally, 2 to 4 days of 70 degree F or warmer temperatures are sufficient to stimulate new leaf growth on an affected plant. If these new leaves seem to be unfolding naturally, the plant should survive and resume normal development.

If the crop damage assessment indicates that a replant decision is called for, some specific information will be needed, including:

• Original target plant population/Intended plant stand

• Plant stand after damage

• Uniformity of plant stand after damage

• Original planting date

• Possible replanting date

• Likely replanting pest control and seed costs

To estimate after-damage plant population per acre, count the number of viable plants in a length of row that equals 1/1000 of an acre and multiply by 1000. (Table 1 shows row length needed for various row widths.) Make several counts in different rows in different parts of the field. Six to eight counts per 20 acres should be sufficient.

When making stand counts, also note plant distribution within the row. Yield loss due to stand reduction results not only from the outright loss of plants but also from an uneven distribution of the remaining ones. Studies have indicated that gaps 14 to 37 inches long reduced yields by about 2% when compared to a uniform stand, whereas 4 to 6 foot gaps reduced yields by about 5%. Therefore, the more numerous and longer the gaps between plants within the row, the greater the yield reduction.

A major consideration in making a replant decision is the potential yield at the new planting date and possibly different planting rate; this can vary depending on the hybrid used, soil fertility and moisture availability. Table 2 shows the effects of planting date and plant population on final grain yield for the central Corn Belt. Grain yields for varying dates and populations in both tables are expressed as a percentage of the yield obtained at the optimum planting date and population. Remember that table values are based on a uniform distribution of plants within the row! Add a 5% yield loss penalty if the field assessment reveals several gaps of 4-6 feet within rows and a 2% penalty for gaps of 1-3 feet.

When making the replant decision, seed and pest control costs must not be overlooked. Depending on the seed company and the cause of stand loss, expense for seed can range from none to full cost. You also need to review herbicide and insecticide programs under late‑planting conditions. For instance, it may be necessary to re-apply herbicides, especially if deep tillage is used. However, try to avoid such tillage depending instead on postemergence chemicals or cultivation for weed control.

Concerning insect control, if insecticides were applied in the row at initial planting, consider re-application if tillage is used before replanting. Also remember that later planting dates generally increase the possibility of damage from insects such as European corn borer, corn rootworm beetle and black cutworm. Therefore, understand that replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season will be based mainly on plant stand and plant distribution. Later in the season as yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.

The cost of replanting will differ depending on the need for tillage and chemical application. The cost and availability of acceptable seed will also be considerations. These factors must be weighed against expected replanting yield gains. If after considering all the factors there is still doubt as to whether or not a field should be replanted, you will perhaps be correct more often if the field is left as is.

Sources:  Guidelines for making corn replant decisions. 1990. National Corn Handbook. NCH-30; Illinois Agronomy Handbook. 1994. University of Illinois.

Table 1. Row length required to equal 1/1000 acre when corn is planted at various row widths.

Table 2. University of Illinois replant chart developed under high yielding conditions - expressed as a percent of optimum planting date and population yield, uniformly spaced within row.(adapted from Nafziger, 1994)

D) Spray Adjuvants Review - Mark Loux and Jeff Stachler CORN Questions

Spray adjuvants include all materials added to a sprayer with the herbicides to modify the properties of the spray particles or the performance of the herbicides. Drift retardants, anti foamers, compatibility agents, pH adjusters, crop oil concentrates, surfactants, and fertilizers can all be considered adjuvants. The latter three of these have a direct effect on the effectiveness of postemergence herbicides, and proper adjuvant selection is essential to maintain consistent control and avoid excessive crop injury. A few points to consider as we get ready to apply postemergence herbicides.

Herbicide labels often contain detailed adjuvant recommendations along with information on adjuvant selection under different environmental conditions. Check labels for this information, and contact your dealer and/or the product representative if you have questions.

Purchase a known brand of adjuvant from a reputable source. Anyone can make and sell an adjuvant without EPA approval, and claims on the adjuvant container are no guarantee of performance. Steer clear especially of adjuvants that claim to do everything or show no information on their composition. Many, but not all, proven adjuvants have a list of ingredients that at least show the percentage oil or active agents. Seed oil concentrates (methylated seed oils) should specify that they contain methylated oil. Seed oil products that do not use a methylated seed oil may be less effective than a petroleum oil concentrate. Avoid products that claim to allow reductions in herbicide rates.

In general, herbicide activity on the crop and weeds is greater with petroleum oil concentrates, compared to nonionic surfactants. Some products specify use of a surfactant unless dry conditions exist, when a petroleum oil concentrate may improve control. Use of a methylated oil may or may not increase activity depending upon the herbicide, weed, and environmental conditions. Some labels do not specify whether petroleum oil concentrate or methylated seed oil should be used, but instead use the general term of crop oil concentrate.

