http://www.ag.ohio-state.edu/~corn/

CORN
Crop Observation and Recommendation Network

For May 17-24, 1999
C.O.R.N. 99-9

In This Issue:

A) Rainfall Shortage: Weed Control Failures Preemergent Herbicides:
B) Timing Of Postemergence Herbicides In Corn:
C) Alfalfa Pests Update:
D) Field Corn Pests Update:
E) Soybean Pests Update:
F) Wheat Crop Remains Excellent, No Major Problems:
G) Weather Conditions Can Favor Development Of Scab:
H) Wheat Virus Diseases:
I) Replant Considerations For Corn:
 

A) Rainfall Shortage: Weed Control Failures Preemergent Herbicides - Jeff Stachler CORN Questions

Another week has gone by with most of the state not receiving enough rainfall to properly incorporate preemergence herbicides. These fields must be scouted immediately to determine what weed control options are available. Fields that had Atrazine products or Balance applied may have an opportunity for "reach back" control if soils are saturated for several days to a week. But do not count on this to occur. The two major options are mechanical or chemical control. Fields where preemergence herbicides were applied in the last 10 to 14 days, a rotary hoeing may still be effective as long as the weeds have not emerged. This will buy some time until significant rainfall occurs. Fields that were rotary hoed last week may need to be rotary hoed again this week or switch to cultivation in the next few weeks.

For those wanting to use chemical weed control some things must be considered. Timing of the postemergence herbicides, the stage of corn development, and the right herbicide for the weeds present. Too early of postemergence application or too late of application to weeds greater than 4-6 inches in height, especially annual grasses, can cause significant yield losses in corn. Fields with full rates of preemergence herbicides and weeds coming up, should be sprayed very soon if reduced rates of herbicides are planned. Generally this works well but, significant rainfall is still needed to activate the preemergence herbicide or else the postemergence application may be too early. Waiting until weeds approach 4-6 inches requires the use of full rates of postemergence herbicides, but will assure better season-long control.
 

B) Timing Of Postemergence Herbicides In Corn - Jeff Stachler CORN Questions

Before applying postemergence herbicides in corn, scout the field to determine the stage of corn growth and what weeds present and their size. Always read the label to assure the appropriate additives are being added for the herbicides being used. Below are the height restrictions and comments about postemergence herbicides in corn:
 
2,4-D Broadcast up to 8 inches, directed spray after 8 inches. 2,4-D amine is safer to corn but a little less effective for weed control, compared to 2,4-D ester. Controls many broadleaf weeds.
Accent Broadcast up to 6 collar or 20-inch corn, directed sprayed up to 10 collar or 36-inch corn. Controls some annual and perennial grasses and a few broadleaf weeds.
Aim New for 1999. Apply up to 8 collar corn. Will burn corn leaves, but quickly recovers. Excellent on velvetleaf and helps control some other broadleaf weeds.
Atrazine Apply up to 12-inch corn. Controls most broadleaf weeds and some small annual grasses.
Banvel/Clarity Broadcast up to 5 leaf or 8-inch corn, after 8 inches apply 8 ounces or less as a broadcast treatment or apply as a directed spray to decrease the risk of corn injury. Controls most broadleaf weeds.
Basagran No corn height restrictions. Controls some broadleaf weeds, yellow nutsedge and Canada thistle.
Basis Apply before corn reaches 3 collar stage. Controls some annual grasses and broadleaf weeds.
Basis Gold Apply before 12-inch or 6 collar corn. Controls many small annual grasses and broadleaf weeds.
Beacon Broadcast 4 to 20-inch corn, directed spray before tassel emergence. Controls some broadleaf weeds and a few grasses.
Buctril/Moxy Apply before tassel emergence. Controls most annual broadleaf weeds.
Distinct New for 1999. Apply 6 ounce rate to 4 to 10-inch corn and 4 ounce rate to 10 to 24-inch corn. Controls nearly all annual and perennial broadleaf weeds, including Canada thistle and hedge bindweed. Distinct may suppress or control some annual grasses if less than 2 to 3 inches tall and the 6 ounce rate is used, but use it for grass control very cautiously until you feel comfortable with this product.
Exceed Broadcast from 4 to 20-inch or up to 6 collar corn, directed spray up to 30-inch corn. Controls many annual broadleaf weeds.
Hornet Broadcast up to 20-inch or 6 collar corn. Controls some broadleaf weeds and Canada thistle.
Liberty Apply only to Liberty tolerant corn up to 24 inches or 7 collars. Controls nearly all annual grasses and broadleaf weeds.
Lightning Apply only to IMI-tolerant corn before 12 inches tall. Controls many annual grasses and broadleaf weeds.
Permit Apply through the layby stage of corn. Controls some annual broadleaf weeds.
Pursuit Apply only to IMI-tolerant corn. No corn height restrictions. Controls some annual grasses and broadleaf weeds.
Resource Apply up to the 10-leaf stage. Controls velvetleaf and helps with a few other broadleaf weeds.
Roundup Ultra Apply only to Roundup Ready corn up to 8 collar or 30-inch corn. Controls most annual and perennial grasses and broadleaf weeds.
Sencor Apply up to tassel emergence. Controls a few annual broadleaf weeds.
Spirit Broadcast from 4 to 20-inch or up to 6 collar corn, directed spray up to 24-inch corn. Controls many broadleaf weeds and a few grasses.
Stinger Apply up to 24-inch corn. Controls a few broadleaf weeds and Canada thistle.
For other premixes or tank mixtures not listed above apply those herbicides to the corn based upon the most restrictive herbicide listed above. For additional information consult the Weed Control Guide for Ohio Field Crops, pages 34 to 60, and the herbicide label.

