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For April 26 - May 2, 1999
C.O.R.N. 99-6
In This Issue:
A) Burn Down Weed Control
B) ALS-Resistant Common and Giant Ragweed in Ohio
C) Herbicides for Wheat
D) Wheat growth, disease and fungicides
E) Cutworm & Armyworm Adults Flying
F) Alfalfa Weevil Activity
The early emerging weeds such as Triplex, lambs quarters, giant and common ragweed, Canada thistle and smartweed are off to a great start this spring. The next group of weeds such as velvetleaf and giant foxtail began emerging about 10 days ago. At this point in time Atrazine in corn and Sencor and Canopy in soybeans can effectively control emerged grasses as long as they are less than two inches and less than 0.5 to 1.0 inches tall, respectively. Add crop oil concentrate and a nitrogen source to improve control. All no-till fields should be scouted to determine the need for Roundup Ultra or Touchdown to control emerged annual grasses or perennial weeds.
If weeds are present in planted fields and the corn and soybeans are just starting to emerge, do not use Roundup Ultra or Touchdown, as the crops can be killed. The use of Gramoxone Extra would be much safer to the crop for this type of application, especially for corn. There may still be some risk of soybean loss with the Gramoxone Extra if too much of the plant is above the soil.
It has been mentioned that Canada bluegrass is becoming more prevalent in no-till fields. High rates of Roundup Ultra or Touchdown are needed to control Canada bluegrass. Now is not the right stage of development for herbicide applications to be effective and it would be nice to have the daytime temperatures above 70 degrees F and nighttime above 50 degrees F for at least 3 days before and after application to maximize control. If planting Roundup Ready soybeans, apply less than one quart of Roundup Ultra or Touchdown for Burn down, unless other species require more, and then apply 2.0 quarts of Roundup Ultra when the bluegrass is in the boot to early head stage of development. This program may not provide complete control. The best time to control the Canada bluegrass is in the fall with maximum growth, sometime about mid-October, and the use of 2.0 quarts of Roundup Ultra or 3.2 pints of Touchdown. Add ammonium sulfate to increase activity.
Research conducted by OSU and Dow AgroSciences has confirmed the presence of ALS-Resistant common and giant ragweed. ALS herbicides with ragweed activity include Classic, Canopy, Canopy XL, Synchrony STS, Exceed, Spirit, Beacon, Scepter, Squadron, Detail, Steel, Pursuit, Raptor, Lightning, and FirstRate. These herbicides fall into three different classes of chemistry and the ragweeds may be resistant to all three classes of chemistry, called cross-resistance. The areas at most risk at this time are fields in which soybeans have been grown for 2 or 3 consecutive years and an ALS herbicide applied each season. Common ragweed is more likely to be resistant than giant ragweed at this time.
There is no need to change your herbicide program at this time, unless otherwise confirmed or if at high risk for ALS-resistance. If the ALS herbicide does not control the ragweeds with the first application, then do not respray with the same or different ALS herbicide. Flexstar, Cobra, or Roundup Ultra on Roundup Ready soybeans are the only other herbicides that will control common and giant ragweed postemergence in soybeans.
If poor ragweed control is observed in the field and it is suspected of being ALS resistant please call Jeff Stachler at (614-292-1393).
Wheat has progressed beyond the jointing stage. Banvel or Clarity should no longer be used on wheat in Ohio as severe injury may occur. Check the stage of wheat before applying any herbicide as time for herbicide applications is quickly disappearing. The safest herbicides that can be used anywhere in Ohio at this time are Buctril and Stinger. Other products may be used but the wheat must be staged before applying the herbicide.
Wheat fields throughout the state appear to be in excellent condition with very high tiller numbers. Tiller numbers have ranged from 35 to 125 per foot of row depending on planting date seeding rate and fertility. Cooler temperatures over the past five days have slowed the development of the wheat crop down a bit. Wheat growth stage is one week later than last year, but about one week earlier than the long term average. Wheat growth stages in Ohio range from flag leaf emergence (growth stage 8) in the southern part of the state to fields in first node (growth stage 6) to second node stage (growth stage 7) in more northern areas. We are well beyond the time for herbicides that are restricted to applications before growth stage 6 (first node visible). Check the label and the growth stage of the wheat before applying any herbicides.
Overall disease level in Ohio is still quite low. Recent surveys have indicated that powdery mildew is common in many fields, especially on susceptible varieties. Stagonospora leaf blotch has also been detected on the lower leaves in some fields. It also appears that Barley Yellow Dwarf my be common in many fields as well. We are now attempting to determine the presence of the virus in plants with symptoms to confirm our diagnosis. It is always important to conduct the correct laboratory tests to confirm virus diseases because symptoms mimic so many other problems unrelated to virus infections. We have collected plants from fields of a number of different varieties. This virus disease is transmitted by aphids that visit fields early in the fall. Generally, the early planted fields are most affected, but the extended warm temperatures last autumn may have extended the time period when aphids were active. We should expect considerable variation in the amount of virus disease among fields because of differences in planting dates and aphid movements. Symptoms of Barley Yellow Dwarf include stunting of plants and yellowing of leaf tips. Some varieties have leaf tips that also turn purple. The amount of yield loss associated with this virus disease is variable, but it is related to the extent of the stunting of the plants and the percentage of plants affected in the field. There is nothing we can do at this time to prevent yield losses in affected fields. We will know more about the extent of the problem as plants move into the flag leaf emergence to boot stage of development.
