CORN
Crop Observation and Recommendation Network

June 26 - July 2, 2000
C.O.R.N. 2000-19

In This Issue:

A. More rain expect more root rots
B. First year corn rootworm injury detection
C. Corn borer larvae beginning to tunnel
D. Leaf cupping and puckering symptoms on soybeans
E. Harvest wheat early: avoid low test weight & disease problems
F. Early corn diseases and possible problems later
G. Flooding and ponding damage to corn
H. East central ohio crop diagnostic clinic
I. Follow corn development with "corncam"

A) More Rain Expect More Root Rots - Anne Dorrance CORN Questions

The perpetual rains throughout the northern parts of Ohio will continue to favor many soil-borne pathogens of soybeans. This is an excellent year to evaluate your soybean varieties for their ability to stand up to the disease pressure. Diagnosis of Phytophthora sojae will also become easier on older plants as Phytophthora also causes a stem rot phase. Approximately a week to 10 days after a heavy rain, the above ground symptoms that you will see is wilt followed by an overall yellowing of the plant. When wilting is evident - look at the base of the plant. Phytophthora is the only soybean pathogen that will colonize the roots and move up the stem from the base of the plant forming a chocolate brown lesion. We have also been finding Rhizoctonia and Macrophomina in dying seedlings. These fungi both cause red lesions on the stem at the soil-line. All of these fungi require moist conditions. If the fields are in a condition that they can drain quickly, this will limit the time available for these fungi to attack plants. After the rains, check to be sure that all the drain tiles are functional.

	Phytophthora root rot
	Phytophthora stem rot
	Rhizoctonia stem rot

B) First Year Corn Rootworm Injury Detection -Hal Willson CORN Questions

In a few weeks, we will begin operating yellow stick traps in soybean fields to monitor adult corn rootworm activity that may impact first year corn. Survey programs using the Pherocon AM traps in 1998 and 1999 have demonstrated a gradient of first year corn rootworm activity that is most intense in the northwest Ohio counties bordering the Indiana border and detectable at very low levels to the third tier of counties away from the Indiana line. To date, the survey effort has demonstrated that most of Ohio does not have a first year corn rootworm problem, and that very few areas would benefit from a treatment of first year corn for rootworm.

The Ohio rootworm survey is based on the deployment of a minimum of four yellow sticky traps in soybeans on a biweekly schedule starting in mid-July and terminating at the end of August. Traps have been distributed to many county extension programs in western Ohio especially in the northwest district.

Adult corn rootworms may be detected early this year in continuous corn fields or first year corn fields having first year corn rootworm activity. During the past week, rootworm pupae were detected in continuous corn plots exhibiting significant rootworm infestations last year. However, collection of adult corn rootworm beetles in soybeans is unlikely to occur until late July when the beetles begin to migrate from the field corn sites in which they developed.

At the present time, it is important to maintain a watch for rootworm injury and early adult rootworm activity in first year corn that may be suspected to support first year corn rootworm. If lodging is detected in a field of first year corn, the root systems should be inspected closely for feeding injury and the presence of rootworm larvae. In general, a lodged plant needs to have a root rating of 4 on the Iowa 1-6 scale to lodge. A field may have a sub-economic average root rating of 2, but a small proportion of the plants will exhibit root rating of 3, where one or more roots will be eaten to less than 1 1/2 inches in length, or a root rating of 4 where the equivalent of one node may be lost. An abundance of adult rootworm beetles in a first year corn field may also be a sign of first year corn rootworm activity. In general, adult rootworm beetles begin to appear abundant in first year corn after they migrate from continuous corn fields. However, if adult rootworm beetles are observed relatively early in a first year corn field, root systems of the field should be sampled and examined for rootworm injury to confirm the presence of possible first year corn rootworm activity.

C) Corn Borer Larvae Beginning To Tunnel -Hal Willson CORN Questions

Inspection of waist high field corn will likely detect the presence of a few 1st brood corn borer larvae in the 3rd and 4th instar stage that have begun to tunnel mid-ribs and the stalks. Although most fields that are in the early and mid-whorl stage of development exhibit minimal levels of corn borer activity, close inspection of plots in the late whorl stage in Crawford and Wood counties during the past week did result in the detection of larvae in the inner whorls of a few plants. In general, 1st brood corn borer activity is minimal this year, but populations are surviving in a few fields of early planted corn that may lead to a substantial 2nd brood if weather conditions are favorable for development of the forthcoming generation.

