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

CORN
Crop Observation and Recommendation Network

July 21 to July 27, 2001
C.O.R.N. 2001-23

In This Issue:

A) Twospotted Spider Mites On Soybeans
B) Soybean Aphid Update
C) Getting Wheat Stubble Ready For Weed Control
D) How's Your Marestail Control?
E) Why Is Corn So Short This Year?

A) Twospotted Spider Mites on Soybeans (Bruce Eisley and Ron Hammond) CORN Questions

The recent dry weather in Ohio has resulted in concern with twospotted spider mites (TSSM). Numerous calls have been received indicating the presence of mites in many fields. Visits to NE Ohio revealed a few fields with ongoing TSSM outbreaks that warrant treatment. We observed other fields with increasing mite populations that are waiting to explode. These fields have areas, especially along field edge, with very large egg populations. The presence of TSSM along with continued drought conditions are a cause for concern.

Where TSSM is a problem, discoloration of foliage will be evident, TSSM feeding sites will exhibit speckling, and all stages of mites (adults, nymphs, and eggs) will be readily detected by inspecting the underside of impacted foliage with a 10X hand lens.

The decision to treat should be based on the following concerns:

Does a soybean stand exhibit significant stress and foliage discoloration that can be linked to the mites and not attributed primarily to drought or herbicide stress? It should be noted that either drought or herbicide stress may enhance spider mite development.
Is the healthy green foliage showing signs of TSSM feeding such as stippling of the upper-side of leaves and expanding discoloration of the under-side of leaves? If fresh signs of TSSM feeding are widespread and easily detected throughout a field, then the infestation is well established and economic losses are likely to result if hot and dry growing conditions continue.
Finally, are all stages of mites present? A key factor is the easy detection of mite eggs which will require the use of a hand lens. If one inspects the underside of a leaf below a point of leaf discoloration and many eggs are detected, then a significant hatch is forthcoming and the TSSM population is about to multiply leading to additional foliage injury. Under a hand lens, the eggs appear like little marbles and should be readily detected when the adult mites (with two black spots) and nymphs (with 6 or 8 legs) are active.

If TSSM is found only in hot spots or mites mostly along the field's edge, the entire field should also be examined for the presence of mites. If eggs or light populations are found throughout the field, growers should give strong consideration to spraying the entire field.

A concern this year relates to the spraying of TSSM in fields also having soybean aphids. Although aphid populations are probably not at levels warranting treatment, our concern is that spraying for TSSM might allow the aphids to increase in population size. This could occur for various reasons, especially if beneficials insects, such as lady beetles, are killed. There is evidence that lady beetles greatly assist in keeping aphids at low levels. Because the aphid situation is so new, we do not know what will happen. Although numerous materials have mites on their labels, the two most likely ones that will be used against TSSM are Lorsban and dimethoate. Some of the other materials are listed for suppression only. In unreplicated field tests in Michigan in 2000, Lorsban appeared to be slightly more effective that dimethoate in lowering aphid numbers. However, another state reported similar efficacy with both materials. If soybean aphid is common in fields that will be treated for TSSM, Lorsban might be a better choice than dimethoate based on this information. We would advise that growers keep a close watch on TSSM-treated fields that also have soybean aphid present.

B) Soybean Aphid Update (Ron Hammond and Bruce Eisley) CORN Questions

We are finding aphids in most fields we examine throughout the state. They range from very few aphids, being hard to find, to moderate populations of 5-15 aphids per plant in extreme NE counties of Ohio. At this time, we have not found any fields approaching a level that requires treatment. However, growers should keep monitoring their fields, and the CORN newsletter, for any changes. The following 32 counties have confirmed soybean aphid populations: Ashtabula, Auglaize, Butler, Clark, Columbiana, Coshocton, Darke, Defiance, Delaware, Erie, Fairfield, Franklin, Fulton, Geauga, Hamilton, Henry, Hocking, Holmes, Madison, Mahoning, Mercer, Miami, Paulding, Pickaway, Portage, Preble, Ross, Sandusky, Seneca, Trumbell, Tuscarawas, Van Wert, Wayne, Williams, and Wood. However, we assume that all counties in Ohio probably have the aphid, and it will only take time to find them. We have set up a web site related to the soybean aphid in Ohio. The address is http://www.ag.ohio-state.edu/~ipm/soyaphid/index.htm.