Methylated seed oils are most likely to improve control under dry conditions, when weeds are large, or when antagonism is possible in tank-mixes.  In general, ALS inhibitors respond best to use of methylated seed oils under these conditions. The activity of Flexstar is also most consistent with a methylated oil. Use of a methylated oil may increase crop injury, but this is herbicide dependent. Injury from Raptor can increase considerably when methylated oil is used instead of surfactant, but injury from Pursuit does not increase nearly as much. In a two-year study we conducted with Accent, methylated seed oil increased activity on weeds compared to surfactant and petroleum oil concentrate under dry conditions, but there was no difference in corn injury.

Postemergence grass herbicides for soybeans (Fusion, Select, etc) are most effective with a petroleum oil concentrate. Activity does not increase with methylated oils.

Glyphosate products are most effective with a nonionic surfactant. Most glyphosate products except Roundup Ultra, Glyphomax Plus, and Touchdown require the addition of surfactant at a rate of 0.5 to 1% by volume. The Touchdown label does not require additional surfactant, but suggests that more surfactant can be added. Use of ammonium sulfate can improve control of some weeds (velvetleaf, for example) and overcome the negative effect of hard water on glyphosate activity. Where hard water is not a problem, the addition of ammonium sulfate is likely to be less beneficial for postemergence treatments, compared to spring burndown treatments.

Use of a fertilizer solution (28%, 10-34-0) or ammonium sulfate is specified with many postemergence herbicides for control of specific weeds or to improve activity in general. Where a rate range is specified on the label, the higher rates are typically used under dry conditions. Increasing rates when not necessary can increase crop injury. Ammonium sulfate seems to cause less crop injury than fertilizer solutions.

There does not appear to be any benefit to adjusting the pH of spray solutions, and avoiding the use of pH adjusters will result in a more economical herbicide program.

It has been difficult for us to assess the value of silicon surfactants. They appear to be very effective wetting agents, and some research has shown them to allow a shorter rain-free period. We are not sure this justifies a higher cost, given that it is extremely difficult to find any research that shows improved weed control with silicon products compared to other adjuvants.

E) Postemergence Reminders - Mark Loux and Jeff Stachler CORN Questions

Avoid application during periods of abnormally cold or dry conditions if possible. Many herbicides are less effective when night temperatures drop below 45 to 50 degrees. The growth of weeds also slows during cold periods, so there is likely to be little drawback to waiting until warmer conditions resume. Some herbicides, such as Liberty, recommend no application during evening hours due to reduced activity. Sunlight is necessary for the activity of many contact herbicides, and activity may be slower or reduced during periods of cloudy weather.

Extended dry conditions are more difficult to deal with than short periods of cold weather. Under dry conditions, weeds tend to become progressively more difficult to kill with herbicide even if they do not appear to increase much in size. Our suggestions for dry weather

1. adjust adjuvant rate and type to maximize activity.

2. apply grass and broadleaf herbicides separately to avoid antagonism.

3. if rain is in the immediate forecast you might consider waiting to spray until after the rain. However, small weeds are much easier to kill than large weeds under dry conditions, so we suggest applying postemergence herbicides when weeds are small if no significant rain appears to be in the immediate forecast.

It can be difficult to predict crop response from postemergence herbicides. We have observed the most severe injury when the crop has not been exposed to stress conditions prior to treatment, or when extremely hot conditions occur around the time of application. For example, if moderately cool and wet conditions have occurred since planting, the crop is more likely to be injured compared to when it has undergone some periods of hot/dry weather. Extended periods of dry weather may actually make the crop less sensitive to herbicides. However, the crop may also fail to rapidly outgrow any injury suffered if dry conditions persist after application.

Injury to corn becomes more likely as the corn becomes more advanced in growth stage.  Sensitivity to growth regulators increases when corn reaches the 5-leaf stage (or at about 8 to 10 inches tall), and lower rates and/or directed applications should be used to avoid injury. In general, sensitivity of corn to ALS inhibitors increases as corn approaches the 6-collar stage or a height of about 20 inches. Some ALS inhibitors can be applied past this stage using drop nozzles. We have observed increased sensitivity of corn to ALS inhibitors during periods of cold weather, independent of corn size.  

In general, we have observed soybeans to become more tolerant of herbicides as they become larger. Soybeans planted early seem to be able to outgrow fairly severe injury with little to no yield loss if rain is sufficient for crop growth. The greatest potential for yield loss in soybeans seems to be when late-planted soybeans suffer moderate to severe injury and have insufficient time to outgrow symptoms prior to seed set. Try to avoid herbicide injury to double-crop soybeans for this reason.

