CORN
Crop Observation and Recommendation Network

July 12 to July 18, 1999
C.O.R.N. 99-17

In This Issue:

A) Field Corn Pest Activity
B) Soybean Disease Update
C) Assessing Potential For Nitrate Problems In Corn
D) Checking The Nutrient Status Of Corn At Silking
E) Injury To Soybeans From Dicamba Volatility Or Drift - Does It Affect Yield?
F) 1999 Manganese Deficiencies

A) Field Corn Pest Activity - Hal Willson CORN Questions

First Year Corn Rootworm Survey:

Monitoring of first year corn rootworm activity in soybeans with yellow Pherocon sticky traps should be initiated this week - preferably on Thursday or Friday (July 15th or 16th) which are the target dates for initiating the statewide biweekly survey this year. A minimum of four traps should be posted per soybean site monitored. Traps should be changed on a biweekly schedule at the end of July, at mid-August and finally the survey should be terminated at the end of August.

During the past week, adult western and northern corn rootworms have been active in corn. Peak movement of the adult rootworms from the field of origin to adjacent corn fields or other fields of corn will most likely occur when the silks in the field of origin begin to turn brown. Thus, the target date for setting the traps in soybeans during mid-July will detect peak rootworm activity in soybeans despite the fact that they are emerging relatively early this season.

Collection of an average of two or more western corn rootworm (WCR) adults per trap per day indicates an economic level of first year corn rootworm (FYRW). Collection of an average of only one WCR adult per trap per day also may indicate a potential FYRW problem, since adult rootworm activity, since traps in soybeans without FYRW rarely collect more than a few rootworm beetles per week.

Additional measures that may be taken to detect first year corn rootworm activity include the following:

• First year corn root systems may be sampled to detect rootworm injury. Occasional detection of feeding scars is not unusual, but detection of significant rootworm pruning of root systems in first year corn indicates a potential FYRW problem. When significant rootworm injury exists, a proportion of the plants should exhibit some goose-necked lodging. However, where lodging is evident, it is important to distinguish lodging due to rootworm from that due to wind damage.
  
• Detection of an unusual level of adult corn rootworm activity in first year corn also may indicate a potential FYRW problem. In general, visual counts of one or more adult corn rootworm in first year corn fields are very rare - especially if the first year corn field is not a late planting or is not adjacent to any continuous corn. Visual counts of adult rootworm in corn should include inspection of at least 10 or 20 plants at random.
  
• Adult rootworm activity in soybeans may also be sampled using a sweep net. Given variations in daily activity, sweep net samples are not as accurate as the use of yellow sticky traps. However, where a potential FYRW problem exists, collection of adult WCR will likely exceed one or more adults per sweep.

 Defoliators In Soybeans:

Above normal activity of Japanese beetles in soybeans have been reported from a number of counties. When evaluating Japanese beetle infestations in soybeans, one should be aware that infestations tend to be very aggregated and field perimeters often exhibit more activity than that of the interior area of a field.

Ron Hammond has reported cases of green cloverworm outbreaks in Wayne County. Such outbreaks are uncommon in Ohio, but when such outbreaks occur then tend to be rather severe and may warrant rescue treatment if defoliation is severe.

Spider Mites On Soybeans:

Some areas in the state remain very dry and two spotted spider mite problems may persist in some areas warranting rescue treatment. In contrast, rainy conditions and cooler evenings have reduced mite activity in some fields. Areas of field having significant infestations at this time are likely to be the dry areas such as elevated light soils. Inspection of only the field perimeter may miss problem areas. When considering treatment for mites, attention should be given to the presence of all stages of mite development, especially the egg stage (which requires the use of a hand lens to detect). If mite numbers are increasing, eggs will be present.

B) Soybean Disease Update - Ann Dorrance CORN Questions

Phytophthora:

Despite Dry Weather - Phytophthora root and stem rot are still a problem in some areas of Ohio. Some fields in Ohio have received adequate rainfall to provide enough moisture for the development of Phytophthora root rot. Once the soil is saturated the oospores will germinate and produce sporangia.

These sporangia contain the swimming zoospore which move through the water to soybean roots. The zoospores can then infect the roots. When scouting fields, plants will appear to be a pale yellow and begin to wilt. At the base of the stem, a canker will form - from below the soil-line and grow up the plant. Unfortunately, many of the calls we have received this year are from varieties that have Rps1-k. This is a good indication that the Phytophthora sojae populations in that field have changed and can now defeat this resistance gene. In fields where this occurs, producers should select varieties with the highest levels of partial resistance (also called field tolerance).

