CORN
Crop Observation and Recommendation Network

July 21-28, 2003
C.O.R.N. 2003-23

In This Issue:

A) Corn Pollination Highlights
B) Testing Corn Leaf Tissue - Is It Important?
C) Wheat Head Scab Forecasting Results for 2003
D) Dying Soybeans - A "Complex" Issue
E) Buckwheat
F) Soybean Aphid Update
G) Late-Season Weed Control Issues in Soybeans
H) Can Large Marestail Be Controlled in Non-Gmo Soybeans?
I) Do You Have Herbicide Resistant Weeds?
J) Wheat Stubble - Friend or Foe?

A) Corn Pollination Highlights - Thomison CORN Questions

The flowering stage in corn is the most critical period in the development of a corn plant from the standpoint of grain yield determination. Stress conditions such as drought or hail damage have the greatest impact on yield potential during the reproductive stage. The following are some of the key steps and phases in the corn pollination process.

Pollen shed usually begins two to three days prior to silk emergence and continues for five to eight days with peak shed on the third day. On a typical midsummer day, peak pollen shed occurs in the morning between 9:00 and 11:00 a.m. followed by a second round of pollen shed late in the afternoon.

The tassel is usually fully emerged and "stretched out" before any pollen is shed. Pollen shed begins in the middle of the central spike of the tassel and spreads out later over the whole tassel with the lower branches last to shed pollen.

Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels. Pollen is light and is often carried considerable distances by the wind. However, most of it settles within 20 to 50 feet.

Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rainstorm as little to none is shed when the tassel is wet. Also, silks are covered with fine, sticky hairs, which serve to catch and anchor pollen grains.

Under favorable conditions, pollen grain remains viable for only 18 to 24 hours. However, the pollen grain starts growth of the pollen tube down the silk channel within minutes of coming in contact with a silk and the pollen tube grows the length of the silk and enters the female flower (ovule) in 12 to 28 hours.

A well-developed ear shoot should have 750 to 1,000 ovules (potential kernels) each producing a silk. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.

Pollen of a given plant rarely fertilizes the silks of the same plant. Under field conditions 97% or more of the kernels produced by each plant are pollinated by other plants in the field. The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains, which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are usually only a problem under conditions of extreme heat and drought. Poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). However, modern hybrids seldom exhibit this problem in Ohio unless they experience extreme drought stress.

B) Testing Corn Leaf Tissue - Is It Important? - Watson CORN Questions

Plant tissue analysis is a diagnostic tool that often has been overlooked by growers. Determining the concentration of nutrients that is actually in the corn plant can provide important information about problem areas and management practices for corn production. Plant analysis can be used to diagnose nutritional problems that may exist in certain areas of the field, or it can be used to monitor the crop to evaluate the overall nutrient status of the crop. Using plant analysis to evaluate your corn crop is very useful.

Correct sampling is important to ensure useful analytical data. The ear leaf should be sampled for testing when the corn is in the initial silk stage of growth. The nutrient concentrations in the ear leaf have been shown to be most highly correlated with corn yield. Approximately 10-20 leaf samples should be taken randomly across each acre of the field. If there is an area of the field that is suspect be sure to test that area separately. Do not sample dead or diseased plants. Be sure to handle the plant tissue after sampling in accordance with the instructions provided with the sample kit.

Interpretative guidelines for corn for each nutrient:

C) Wheat Head Scab Forecasting Results for 2003 - Lipps and Mills CORN Questions

Over the past three years we have been testing forecasting models to predict the risk of head scab in Ohio. The forecasting system has been developed to act as an early warning in the event of a major scab epidemic that could drastically affect the wheat crop in the state. Information on the forecasting models is available on the Ohio Field Crop Disease web site http://www.oardc.ohio-state.edu/ohiofieldcropdisease.

During 2003 the head scab risk forecasting models predicted moderately high risk of disease for fields in southern Ohio and for low risk of scab for other locations. The higher disease prediction in southern Ohio was due primarily to the relatively warm temperatures that occurred in early May prior to flowering. Buy mid May, temperatures became cooler which reduced the risk prediction for the wheat in northern Ohio that flowered during late May.

To determine if the risk forecasting models were accurate in their predictions, OSU Extension county agents volunteered to assess the levels of head scab in their counties. A total of 148 fields were surveyed in 30 counties. Head scab was assessed by estimating disease incidence (percentage of heads with scab). Across the state, the average incidence of scab per field was 8.9% with a range of 0 % to 73.3% . Over 50% of the fields surveyed had incidence levels below 5% and 76.4% had incidence levels below 10%. Only 8.1% of the fields surveyed had moderate to high levels of scab (20% and up). Although the predictions were considered relatively accurate, some fields in certain counties (Henry, Licking, Muskingum, Pickaway, Ross and Wayne Co.) had higher levels of disease than expected and in other counties (Butler, Champaign, Clinton, Shelby and Miami), levels of disease were less than expected.