D) Leaf Cupping And Puckering Symptoms On Soybeans - Mark Loux And Jeff Stachler CORN Questions

As in past years, we have received a number of calls about cupping or wrinkling of soybean leaves. Some of this has been a result of drift due to the many windy days this year and some due to other problems. We have put together the following information to address this issue.

These types of symptoms have been observed around the Midwest over the past five years or so in increasing frequency. A number of factors can cause these symptoms, and it can be difficult to pinpoint the exact cause. Herbicides are often blamed, but are not always the culprit, unless a clear drift or spray pattern is observed in the field.

One of the first herbicides to get blamed in many fields is Banvel, which may have been applied in a nearby corn field. Products containing dicamba include Banvel, Clarity, Marksman, Celebrity Plus, Distinct, and Northstar. Exposure of soybeans to low concentrations of dicamba through drift or volatility, or even dicamba residues in spray tanks may be the culprit in some fields. The potential for volatility varies among these products, but all can drift if applied during windy conditions. However, many of the affected fields seem to be far enough away from treated corn fields, or Banvel was not used in the area, and this possibility can be ruled out. The most typical symptoms from exposure of soybeans to Banvel or Clarity are puckering of the new leaves that are emerging 7 to 10 days after exposure. This may be accompanied by stunting of the plant. Soybeans may show these symptoms on several trifoliates, and then recover completely.

Research indicates that soybean yield is not generally reduced when minor symptoms occur, and yield loss is more likely if soybeans are in the reproductive stage at the time of exposure (although still unlikely unless symptoms are severe). 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 to 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"

We have heard reports of and observed fields where leaf puckering or cupping was uniform over the entire field. Other fields have shown symptoms only in some areas. In OSU research plots, we have occasionally observed puckering in Roundup Ready soybeans following application of Roundup. Spider mites and leafhopper have been known to cause cupping and wrinkling of soybean leaves. Many of the fields with puckering were previously treated with a postemergence herbicide other than Roundup. Pursuit, Classic, and Pinnacle seem to be the herbicides most often used in fields with the symptoms, but other herbicides have also been used. One working theory about these symptoms when the postemergence herbicide causes injury to the terminal buds on soybeans, apical dominance is altered, and plant hormones are redistributed within the plant. The result is the appearance of injury that is similar to that from plant growth regulator herbicides (dicamba, 2,4-D). New shoots may occur at nodes below the injured zone, the plant may take on more of a bushy appearance, and leaves may be wrinkled and cupped. However, most of the fields have not exhibited the increased "bushiness" such as might occur if apical dominance was lost.

While herbicides may be responsible for some of the puckering, cupping, and wrinkling that has been observed, we suspect that environmental conditions and soybean variety may have a significant role. This is based on the observation of uniform cupping in fields where no postemergence herbicide was used. Some varieties may be more likely to show symptoms than others. We have not been able to come up with a good explanation for this phenomenon. However, the good news is that leaf cupping and wrinkling should not affect yields, and soybeans generally compensate well from other herbicide-related problems given enough time and moisture.

E) Harvest Wheat Early: Avoid Low Test Weight & Disease Problems -Pat Lipps CORN Questions

Most of Ohio's wheat crop has escaped several near disasters this year from weather and disease. In general the crop is in good condition. Harvest is now underway in southern Ohio and most producers are getting ready to begin harvest as soon as the weather and the grain is sufficiently dry. Reported yields in southern Ohio have ranged from 65 to over 100 bu/A so far. Continued rain showers through the later stages of crop maturity will pose a threat to grain quality. Disease causing fungi on the heads will continue to colonize glumes and kernels. Any heads affected by head scab will support the growth of Fusarium as long as they are above about 20% moisture. Thus, the scab fungus has the opportunity to grow and produce Vomitoxin in these heads as long as they remain moist. Stagonospora nodorum, that causes glume blotch, also will continue to colonize glumes and developing kernels. Although Stagonospora does not produce a toxin, it can infect seed and cause seed shriveling. The result of continued wet weather can only reduce the quality of the grain from this point on. If the maturing crop remains wet we can expect lowered test weights and possible seed quality problems. Wheat growers should make every effort to harvest the crop as soon as possible to maintain grain quality and test weight. Although most fields have only very low levels of head scab, a few fields may have significant numbers of heads affected. It would be to the advantage of the grower to turn up the fan speed during combining to discard as many shriveled seed as possible to improve test weight. Grain buyers will likely target lower test weight grain for dockage.