Sclerotinia Sclerotiorum:

The fungus that causes Sclerotinia stem rot forms very small, pink fruiting bodies from the sclerotia called apothecia. Now is the time to scout for these fruiting bodies in fields that have a history of white mold. For Sclerotinia stem rot to occur we need to have two periods of moisture. One before flowering to allow for development of these apothecia and another during flowering to allow for infections of blossoms to occur. Sclerotinia sclerotiorum is a fungus that prefers cool temperatures - canopy temperatures of less than 86oF. It is too late in the season to utilize many of the management tools we have available ie: use of resistant varieties and fungicide application.

C) Assessing Potential For Nitrate Problems In Corn - Peter Thomison  CORN Questions

Severe drought stress in parts of Ohio has raised questions concerning the potential for toxic levels of nitrates in corn harvested for silage. Nitrates absorbed from the soil by plant roots are normally incorporated into plant tissue as amino acids, proteins and other nitrogenous compounds. Thus, the concentration of nitrate in the plant is usually low. The primary site for converting nitrates to these products is in growing green leaves. Under unfavorable growing conditions, especially drought, this conversion process is retarded, causing the nitrate to accumulate in the stalks, stems and other conductive tissue. The highest concentration of nitrates is in the lower part of the stalk or stem. For example, the bulk of the nitrate in drought-stricken corn plants can be found in the bottom third of the stalk. If moisture conditions improve, the conversion process accelerates and within a few days nitrate levels in the plant return to normal.

The highest levels of nitrate accumulate when drought occurs during a period of heavy nitrate uptake by the corn plant. A drought during or immediately after pollination is often associated with the highest accumulations of nitrates. Extended drought prior to pollination is not necessarily a prelude to high accumulations of nitrate. The resumption of normal plant growth from a heavy rainfall will reduce nitrate accumulation in corn plants, and harvest should be delayed for at least 3 to 4 days after the rainfall.

Not all drought conditions cause high nitrate levels in plant. If the supply of soil nitrates is in the dry soil surface, plant roots will not absorb nitrates. Some soil moisture is necessary for absorption and accumulation of the nitrates.

If growers want to salvage part of their drought damaged corn crop as silage, it's best to delay harvesting to maximize grain filling. Even though leaves may be dying the stalk and ear have enough extra water for good keep. Kernels will continue to fill and the increases in dry matter will more than compensate for leaf loss unless plants are actually dying or dead. Moreover if nitrate levels are high or questionable, they will decrease as plant get older and nitrates are converted to proteins in the ear.

The kernel milkline can be used as a guide in determining the best time to cut corn for silage. When the kernel milkline has moved 1/4 to the distance from the top (or crown) of the kernel to the base, the whole plant contains approximately 60-70% moisture -- usually the recommended moisture range for making corn silage.

Test kits for qualitative plant tissue analysis of N-P-K are available. Dr. Jay Johnson noted that Spectrum Technologies markets one such kit. Their address is:

Spectrum Technologies, Inc.
23839 West Andrew Road
Plainfield, IL 60544
800/248-8873 or 815/436-4440
fax 815/436-4460
e-mail specmeters@aol.com

Testing nitrate levels with such kits generally involves splitting the corn stalk, adding a small amount of the nitrate powder (match-head size), working the nitrate reaction powder into the sap with a knife blade, and waiting several minutes for a final reading (to allow color development). Since the nitrate powder reacts with nitrates in the plant sap, it is important to have fresh plant tissue when testing. Normally with drought stricken corn plants, the highest concentration of nitrate-N is in the bottom third of the stalk. Therefore if the corn is to be harvested for silage it is important to test the nitrate levels at the cutting height of the forage chopper to determine the highest levels of plant nitrate.

Quick tests for nitrates should be used to determine if the plant is low in nitrates. If the plant is marginal to high in nitrates then a lab analysis is recommended before feeding. To get a quantitative nitrate determination, send or deliver a sample of 6-10 plants (cut at the stalk height of the forage chopper) to a plant analysis laboratory. Lists are available from your county extension office. Results of the test can generally be obtained over the phone, fax or email when tests are completed.

For more information on feeding corn with varying nitrate-nitrogen levels, consult Dairy Guide Leaflet D6111.

D) Checking The Nutrient Status Of Corn At Silking - Peter Thomison CORN Questions

Leaf tissue analysis, used in conjunction with other data (e.g. soil test results) and observations, can be an effective aid in evaluating the mineral nutrient status of the soil-plant system. It can be especially useful in determining plant micronutrient status. The silking stage in corn is a good time to collect ear leaf samples for tissue nutrient analysis. Fields should be sampled when 50% or more of ears show silks. However, don't sample after silks turn brown. The entire leaf directly below the ear is the part of the plant to be sampled, i.e. broken off at its base (do not include portions of the leaf sheath or collar).