Differences in disease levels between fields can be due to differences in variety susceptibility, proximity of the wheat plants to a source of fungal spores in corn residue, and the total amount of time wheat heads remained wet during flowering. Additionally, relative maturity of the variety and planting date can alter the flowering date of a field causing great differences in disease levels.

We have received reports of vomitoxin levels from 0 to 8 ppm in areas where scab occurred. Vomitoxin, also known as deoxynivalenol or DON, is a toxin produced by the Fusarium fungus causing head scab on wheat. This toxin is produce by the fungus when sufficient moisture is available in infected grain and the temperature is favorable for the growth of the fungus. The scab risk forecasting models were not developed to predict vomitoxin levels in harvested grain. Vomitoxin can accumulate in grain during the later periods of grain filling, ripening and right up to the time of harvest.

Weathering from extensive rains in mid to late June (from 10 to 17 days of rain depending on location) kept wheat heads wet and favored late season colonization of tissues in the head contributed to the overall levels of vomitoxin in the grain of some fields by harvest. Therefore, it is possible to have relatively low levels of scab in a field and have higher levels of vomitoxin, especially when wet weather predominates during ripening of the crop.

D) Dying Soybeans - A "Complex" Issue - Dorrance CORN Questions

We have received LOTS of calls and samples this past week of soybeans prematurely dying. The symptoms are typically a wilt, with the top dying back and the leaves are yellowing. In a few cases the leaves have brown areas between the veins similar to SDS (Sudden Death Syndrome). When the tops are cut open, they are green on the inside, this eliminates some virus pathogens. The next major symptom is the roots - there are none, or those that are present, I can easily pull the outer cortical tissue off. In most of the cases, the beans have been sitting in saturated soil for a long period of time. We've isolated from some of these roots and we have a menagerie of root nibblers: Phytophthora, Pythium (both water molds) and Fusarium (mainly a secondary root nibbler). I think what has happened is that these soybean plants have a combination of flooding injury and root rots. In other words, this season has been very stressful, and the beans are now showing all of the symptoms. The problem that has caused all of this (besides the lousy weather) is poor soil drainage. One of the other symptoms - is that you can see the tile lines in some of these areas. Where the beans sit on top of the tiles, they are taller and greener. Where the beans are between the tiles - they are stunted and yellow. All of these pathogens require saturated soil conditions, in addition to variety selection - good soil drainage in Ohio soils is key to achieving optimum yields. To avoid these yield losses in the future: mark these fields where the symptoms are the worst - and target these fields for future repair or re-tiling. If the areas are compacted, its time to begin repairing this damage. These soybean plants look very bad, as the rains continue we will see more death due to Phytophthora and Pythium. If some of these plants begin to reroot - they may recover, but that's only if we can avoid these flooding rains for the rest of the season.

E) Buckwheat - Lentz CORN Questions

Flooding and/or wet conditions have destroyed or prevented the establishment of traditional row crops in some parts of the state. Some producers have asked if buckwheat would mature in time to be planted in these fields. Generally, July 15 is the last date we would recommend planting a short season crop such as buckwheat. To insure optimal yields, buckwheat should have 120 frost free days after emergence. In addition, since buckwheat is a specialty crop, it would be prudent to make sure a buyer is available before planting the crop. Traditionally buckwheat is bought by contract agreements. More details about buckwheat may be found in Extension Bulletin AGF-116-01.

F) Soybean Aphid Update - Hammond & Eisley CORN Questions

Visits to fields last week indicated that aphid populations are on the increase in Ohio. The soybean aphid density in a field we are watching in Wood County field went from 6-8 aphids per plant two weeks ago up to 25-30 aphids per plant, while a Coshocton County field went from 2-3 aphid to 10 aphids per plant. Also, the percentage of plants with aphids also went up, now at 100% to 82% of the plants being infested in Wood and Coshocton, respectively. And all this was during a time period of rain and storms.

Also of significance was the discovery of soybean aphids in Fayette County, north of Washington Courthouse, in southern Ohio where a moderate population was found. Although a third to half of the plants did not have any aphids, we found numerous "hot spots" where plants were covered with 1000s of aphids and coated with honeydew. These hot spots, usually about a 3 by 3 ft area, were easily located because the plants were already misshapen and stunted, with yellow leaf margins.