F) Early Corn Diseases And Possible Problems Later -Pat Lipps CORN Questions

Anthracnose and crazy top: Although the corn crop has appeared to deal with the extremely wet weather in May and June better than soybeans, growers should be aware that several diseases are lurking in the fields. Extremely wet weather has favored the development of Anthracnose on the lower leaves of plants in many fields, especially in continuous reduced tillage corn. The typical lesions are small, oval spots with distinct dark brown borders. Lesions are usually near the tip of the leaves and affected leaf tips generally have a yellow orange appearance. The leaf blight phase usually is not yield limiting, but the leaf lesions provide spores that can infect the stalks later in the season. Continued wet weather will indicate a high probability of Anthracnose stalk rot this fall.

Lots of corn fields have had standing water in them sometime over the past month. Any fields that have been saturated for 48 hours or more soon after planting or before corn plants were in the four-leaf to five-leaf stage may have plants affected by Crazy Top. Spores of the Crazy Top fungus swim in flood water to young corn plants, infect and grow systemically within the plant. Generally, the first symptoms of the disease begin to appear several weeks to a month after infection. Infected plants first show excessive tillering plus rolling and twisting of the upper leaves. As plants age, ears and tassels may be transformed into a mass of leafy structures. These strange looking plants frequently do not produce grain. The extent of the damage is entirely dependent on the percentage of plants infected at the early stage of development.

G) Flooding And Ponding Damage To Corn - Peter Thomison (Reprint-Corn 2000-14) CORN Questions

The recent thunderstorms and heavy downpours that hit parts of Ohio during the past week resulted in localized flooding in some corn fields, and ponding in others. The following are some tips to consider when evaluating possible injury from ponding and flooding.

The extent to which flooding injures corn is determined by several factors including: (1) plant stage of development when flooding occurs, (2) duration of flooding and (3) air/soil temperatures. Prior to the 6-leaf stage (measured by visible leaf collars) or when the growing point is near or below the soil surface, corn can survive only 2 to 4 days of flooded conditions. The oxygen supply in the soil is depleted after about 48 hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions; e.g. nutrient and water uptake is impaired, root growth is inhibited, etc. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival. Once the growing point is above the water level the likelihood for survival improves greatly. Corn in southern Ohio that had not yet reached the 6-leaf stage, had great potential for flooding and ponding injury.

Even if flooding doesn't kill plants outright, it may have a long term negative impact on crop performance. Excess moisture during the early vegetative stages retards corn root development. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Flooding and ponding can also result in losses of nitrogen through denitrification and leaching.

If flooding in corn lasts less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. Also look for new leaf growth 3 to 5 days after water drains from the field. Sometimes the growing point is killed by bacterial infections during and after flooding, but plant growth continues in the form of non-productive tillers (suckers).

Additional disease problems that become greater risks due to flooding and cool temperatures are corn smut and crazy top. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage is difficult until later in the growing season.

H) East Central Ohio Crop Diagnostic Clinic CORN Questions

The mid season session of the East Central Ohio Crop Diagnostic Clinic will be held on Wednesday, July 12 from 5:30 p.m. until 8:30 p.m. at the Roger Kilberger Farm in northeast Fairfield County. The farm is one-fourth mile off of S.R. 204 on Shell Beach Road. Take S.R. 204 east of Millersport 3 miles to Shell Beach Road.

Rootworm resistant corn, various rates and adjuvants with roundup will be compared. Disease and growth and development in corn and soybeans will also be a part of this hands on in-field clinic. CCA credits and pesticide applicator credits will be available. Cost is $5.00 for the program and a light supper to follow. For additional information, contact Licking Extension at 740-349-6900 or e-mail lick@agvax2.ag.ohio-state.edu.

I) Follow Corn Developement With "CORNCAM" -Peter Thomison CORN Questions

"CornCam" on the Iowa Farmer Today (IFT) Web page gives web surfers an opportunity to watch a corn crop grow and to monitor the life cycle of corn. Iowa Farmer Today has a web camera trained on a cornfield north of Prairieburg in Northeast Iowa to monitor the progress of this field through the growing season. The camera transmits images once every 15 minutes to the IFT Web page. CornCam address: http://www.iowafarmer.com/corncam/corn.html (includes links to a number of other good corn related web sites)

So.......Watch corn grow! Cheer as the mighty cornstalks battle wind, hail, and rainstorms! Count the ears! See it tassel!

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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.

Editor: David A. Jones        Web Editor: Tom Rosati

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