The number of plants to sample depends on the general condition of the plants, soil uniformity, and purpose of the plant analysis. However, to minimize variation in nutrient concentrations from plant to plant, sampling should be from no less than 20 plants randomly selected in the area or field represented by the analysis. When a nutrient deficiency is suspected (even without visual plant symptoms) or there is a need to compare different areas in a field, ear leaf samples should be collected SEPARATELY from both the affected plants and adjacent normal plants.

Knowing what not to sample is just as important as knowing what should be sampled. Don't sample plants that -

• are diseased, insect damaged, or dead,
• are mechanically injured,
• have been stressed by too much or too little moisture (i.e. flooding or drought),
• have been stressed by abnormally high or low temperatures, and
• show nutrient stress (e.g. severe stunting) unless being comparatively sampled with more normal areas.

When leaf nutrient analysis is necessary to perform on plants which were stressed by drought or other environmental conditions prior to sampling, record weather and other conditions so later interpretation of the nutrient analysis can take environmental conditions into account.

Brown paper bags are ideal for collecting and drying tissue samples. Leaf tissue should be loose and remain loose in bag for adequate drying. Do not wad leaves tightly or tie in a bundle since heating and molding may occur. Allow about 48 hours for drying.

Sufficiency ranges for corn ear leaf sampled at silking are shown in Table 1 below.

Table 1. Plant Analysis for Nutrient Levels in Corn (ear leaf sample taken at silking)

E) Injury To Soybeans From Dicamba Volatility Or Drift - Does It Affect Yield?  - Mark Loux  CORN Questions

A recent article in CORN addressed the possible causes of soybean leaf cupping. One possible cause in the exposure of soybeans to low concentrations of dicamba through drift or volatility, or even dicamba residues in spray tanks. Products containing dicamba include Banvel, Clarity, Marksman, Celebrity, Distinct, and Northstar. The potential for volatility varies among these products, but all can drift if applied during windy conditions. It is difficult to predict soybean yield losses from herbicide injury at this time of the growing season, since late-summer rains have such an important role in determining soybean yield. Our research with postemergence soybean herbicides indicates that soybeans can tolerate considerable early-season injury with little or no impact on yield, when rainfall and other environmental conditions are generally favorable for crop growth after the injury has occurred. Yield loss seems to be most likely when herbicides are applied after about the beginning of July, and soybeans are small at the time of application (which might occur from late planting or poor early-season growing conditions). Where this has occurred, soybeans may not recover well enough to attain the size needed for maximum yield potential. A summary of research on the effect of dicamba on soybeans was prepared by Dr. Karen Renner at Michigan State University in 1991, and this is presented below. It would appear to us based on our past experience and this summary that dicamba injury to soybeans does not necessarily result in yield loss. However, severe injury to small soybeans this late in the season can result in yield loss if soybeans fail to outgrow injury and remain small through July. Weather conditions and tillage practices will certainly have a role in the effect of dicamba on soybeans also. Dr. Renner's article (MSU Field Crop Advisory Team Alert, June 26, 1991) with some editing from me, entitled "Banvel Drift to Soybean Fields -When is There a Yield Reduction?":

"Soybeans are very sensitive to dicamba. Foliar symptoms or leaf crinkling, cupping, and malformation can occur at rates as low as 0.009 lb ai (active ingredient) per acre. A 1 pint/A application of Banvel/Clarity in corn is equal to 0.5 lb ai/A and thus visual injury to soybeans can occur if only 2% of the spray solution drifts or volatilizes rto nearby soybean fields. The question often asked is whether Banvel "drift" will reduce soybean yield."

"Research on dicamba drift on soybeans has been conducted in Illinois, South Dakota, Minnesota, and Ohio over the past 20 years. In 1968 research in Illinois, Banvel was applied either at the prebloom stage (third trifoliate open and expanded, three weeks after planting), or at the bloom stage (eighth trifoliate, six weeks after planting. Applications at the bloom stage were eight times more injurious than applications at the prebloom stage. At the prebloom stage, 0.06 lb ai/A was needed to reduce soybean yield, while only 0.008 lb ai/A caused yield reduction when applied at the bloom stage. Later applications also appeared to persist in the harvested soybean seed and reduced soybean seed germination."