In corn, to avoid yield loss from early weed interference, total postemergence herbicide programs should be appplied before most weeds exceed about 4 inches inches in height, before corn exceeds the V4 stage, and before about 25 days after planting. This is dependent upon weed density, however, and timing is less critical when postemergence herbicides are applied following a preemergence herbicide that has reduced the density of and suppressed the growth of the weeds. Total postemergence programs applied at the 4-inch weed stage should include a herbicide with residual activity to control later-emerging weeds that can reduce yield.

In soybeans, to avoid yield loss from early weed interference, total postemergence herbicide programs should be applied before most weeds exceed about 6 inches inches in height. Timing may be less critical where a preemergence herbicide has been applied for early-season control/suppression of weeds. Most soybean herbicides are labeled for control of weeds in the 2- to 6-inch range, although some can be controlled when larger.

Giant ragweed presents problems in total postemergence programs, since it grows twice as fast as most other weeds and can emerge after early postemergence applications. We suggest a preemergence + postemergence program or split postemergence program for most consistent control of dense giant ragweed populations. In a split postemergence program, the first application can be made when ragweed are no more than 6 inches tall, and followed with another application two to three weeks later. This is also a good strategy for dense foxtail populations.

Antagonism, or a reduction in the activity of grass herbicides, can occur when they are tank mixed with certain broadleaf herbicides. This is most often observed in soybeans, but can occasionally occur with Accent in corn as well. Antagonism will be most evident for grasses that are more difficult to control, such as barnyardgrass, yellow foxtail, and crabgrass, and when grasses are large and/or under drought stress. To avoid problems with antagonism:

1. increase grass herbicide rates and adjust adjuvant type and rates.

2. apply grass and broadleaf herbicides separately.

3. apply when grasses are small and actively growing.

For more information, visit the Weed Control Guide for Ohio Field Crops 2000 Edition - Bulletin 789-99.

F) Armyworms on No-Till Corn - Hal Willson CORN Questions

Field corn planted no-till into grassy habitats should be monitored closely at this point in time for armyworm activity. Fields that may be at risk for significant armyworm infestations include corn planted no-till in rye cover crops and corn planted no-till into old hay fields. A severe infestation of armyworm can reduce stand when an infestation occurs in the pre-whorl stage and cause significant defoliation when corn is hit in the whorl stage. Total destruction of a field of no-till corn can occur if a severe infestation is allowed to develop without application of a rescue treatment.

Detection of foliar feeding injury by armyworm on 15 to 20% of a stand should be regarded as an indicator of a potential problem, and the field should be rechecked within a few days to determine whether the impact of defoliation is increasing and a rescue treatment may be warranted. In general, a severe infestation will impact almost 100% of a stand and defoliation of the plants will exceed 50%, stand height is being reduced, and some plants being eaten down to ground level. If defoliation remains less than 50% and the new growth exhibits minimal feeding injury, the stand will likely recover with minimal impact on yield.

For more information, visit the Fact Sheet on Armyworms.

G) Billbugs Feeding on Corn - Hal Willson CORN Questions

Received a report during the past week from Ron Becker (Wayne Co. IPM program) that a number of fields were exhibiting above normal activity of billbug feeding injury. Billbugs tend to feed on early corn causing numerous feeding holes in the emerging foliage. When a billbug infestation is heavy, some plants will actually appear deformed as the growing tips tend to become entwined in the feeding holes. In general, a corn stand tends to overcome the effects of billbug injury as the stand develops into the early whorl stage and rescue treatment is not warranted.

H) Alfalfa Weevil and Regrowth Following Harvest of 1st Cutting - Hal Willson CORN Questions

Reports have been received that some fields of alfalfa are requiring stubble treatments to control adults and larvae of the alfalfa weevil that are retarding regrowth of alfalfa following harvest of the 1st cutting. In cases where regrowth appears to be retarded 7 to 10 days after harvest due to feeding activity of weevil adults or larvae, application of a stubble spray may be warranted.

Although a harvest of alfalfa will generally decimate the weevil population, surviving larvae will continue feeding on whatever alfalfa remains in the field. In an experiment where we turned under an alfalfa stand in late April to enhance seed maggot on a corn emergence trial, I actually found young weevil larvae feeding on alfalfa stems below the soil surface two weeks after planting.

For more information, visit the Fact Sheet on Alfalfa Weevil.

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.

Contributors to C.O.R.N. this week include: State Specialists: Peter Thomison (Corn Production), Pat Lipps (Plant Pathology), Anne Dorrance (Plant Pathology),Erik DeWolfe (Plant Pathology), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Hal Willson (Entomology) and Ron Hammond (Entomology); District Specialists: Ed Lentz (Agronomy); Extension Agents: Dave Jones ( Allen), Steve Bartels (Butler), Barry Ward (Champaign), Greg LaBarge (Fulton), Gary Wilson (Hancock), Mike Estadt (Pickaway), Glen Arnold (Putnam), Clark Hutson (Seneca) and Roger Bender (Shelby).

Editor: Clark Hutson        Web Editor: Nathan Watermeier

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.

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