Cold night temperatures have probably limited the development of powdery mildew and Stagonospora blotch throughout the state. We have seen Stagonospora leaf blotch on susceptible varieties in different locations throughout the state, the wet weather over the last two weeks have contributed to its spread. We suggest that fields of the following varieties be scouted now for evidence of powdery mildew: Pontiac, Cardinal, Clark, Dynasty, Grant, Gregory, Patterson, Patton, Podach, Kingston, Maners, Stine 481, Stine 480, DG-411, GR983, SR205, RS927, 2510, 2540, RS901, RS927, 537W, 529W, SC1358, SG1550, SG1555, TS8040, TS4020 and W9140. In most cases the development of the disease has not progressed to the level where a fungicide application should be made. Fields should be scouted just prior to flag leaf emergence to determine if the upper leaves are in danger of infection. It is important to note that the disease is present in the field so these field can be scouted at a later date, if necessary.
The economics of using a fungicide are most favorable when controlling disease on a susceptible variety in fields with a high yield potential (70-100 bu/A). Costs for a fungicide plus application are around $14-17/A. Scout fields by randomly selecting 30 or more tillers throughout the field. There are 4 main leaves on each tiller of most modern varieties. Examine each leaf on each tiller and assess disease from the top leaf downward on the stem. A fungicide application is most beneficial when the top two leaves (flag leaf and the one below it) are in danger of becoming infected before flowering. Apply fungicides to susceptible or moderately susceptible varieties. Apply Tilt or Bayleton when disease lesions are just being detected on the second leaf (1% leaf area affected). If Quadris us used, applications should be made before the upper leaves become infected. Fungicides should be applied in 20 gal water/A with ground equipment or in 5 gal water/A by airplane. This amount of water must be applied with the fungicide to ensure good coverage and acceptable performance.
Below is a list of fungicide and fungicide combinations available for use on wheat for powdery mildew, Stagonospora leaf blotch, and leaf rust:
Adult moths of the black cutworm and the common armyworm have been active during the past two weeks based on collections in pheromone traps at the Western Branch station in Clark County. Given the warm condition in early April, it is very likely that significant number of black cutworm moth were carried in on early spring weather fronts from their overwintering habitats in the south, and early emergence of the armyworm moths may have also been influenced by mild early spring conditions.
Black cutworm is a common early season pest of Ohio corn, especially in the southern half of the state that is most influenced by jet stream weather fronts coming up the Ohio River Valley from south. At the Western Branch Station in Clark County, we annually observe cutworm activity. In contrast, periodic infestations in our plots at the Northwest Branch in Wood County are rare.
Cutworm activity in our first year plots at the Western Branch have been relatively mild during the past two years. Although a Regent trial was impacted by a heavy cutworm infestation in 1997 and a Bt-corn trial had about 10% stand loss in 1998, most of our no-tillage first year corn trials in 1997 and 1998 experienced marginal levels of stand loss due to cutworm. However, in previous years, we have had a number of trials where stand losses ranged from 10 to 50%. Years of above normal cutworm activity have often coincided with growing seasons having favorable early spring planting conditions.
When significant infestations of cutworm occur, preventive treatment with a granular soil insecticide with cutworm activity (Aztec, Force or Lorsban) at planting provides the best protection. In addition, application of a broadcast pre-emergence treatment such as Pounce, Warrior or Lorsban will provide good stand protection against cutworm. Routine application of either type of preventive treatment for cutworm is only justified if the annual benefit of stand saved over time outweighs the cost of the treatment.
The standard recommendation for cutworm control in most of the corn belt is to monitor corn closely during the early stages of development and apply a rescue treatment when needed. Although a rescue treatment will generally control the infestation and prevent any additional losses, most of the stand loss due to a cutworm infestation will have already occurred by the time the infestation is detected and a treatment applied.
The bottom line of deciding whether apply or not apply preventive treatments for cutworm depends on the long term risks associated with a geographical area. Thus, the use of a preventive treatment on a farm having a history of cutworm like the Western Branch in Clark County would be justified, but such a treatment on a farm having conditions like the Northwest Branch in Wood County would not be justified.
As for armyworm infestations on corn, application of a rescue treatment when needed is the most effective approach if the infestation is detected early. Significant armyworm infestations are limited to corn planted no-till into a grassy cover crop or old stand of forage. Unlike cutworm infestations, armyworm injury is totally above ground and can be easily detected and controlled if scouted on a regular basis. However, a pre-emergence treatment for armyworm may be justified in a situation where corn is planted no-till into a grassy habitat and cannot be scouted on a regular basis.
Alfalfa stands are rapidly growing and may have economic infestations of alfalfa weevil. During the past two weeks, weevil activity observed in Franklin and Clark Counties was relatively low. However, over the past weekend, weevil activity has begun to accelerate and 2nd, 3rd, and 4th instar stages can readily be detected in alfalfa stands which currently are running about 12 to 14 inches in height.
The most obvious initial indication of weevil activity at this point in time will be foliar tip injury. If an infestation is significant, an entire stand may appear frosted as larvae develop and feed on the foliage. If injury appears to be significant, random samples of ten stems from 3 to 5 locations in a field should be collected and shaken into a bucket. If the average number of larvae collected in the bucket is greater than 3 larvae per stem, then a rescue treatment may be warranted.
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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 (Extension Specialist, Plant Pathology), Anne Dorrance (Extension Specialist, Plant Pathology), (Jeff Stachler (Extension Associate, Weed Science), and Hal Willson (Extension Entomology); EXTENSION AGENTS: Steve Prochaska (Crawford), Roger Bender (Shelby), Larry Lotz (Fayette), Greg LaBarge (Fulton), Dennis Baker (Darke), Howard Siegrist (Licking), and Clark Hutson (Seneca).
Editor: Steve Prochaska 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.
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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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