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

CORN
Crop Observation and Recommendation Network

September 29 - October 5, 2003
C.O.R.N. 2003-33

In This Issue:

A) Glyphosate-Resistant Marestail on the Move
B) Fall Herbicide Applications Will Reduce Marestail Problems in the Spring - But How Much?
C) Fall Slug Sampling
D) Soybean Aphid Information
E) Sampling Fields for Soybean Cyst Nematode

A) Glyphosate-Resistant Marestail on the Move - Mark Loux and Jeff Stachler CORN Questions

Last spring, we confirmed glyphosate resistance in marestail populations from southwest Ohio. At that time, resistance appeared to be confined to southwest Ohio and to fields that were primarily continuous Roundup Ready soybeans, where glyphosate was the only herbicide used in most of these fields over a several year period. However, our assumption was that glyphosate resistance would eventually be found in soybean fields in other parts of Ohio, and also in Roundup Ready soybean fields that were in a rotation with corn and/or wheat. We are currently collecting marestail seed from fields as far north as Interstate 70 where glyphosate resistance is suspected, including fields in Ross, Clark, Greene, Fayette, Preble, and Montgomery Counties. We will be screening these marestail populations in the greenhouse to determine whether they are resistant. Corn has been in the crop rotation in many of these fields, although some have had multiple years of Roundup Ready soybeans between the corn years. A common practice in almost all of the suspect soybean fields is that glyphosate was the only herbicide used. The spread of glyphosate-resistant marestail is not surprising, given the intensive use of glyphosate, the prevalence of glyphosate-resistant marestail in other parts of the country, and the wind-borne nature of marestail seed. In Roundup Ready fields where glyphosate is the only herbicide used, the issue is probably more one of "when" glyphosate resistance occurs in marestail versus "whether" it occurs.

In spite of the apparent spread of glyphosate-resistant marestail, certain practices appear to slow its development and help keep the population of resistant plants in a field low. These practices include:

crop rotation. Include corn and/or wheat in the rotation so that soybeans are not planted two years in a row. Avoid sequential years of Roundup Ready crops.
avoid use of only glyphosate in Roundup Ready crops.
use of multiple herbicide sites of action within and across years. Some examples: mixing 2,4-D ester with glyphosate in fall and spring burndown treatments; in corn, preemergence application of an atrazine premix product prior to postemergence glyphosate application; in soybeans, mixture of glyphosate plus FirstRate/Amplify in postemergence treatments.
use a fall herbicide treatment to control marestail that emerge in late summer and fall (see article that follows).
include a herbicide with residual activity on marestail in fall and early spring burndown treatments to reduce the population of spring-emerging marestail.
apply spring burndown treatments when marestail are less than 2 inches tall, which will maximize the activity of 2,4-D and other herbicides.

B) Fall Herbicide Applications Will Reduce Marestail Problems in the Spring - But How Much? - Loux, Dobbels, and Stachler CORN Questions

Among all of the winter weeds we deal with in Ohio, marestail can be the most difficult to make a single herbicide recommendation for due to its biology and ability to develop herbicide resistance. Marestail can emerge throughout most of the year. Peak times of emergence seem to be: 1) late summer into early fall; and 2) April into early May. However, when wet, cool conditions extend into late spring, we have observed considerable emergence into June. Plants that are larger than the rosette stage in fall do not survive the winter, but most fields with a history of marestail problems will be infested with plenty of very small plants at this time of the year. These plants can overwinter and are likely to reach the "bolt" or stem elongation stage earlier next spring, compared to those that emerge in the spring (marestail become more difficult to control after the stem elongates). A herbicide application this fall will easily control small, emerged marestail plants, and our research indicates that 0.5 lb ai/A of 2,4-D ester is adequate for this purpose. So, most of the commonly used fall herbicide treatments targeted for winter annuals and dandelions will control emerged marestail. However, while control of emerged marestail this fall is beneficial, we know that we can expect another round of marestail emergence in the spring. So, the real question about fall applications is not how to control emerged plants, but rather what can be applied for residual control of marestail into the spring? An added twist to this question - many marestail populations in Ohio are ALS-resistant, which could limit the utility of some common fall programs such as CanopyXL, Python, and Backdraft.

In a study we conducted over the past three years, we applied various herbicides in the fall and early spring to determine the length of residual control of marestail. Neither of our study sites contained glyphosate-resistant marestail, but the 2001/2002 site had areas of ALS-resistant plants. In the 2001/2002 study, most fall treatments were providing at least 90% marestail control in early May. However, differences among fall treatments were clearly evident by early June, which indicates that much of the spring emergence of marestail can occur in May. Perhaps the clearest indicator of these differences was the measurement of marestail population densities, which in early June were as follows for fall applications (in # plants per square yard): untreated - 130; Canopy XL - 78; glyphosate - 61; Valor - 55; Python - 47; FirstRate - 44; Authority - 17; Scepter - 13; Sencor - 13. In contrast, application of Canopy XL, Valor, Sencor, FirstRate, or Authority in early April resulted in 5 or fewer marestail plants per square yard by early June.

In the 2002/2003 study, where the marestail plants were less numerous and the population was not ALS-resistant, fall-applied herbicides were more effective than in the previous year's study. Marestail population densities in early June were as follows for fall applications (in # plants per square yard): untreated - 52; glyphosate - 16; Sencor - 12; Scepter - 12; Python - 8; Authority - 4; Canopy XL - 0; Valor - 0; FirstRate - 0. Early-April application of herbicides resulted in complete control of marestail through early June for all treatments except Python, for which a population of 4 plants per square yard was observed.