Thus, Ohio has populations of aphids in increasing densities in northern, east central, and southern Ohio. This southern location had relatively few aphids in 2001 when aphids were common in the state and a problem in the north. The next 2-4 weeks will be critical in stopping aphid populations that are continuing to increase. Aphids should be treated when the average population is between 250-300 on a majority of plants and increasing. Although none of these fields mentioned are at this level yet, there is a possibility for these densities to go higher. Reports from some other Midwestern states already suggest a much bigger problem from aphids in other states compared with 2001.

Growers in Ohio should be vigilant to potential aphid problem. Not all fields will have problems, and many might not even have aphids present. However, you can only determine this by scouting your fields.

See the CORN newsletter for the week of June 16-22 http://corn.osu.edu/archive/2003/jun/03-18.html for information concerning management of the aphid. The following insecticides are labeled for soybean aphid control:

Asana XL 5.8 to 9.6 fl oz per acre
Furadan 4F 0.25 to 0.5 pt per acre
Lorsban 4E 1 to 2 pts per acre
Mustang 3.4 to 4.3 fl oz per acre
Penncap-M 2-3 pts per acre
Warrior 1.92 to 3.2 fl oz per acre

Growers should also be reminded that if a treatment is necessary during flowering stages of growth, they have to be aware of potential problems with bees. Care should be taken during flowering if bees are actively working a soybean field. Growers should be advised to follow all label directions and state regulations; it is the soybean grower's responsibility!

As aphids are found in high populations in other counties in Ohio, we would appreciate hearing about it. Please send location (county, township, and nearest roads or highways, or GPS coordinates if available), relative densities, and other information to hammond.5@osu.edu.

G) Late-Season Weed Control Issues in Soybeans - Loux CORN Questions

A number of factors have worked in combination to cause late-season weed problems in some soybean fields. These factors include slow soybean canopy development and some stand loss through the early part of the summer, significant mid-season rain to stimulate weed emergence, and problems with timely application of burndown and postemergence herbicides. Weed problems are generally worse in non-GMO soybeans, because of the lack of effectiveness of conventional postemergence herbicides on large weeds, compared to glyphosate. Some thoughts and reminders on late-season weed problems:

A general principle in weed management is that weeds emerging around the same time as the soybeans have the most potential to reduce yield. Weeds that emerge later than about 4 to 6 weeks after soybean emergence have little effect on yield. Some caveats apply here, though: 1) this principle is based on research conducted primarily in the 1980's in conventional tillage, wide-row soybeans; 2) subsequent research showed that very competitive weeds such as giant ragweed can reduce yield even when they emerge later than 4 to 6 weeks after soybeans; and 3) the research was probably conducted under more favorable environmental conditions than we are experiencing this summer (wouldn't take much!). While narrow rows increase the ability of soybeans to suppress late-emerging weeds, anything that delays soybean canopy development or reduces stand, such as no-till, poorly drained soils, wet and cold conditions, or disease, will generally result in greater risk of yield loss from late-emerging weeds.

Even where they have little potential to reduce yield, late-emerging weeds can still produce seed and interfere with soybean harvest. Late-season herbicide applications may be justified if they reduce seed production and problems at harvest, but may be less effective than a harvest aid treatment with regard to the latter.

Postemergence applications at this time of year will result in minor yield loss from tire damage.

Based on the previous points, the potential benefits of a late-season application must be weighed against the cost and probability of effectively controlling weeds or reducing problems at harvest. In Roundup Ready soybeans, there is a high probability that a late-season glyphosate application will adequately control or suppress many weeds at a relatively low cost (this would be the time to take advantage of the respray policy if you purchased a high-dollar glyphosate). In contrast, postemergence herbicides for non-GMO soybeans are often ineffective on large weeds, so late-season applications may be a high-cost proposition with little chance for success.

Be sure to consult labels or the OSU Weed Control Guide for recrop restrictions and other precautions when applying herbicides this late in the season.

H) Can Large Marestail Be Controlled in Non-Gmo Soybeans? - Loux CORN Questions

Marestail is evident in many soybean fields across Ohio. In Roundup Ready soybeans where marestail is not resistant to glyphosate, use rates are based on marestail size. Plants larger than 12 inches should generally be treated with 44 oz of WeatherMax or 64 oz of other glyphosate products. In non-GMO soybeans where marestail are not ALS-resistant, best options are Classic or FirstRate, but these products are most effective on plants smaller than about 6 inches tall. Use the highest label rate of these products, but be sure to follow recrop restrictions to avoid injury to the next crop in the rotation. We do not know of any effective treatments for control of ALS-resistant marestail in non-GMO soybeans.