C) Alfalfa Pests Update - Hal Willson  CORN Questions

Reports of economic alfalfa weevil activity have been received from many areas of the state. In addition, harvest of first cutting is underway. Where weevil activity was high and harvest has been implemented, regrowth should be monitored to determine whether weevil feeding is delaying development. However, in most cases the harvest operation will have a devastating impact on the weevil population, and many late instar larvae are currently pupating.

Alfalfa weevil activity continues to intensify in some fields where alfalfa has not reached the optimal point of harvest. In some cases, weevil activity appeared to be non-economic a week or two ago, but intensified during the final stages of alfalfa growth. In such cases, an additional week of growth is desirable, the weevil is reducing new growth, and a grower is reluctant to run over a field to apply a treatment that may achieve marginal results.
In summary, the options come down to:

  1. harvesting the first cutting a little early to prevent further losses,
  2. applying a rescue treatment that achieves marginal results, or
  3. delay harvest without treatment and hope that weevil activity declines as late instar larvae pupate.
Given such a scenario, selection of an option will finally depend on the value of the crop and feasibility of a timely harvest. During the past week at the Western Branch, we chose to treat one set of plots where optimal yields were regarded as essential and delay harvest without treatment of other plots where the need for optimal yields was questioned.

Potato leafhopper has arrived. During the past week, sweep net samples of alfalfa in Clark county have collected on the average about two adult leafhoppers per ten sweeps. The current presence of adult leafhoppers in tall first cutting alfalfa is not a problem, but regrowth of alfalfa following the harvest of first cutting of alfalfa should be monitored.
 

D) Field Corn Pests Update - Hal Willson CORN Questions

Corn flea beetles can readily be found on emerging corn, and reports have been received that some growers have applied treatments to reduce flea beetle activity. When present in abundance, flea beetles may reduce stand. In addition, flea beetles may transmit Stewart's Wilt on susceptible varieties. In regard to stand loss, flea beetle numbers need to be very heavy at a period of time when plant growth is minimal to cause enough stand loss to justify a rescue treatment. In regard to Stewart's Wilt transmission, only a few beetles are needed to transmit the disease and the disease would likely be transmitted by the time a rescue treatment is applied. Therefore, the primary justification for applying a rescue treatment for flea beetles would be to prevent stand loss, and such action would only be justified when it can be confirmed that the beetles are actually causing stand loss. In many cases, foliar feeding injury due to flea beetle may appear to be severe, but once growing conditions improve plant growth will resume and the perceived impact of flea beetle will be non-significant.

Given above normal early spring temperatures and forthcoming emergence of corn that should follow some mild rains, corn fields should be periodically scouted to detect early cutworm and stalk borer activity. At the Western Branch, significant flight activity of black cutworm was observed in late April, but cutworm flights declined to zero during the past two weeks. Given the complete break in weather fronts between the western and eastern corn belt regions, it appears that migratory flights of cutworm via the Ohio river valley region from Texas and southern Missouri have been totally disrupted.
 