A primary conclusion from this study was that a number of herbicides could provide "season-long" marestail control (through early June) when applied in early April, but this was more difficult to accomplish when they were applied in the fall. The residual control from fall treatments will depend somewhat on how numerous the spring-emerging marestail are, and how late they emerge. In populations where most of the marestail have emerged by early May and the populations are not extremely high, fall applications can provide acceptable control of marestail through early June. Conversely, fall applications will be less effective where considerable numbers of marestail emerge after early May. We believe that the "ideal" marestail control program may be early-April application of 2,4-D plus one of the more effective residual herbicides listed above. However, a fall application with residual activity can greatly reduce spring populations of marestail and reduce the risk of developing glyphosate resistance.

C) Fall Slug Sampling - Ron Hammond & Bruce Eisley CORN Questions

We are getting into the time period when growers and consultants should consider scouting for slug fall populations. While we do not have specific thresholds at this time, sampling will indicate whether a field has an extremely small or very large population. Having a large number of slugs will help growers next spring determine which fields need extra monitoring. Scouting should be done by placing wood boards or roofing shingles on the ground across the field and checking them weekly throughout the fall and counting the number of adult slugs underneath the traps. It is best to count the slugs in the morning. Ten traps in a field would be a good number to begin with. By doing this in a number of fields, a grower can determine which fields are at higher risk next spring. As growers scout their fields, they should make note if sections of the field with heavier weed growth, such as heavy dandelion growth, have higher populations of slugs. As we see more fields with higher fall weed populations, we will need to determine if this enhances slug populations.

D) Soybean Aphid Information - Hammond & Eisley CORN Questions

Having had a significant soybean aphid problem this year, it is time to determine the extent of the problem and whether controls were effective. We are asking extension personnel, growers, and consultants to send us their observations and any data they might have on yield losses to us in order to help us understand the problem. Yield data from strip trials, or sprayed vs. non-sprayed areas would be greatly appreciated. Please send any information to hammond.5@osu.edu, or call Ron Hammond at 330-263-3727. Information concerning variety, planting date, row width, insecticides used including dates, application methods and rates, area sprayed, etc., is needed.

E) Sampling Fields for Soybean Cyst Nematode - Anne Dorrance & Dennis Mills CORN Questions

Soybean cyst nematode is still a pest in many of Ohio's production fields. Light infestations of SCN in fields will have no above ground symptoms, which is part of the challenge, but yields will be anywhere from 5 to 10 bushels off. Higher infestations on susceptible soybean cultivars will have more severe symptoms, such as: soybeans will be irregular in height, mixtures of tall and short soybeans, early yellowing and very low yields. This pest is managed with crop rotation, which reduces the SCN population levels and planting resistant cultivars. However, SCN readily adapts to ALL sources of resistance so it is important to manage the type of resistance that is planted in a field. The correct management plan starts with knowing what level of infestation is present.

The best time to sample fields for soybean cyst nematode is in the fall after the soybeans are harvested. Soybean cyst nematode populations can increase as much as 10 to 30 fold per growing season. Soybean cyst nematodes will not be distributed evenly throughout a field. Techniques for sampling soil for SCN by the Soybean Cyst Nematode Coalition are as follows:

1. Use a one inch diameter soil probe to collect samples (6-8 inches in depth)
2. Following a zig zag pattern, collect 10 to 20 soil cores per 10 to 20 acres
3. Collect cores from areas of similar soil type and crop history
4. Dump cores from each 10 to 20 acre area into a bucket or tub and mix thoroughly
5. Place 1 pint (2 cups) of mixed soil in a soil sample bag or plastic zippered bag and label with a permanent marker; and
6. Store sample in cool, dark place until shipped to the lab doing SCN analysis.

This level of sampling is necessary to obtain relatively accurate counts of the nematode population (egg and cyst) and to make meaningful recommendations for management. Charges this year at the C. Wayne Ellett Plant & Pest Diagnostic Clinic are $15.00 for each soil sample. Forms to accompany samples can found at the county extension offices or http://www.ag.ohio-state.edu/~plantdoc/cweppdc/scnform.html

Mail samples to:

C. Wayne Ellett Plant & Pest Diagnostic Clinic
Room 110, Kottman Hall
2021 Coffey Road
The Ohio State University
Columbus, OH 43210

Additional Ohio SCN testing labs:

Brookside Laboratory Inc
308 S Main St
New Knoxville, OH 45871
Phone: (419) 753-2448
Fax: (419) 753-2949
Email: mflock@blinc.com
Cost: $13.00 per sample
Results reported: egg and cyst counts per 100cc of soil

Geophyta Inc
2685 CR 254
Vickery, OH 43464
Phone: (419) 547-8538
Fax: (419) 547-8538
Email: geophyta@nwonline.net
Cost: $15.00 per sample
Results reported: cyst counts per 100cc of soil

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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.

State Specialists: Anne Dorrance & Dennis Mills (Plant Pathology), Mark Loux, Jeff Stachler & Anthony Dobbels (Weed Science), and Ron Hammond & Bruce Eisley (Entomology); District Specialist: Ed Lentz (Agronomy); Extension Agents: Harold Watters (Miami), Roger Bender (Shelby), Steve Foster (Darke), Barry Ward (Champaign), Clark Hutson (Seneca), Glen Arnold (Putnam), Howard Siegrist (Licking), Dusty Sonnenberg (Henry), Greg La Barge (Fulton), and Andy Kleinschmidt (Van Wert).

Editor: Andy Kleinschmidt 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.