C) Getting Wheat Stubble Ready for Weed Control(Jeff Stachler & Mark Loux) CORN Questions

If perennial weeds like Canada thistle are in wheat stubble it is now time to get them ready to spray this fall. Mow these fields before July 31st as close to the ground as possible. This will control some of the annual weeds and get the perennial weeds to start all over again. For areas that are really dry, mow as soon as possible and if it stays dry there may not be enough regrowth until October to get good control, but it will be better than nothing. We will discuss herbicide options at the end of summer for these fields.

If annual weeds like common ragweed and giant foxtail are the only weeds in the wheat stubble, then make sure the plants have recovered fully from wheat harvest and spray before the weeds begin to flower. For control of annual grasses apply glyphosate at 0.56 lb acid equivalent(ae)/A and for annual broadleaf weeds apply glyphosate at least at 0.75 lb ae/A, but consider 1.125 lb ae/A. Those areas of the state that are dry should consider not applying the glyphosate until the weeds are no longer under stress. The purpose of this application is to reduce the weed seedbank by not letting these weeds set seed.

D) How's Your Marestail Control? (Jeff Stachler and Mark Loux) CORN Questions

If you have marestail left in your soybean fields and you are wondering if it is herbicide tolerant or resistant then contact Mark Loux at 614-292-9081 or Jeff Stachler at 614-292-1393. We are wanting to screen several different marestail populations from around the state for different levels of tolerance to several herbicides. If you want to help, give us a call.

E) Why Is Corn So Short This Year?(Peter Thomison) CORN Questions

Across much of Ohio, the height of corn at pollination this year is noticeably shorter than normal. Aside from drought stress and N deficiencies, which are contributing to stunting in some areas of the state, there are other factors responsible for reduced plant height. The following is information from a recent article written by Dr. Bob Nielsen, which addresses these factors.

The causes of shorter than normal corn can be traced back to planting date and temperature during stalk elongation. Remember that stalk elongation begins at about the V5 stage of development (five visible leaf collars). Prior to that stage, most of the plant's energy is directed to root development and leaf initiation. After that stage, the plant enters its so-called grand growth phase wherein above- and below-ground growth accelerates to an exponential pace that peaks near tasseling.

Elongation of the stalk occurs primarily by cell expansion near the bases of the internodes at what are called the intercalary meristems. Stalk elongation is influenced by a number of factors, including daylength and temperatures. Longer daylengths tend to increase internode lengths and overall plant height. Cold temperatures are thought to increase the rigidity of basal internode cell walls, thus limiting cell expansion and internode elongation.

Given these physiological causes of short plants, one can think about this year's corn crop and begin to understand why some of it so short at tasseling. According to the USDA Crop Reporting Service, Ohio's corn planting progress finished five days ahead of last year and twenty-six days ahead of the 1996-00 average. Early-planted corn normally reaches the V5 stage at dates earlier than does later-planted corn. Stalk elongation in early-planted corn, therefore, begins in a time period that is characterized by shorter day-lengths and generally cooler temperatures than corn planted later in the season. As described above, both of these factors contribute to shorter internodes and plant heights.

Now consider the three- to four- week period which began about May 10-11 when temperatures were significantly lower than normal throughout much of the state. Much of the early-planted corn was beginning or well within the stalk elongation period while most of the later-planted crop was younger than V5. This extended period of cool temperatures influenced the elongation of internodes in the lower third of the stalk and accentuated the expected typically shorter heights of early-planted corn.

Are there yield consequences of unusually shorter corn? There are probably no negative consequences from reduced height per se, unless the short height is dramatic enough to significantly reduce crop canopy cover and harvest of sunlight. Conversely, shorter corn may be beneficial from the standpoint that the risk of stalk lodging is decreased due to the lower center of gravity.

(If you'd like to read Dr. Nielsen's article in its entirety, it's available on-line at :) http://www.agry.purdue.edu/ext/corn/cafe/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, Ann Dorrance, and Dennis Mills (Plant Pathology), Jeff Stachler (Weed Science), Ron Hammond and Bruce Eisley (Entomology), and Peter Thomison (Corn Production.) Extension Agents: Dave Jones (Allen), Barry Ward (Champaign), Steve Prochaska (Crawford), Gary Wilson (Hancock), Howard Siegrist (Licking), and Roger Bender (Shelby).

Editor: David Jones 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.

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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.