E) Soybean Pests Update - Hal Willson  CORN Questions

Reports have been received that bean leaf beetle is causing significant injury on early planted soybean fields. As usual, the first soybean fields to emerge will be jumped on by the over-wintering adult bean leaf beetles. Rescue treatment of heavily infested fields may be warranted if the beetles are causing losses in stand. As additional soybean fields emerge, the bean leaf beetle populations will become dispersed and the risk for significant injury to emerging stands will be reduced.

Sweep net sampling of alfalfa has indicated that the abundance of over-wintering bean leaf beetles may be above normal. If a significant number of bean leaf beetles become established on an early planted soybean crop, production of significant first and second generations of bean leaf beetles may follow. The last year of significant bean leaf beetle activity was in 1994, and severe pod injury due to bean leaf beetle leading to significant moldy bean problems was last observed in 1991.
 

F) Wheat Crop Remains Excellent, No Major Problems - Pat Lipps  CORN Questions

Wheat throughout the state is in some stage of heading or flowering (GS 10.1 to 10.5.1) Most areas of Ohio remained dry, but in those areas that received some moisture powdery mildew continued to develop on susceptible varieties.

We have reports of powdery mildew at or above threshold levels in some fields of susceptible varieties in north central Ohio. Mildew does not need rain to develop, just lush stands of wheat and high relative humidity. If you detect powdery mildew on the leaf below the flag leaf or on the flag leaf an application of a fungicide would be warranted, especially in fields with high yield potential. Visit last week's CORN on the web to see a picture of powdery mildew on a plant at threshold level.

The lack of rain has still greatly limited the spread of Stagonospora leaf blotch. However, we are seeing a few small chocolate brown spots on plants of susceptible varieties. We will not see much development of this disease until we get significant rain showers to spread the spores. If wheat leaves remain clean for the next two weeks, yield losses from Stagonospora will be limited.
 

G) Weather Conditions Can Favor Development Of Scab - Pat Lipps  CORN Questions

The wheat crop is now in its vulnerable stage for Fusarium head scab development. If the weather turns wet, with 2 to 3 days of consecutive rain showers then head scab could develop. The weather that occurs in Ohio over the next week will be critical to head scab development. We will not be able to detect symptoms in the fields for about 2 weeks after flowering.
 

H) Wheat Virus Diseases - Pat Lipps CORN Questions

Four different virus diseases have been detected in Ohio wheat this year. Luckily, most situations are restricted to certain spots in a few fields. Now is the best time to see virus symptoms in wheat.

Barley Yellow Dwarf is usually found in early planted fields and affected plants are generally scattered throughout the stand. Reddish and yellow flag leaf tips that stand erect in the field are common symptoms. Aphids carry the virus from plant to plant. Yield loss is proportional to the percentage of plants affected and the severity of the stunting. All varieties are susceptible to some level.

Wheat Spindle Streak Mosaic is the most common virus disease found in the state. This virus is spread by a soil-borne fungal vector. Generally most plants in a field show symptoms, giving the field a yellow coloration. Small, yellow spindle-shaped dashes in the upper leaves are characteristic of this disease. There are numerous resistant varieties that eliminate any chance of disease development or yield loss. Presence of this disease indicates that a susceptible variety was planted.

Soil-borne Wheat Mosaic was detected in Ohio for the first time this year and appears to be somewhat rare. This virus is spread by the same soil-borne fungus that wheat spindle streak mosaic is spread by. Affected plants are usually in lower areas of the field, but wet fields can have affected plants throughout. The most visible symptom is severe stunting (termed rosetting) on highly susceptible varieties. Affected plants do not grow much in the spring and remain stunted. A mild mosaic symptom (yellow and green colored areas) are seen in the youngest leaves. Yield losses are great because affected plants produce little grain.

Wheat Streak Mosaic is also rare in Ohio, but we have seen more cases in the last 4 years than in the previous 15 years. This virus is carried by a mite, which moves from volunteer wheat, or sometimes corn, in the fall to the newly planted wheat crop. Infected plants have thin green and yellow streaks on the leaves, with the older leaves showing more severe symptoms than younger leaves. Many times affected plants do not produce heads, so this virus disease can cause severe yield losses. Destroying volunteer wheat three weeks prior to planting the new wheat crop is essential for limiting wheat streak mosaic.
 