"In South Dakota from 1974 to 1977, Banvel was applied to six soybean cultivars at either early bloom, midbloom, or the early pod fill growth stage to simulate potential Banvel drift scenarios. Visual injury began to occur one to two weeks following each application. Leaves that developed soon after exposure showed the greatest injury. The degree of leaf injury did not vary with the time of Banvel exposure. Soybean height was reduced from application at all growth stages. Yield reduction occurred when Banvel was applied at early pod fill, as pods were deformed and soybean seed germination was reduced. Variability in response was noted among the six soybean cultivars, but none appeared tolerant."

"In Minnesota research in 1975, injury to soybeans exposed to Banvel at the first trifoliate leaf stage had to reach an injury index of 60 before yield reduction occurred (the index for injury ranged from 0 to 100; an index of 60 = greatly reduced terminal leaf growth and vigorous malformed auxiliary shoot growth, 70 = terminal bud dead). Soybeans that had an injury rating of 60 were within 16 to 20 feet of a 0.25 lb ai/A Banvel application. In another experiment, soybeans at the first trifoliate leaf stage that were 165 feet from an application of 0.5 lb ai/A of Banvel had an injury index of 20 (cupping of terminal leaflets and slight crinkle of second leaf but growth rate still normal) to 40 (malformation and growth suppression of terminal leaves, reduced terminal leaf size) three weeks after exposure. When soybeans were exposed at the first trifoliate, terminal bud death and malformed auxiliary shoot growth had to occur for soybean yield to be reduced. Leaf malformations alone did not reduce soybean yield when soybeans were exposed at the first trifoliate stage."

"In Ohio research in 1980 and 1981, Elf and Williams soybeans were treated with Banvel at low application rates to simulate drift. When treated at the prebloom stage, leaf crinkling and cupping and a reduction in terminal leaf size occurred from application of 0.0006 lb ai/A seven weeks after planting. Severe leaf margin damage and terminal bud kill occurred when soybeans were exposed to 15 to 30 times this rate. Terminal buds were killed at the 0.02 lb ai/A rate at prebloom or mid-bloom timing for Elf, and 0.009 lb ai/A for Williams. Yield of Elf was not reduced until 0.02 or 0.07 lb ai/A were applied prebloom or mid-bloom, respectively. For Williams, yield loss occurred at 0.04 and 0.02 lb ai/A for the prebloom and mid-bloom, respectively. This would equal 8 and 4% of a 1 pint/A Banvel application. Plant height was not a reliable predictor of yield reduction, since height varied between years, cultivars, and by time of application. Soybean response to Banvel varied with the growth stage at time of application. Foliar symptoms were most pronounced and occurred at lower rates at the prebloom stage (7 weeks after planting). Most foliar symptoms, i.e. crinkling and cupping of terminal leaves, leaf margin injury and size reduction, and distorted venation patterns, occurred at rates much lower than those required to cause yield reductions. Severe injury symptoms such as terminal bud kill, splitting of the stem, swelling of the petioles, and curled malformed pods were associated with yield reductions"

Happy scouting!

F) 1999 Manganese Deficiencies - Jim Beuerlein  CORN Questions

Manganese deficiencies have become visible in areas of many 1999 soybean fields in Central and Northern Ohio. When deficiencies are minor but just visible, yield losses of up to 25 percent will occur. Yields in areas of fields where most leaves are yellow and remain yellow throughout the season are less than 50 percent of normal. More severe deficiencies can result in death of plants. Yield losses of up to 10 percent can occur without deficiency symptoms being visible.

Deficiencies typically occur in lake bed soils, glacial outwashes, peat and muck soils, and depressions in fields. High soil pH, organic matter, clay content and dry soil conditions increase the severity of the problem. An application of 1 to 2 pounds of spray grade manganese sulfate will usually correct the problem. Rainfall adequate to thoroughly wet the top 6 inches of soil will usually dissolve enough manganese in the soil so that newly forming tissue will not be deficient. However, manganese is not mobile in the plant and leaves with deficiencies will not receive manganese removed from the soil or transferred from other parts of the plant, making foliar application necessary.

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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:

Hal Willson, (Entomology), Ron Hammond (Entomology), Peter Thomison (Corn Production), Anne Dorrance (Plant Pathology), Mark Loux (Weed Science), and Jim Beuerlein (Soybean Production); Extension Agents: Dave Jones (Allen), Barry Ward (Champaign), Steve Prochaska (Crawford), Dennis Baker (Darke), Gary Wilson (Hancock), Clark Hutson (Seneca), and Roger Bender(Shelby)

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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