I) Replant Considerations For Corn - Peter Thomison CORN Questions

Dry, crusted surface soils may have caused poor emergence in certain areas of the state. The threat of stand reductions or uneven emergence force the unwanted and difficult decision of whether or not to replant. Given how early planting was this year, prompt replanting may result in little yield loss. Nevertheless a replant decision 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 follow when making a replant decision. Much of this information is available in Agronomy Fact Sheet (AGF-124-95) "Guidelines For Corn Replant Decisions" - also accessible at http://ohioline.ag.ohio-state.edu/agf-fact/0124.html
 

When To Assess Crop Damage

The first guideline is not to 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.

Facts Needed For A Replant Decision

If the crop damage assessment indicates that a replant decision is called for, some specific information will be needed, including: 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.
 
Table 1. Row length required to equal 1/1000 acre when corn is planted at various row widths.
Row Width 
(inches)
1/1000 acre 
(feet)
20
26.1 
28
18.7 
30
17.4
36
14.5
38 
13.8
40
13.1
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.

Guidelines For Making The "Right" Decision

A major determinant 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 effects of planting date and plant population on final grain yield for the central Corn Belt. Grain yields for varying dates and populations are expressed as a percentage of the yield obtained at the optimum planting date and population.
 
 Table 2. Central Corn Belt grain yields for corn planted at various dates and population rates, expressed as a percent of optimum planting date and population yield (uniformly spaced within row).
Planting Date Plants per acre at harvest
 
12,000
14,000
16,000
18,000
20,000
22,500
25,000 
4/20 
72 
78 
83 
87 
90 
93 
95 
4/25 
75 
81 
86 
90 
93 
96 
98 
5/1 
77 
83 
88 
92 
95 
98 
100 
5/6 
78 
83 
88 
92 
95 
98 
100 
5/11 
77 
83 
88 
92 
95 
98 
99 
5/16 
75 
81 
86 
90 
93 
96 
98 
5/21 
73 
78 
83 
87 
91 
94 
95 
5/26 
69 
75 
80 
84 
87 
90 
92 
5/31 
64 
70 
75 
79 
82 
85 
87 
6/5 
59 
64 
69 
73 
77 
80 
81 
6/10 
52 
58 
63 
67 
70 
73 
75
*Adapted from NCH-30 "Guidelines for Making Corn Replant Decisions"
Here's how Table 2 might be used to arrive at a replant decision. Let's assume that a farmer planted on April 25 at a seeding rate sufficient to attain a harvest population of 25,000 plants per acre. As a result of poor seedbed conditions and an unfavorable environment during emergence, the farmer determined on May 15 that his stand was reduced to 16,000 plants per acre. Table 2 shows that this would likely result in a 12 percent final grain yield loss (i.e., 98 percent expected yield for a 25,000 plant per acre final population planted April 25 minus 86 percent expected yield when stand is reduced to 16,000 plants per acre).
If he could replant the next day (May 16) at his original seeding rate, the farmer's final yield (according to Table 2) should be 98 percent of optimum, which is the same as that expected from the April 25 planting. Being 12 percent better than the present reduced stand projected yield, replanting appears profitable.

But what if stand reduction occurred later or conditions delayed replanting until May 31? Table 2 shows that replanting at the initial seeding rate would yield 87% of optimum, which is only 1 percent better than the expected yield from the reduced population. Thus, replanting definitely could not be justified.

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.

Determining Seed And Pest Control Costs

When making the replant decision, these factors 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 reapply 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.

Adapted from: Guidelines for making corn replant decisions. 1990. National Corn Handbook. NCH-30.

More Information

Where tables are included every effort is made to transmit the tables intact. Due to the setup of different e-mail receiving programs tables may not be readable due to shifts in lines and font sizes etc. Readers can visit our web site when table formats are unreadable. The newsletter is posted on the web site by Tuesday noon at: http://www.ag.ohio-state.edu/~corn/index.html

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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 & Anne Dorrance (Plant Pathology), Hal Willson (Entomology), Peter Thomison (Corn Specialist), and Jeff Stachler (Weed Science); DISTRICT SPECIALIST: Ed Lentz (Agronomy, Northwest); EXTENSION AGENTS: Glen Arnold (Putnam), Dennis Baker (Darke), Roger Bender (Shelby), Clark Hutson (Seneca), Dave Jones (Allen), Larry Lotz (Fayette), Steve Prochaska (Crawford), Steve Ruhl (Morrow), Howard Siegrist (Licking), Barry Ward (Champaign), Ray Wells (Ross), and Gary Wilson (Hancock).

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.

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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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