I) Do You Have Herbicide Resistant Weeds? - Stachler CORN Questions

As you scout fields after an herbicide application, if you see dead plants next to injured or completely healthy plants, you should be concerned that herbicide resistant biotypes of those species are becoming more prevalent in the field. The only way to confirm herbicide resistance is to collect weed seeds from the field and grow them in the greenhouse with known sensitive biotypes. If you believe you have a weed species that is resistant to an herbicide, especially glyphosate, and that species has not already been confirmed resistant in Ohio, feel free to contact Mark or me. We are still interested in knowing about new fields of glyphosate-resistant marestail, so we know how far north and east it is traveling. Jeff Stachler can be reached at 614-292-1393.

J) Wheat Stubble - Friend or Foe? - Stachler and Loux CORN Questions

The summer and fall after wheat harvest is an excellent time to control many annual, biennial, and perennial weeds, and reduce weed populations in future years. Benefits of controlling weeds at this time include the following:
- Reduces weed seed production and prevents increases in the soil weed seedbank for many summer and winter annual weeds.
- Prevents increase in the seedbank of ALS-resistant ragweeds and glyphosate and ALS-resistant marestail as well as other resistant weeds.
- Controlling volunteer wheat, quackgrass, and other grasses reduces the risk of several wheat diseases.
- Controlling purple deadnettle reduces its potential to serve as a host for soybean cyst nematode.
- Late summer and fall allows application of glyphosate/2,4-D when perennials are in an appropriate growth stage to get maximum movement of herbicide into their roots.
- Late-fall applications are an excellent tool to manage dandelion and winter annual weed populations, which have been increasing in recent years.
When planning a strategy for weed control in wheat stubble, it is essential to decide which weeds are most important to target, since there is no single application timing that works well for all weed types. The single biggest challenge may be determining how to prevent summer annual weeds from going to seed, while also targeting cool-season perennials and winter annuals with a late-fall application. There are really three different application times to properly control all species. Those are before mid-August, mid to late-September, and mid-October to early November. Some things to consider:
- Summer annuals (ragweeds, foxtails, etc) and marestail should be controlled by about mid-August to prevent seed production. This can be accomplished with herbicides. Mowing by early August will greatly suppress these weeds, but some of them will still regrow and produce seed if not killed by frost or a later herbicide application (although seed production will be much less than if they are not disrupted in August).
- Winter annuals emerge in late summer through mid-fall, so a mid- to late-fall herbicide application is a very effective tool for control.
- Cool-season perennials such as Canada thistle, quackgrass, and dandelion, and biennials such as wild carrot and poison hemlock survive light frosts, and are most effectively controlled with herbicide applied through late-October or early November. Thistle and quackgrass plants should be at least 8 to 12 inches tall for best results.
- Warm-season perennials such as hemp dogbane, milkweeds, bindweeds, and johnsongrass will complete their life cycles by late summer and do not tolerate frost well, so herbicide applications must be applied before a frost when plants are in the appropriate growth stage (bud to flower of boot to seedhead).
- Tank-mixtures of 2,4-D with glyphosate can antagonize the activity of glyphosate on Canada thistle and perennial grasses. Antagonism can be overcome by increasing the glyphosate rate.

To control summer annual weeds, an application of glyphosate (11 to 22 oz of WeatherMax or 16 to 32 oz of other glyphosate products) and/or 2,4-D ester (1-2 pts/A) will be necessary. A combination of glyphosate plus 2,4-D can be more effective than either herbicide alone, allows for lower rates of glyphosate, and slows the rate of development of glyphosate-resistant weed populations. If the weeds are large or tough to control species such as velvetleaf, smartweed, horseweed/marestail and morningglory, high rates of glyphosate may be necessary when applied without 2,4-D. Mowing the wheat stubble at a height of 4 to 6 inches when any weed species has started to flower and then mowing as low as possible when these species begin flowering again is another method of reducing, although not eliminating, seed production of summer annual weeds in wheat stubble. This strategy will not be effective for control of most perennials.

Mowing wheat stubble fields as soon as possible (before first week of August) will allow for maximum regrowth of perennial weeds. Mowing prevents summer annual weeds from producing large amounts of seeds, allows for regrowth of perennial weeds, and helps reduce the wheat residue that can interfere with spray coverage. Applying glyphosate and/or 2,4-D in July or August will prevent seed production by annual weeds, but can actually reduce the control of perennial weeds, which are more difficult to control in the following year's crop.

The most effective timing for perennial weed control is generally mid-September through late October depending upon the species. Herbicide effectiveness at that time will be maximized by applying after several days of warm weather when perennial weeds are fairly large and/or in the bud to flower stage. Perennial weed regrowth should be at least 8 to 12 inches in order to obtain maximum control of the roots or rhizomes for next year. For warm-season perennials such as johnsongrass, hemp dogbane, milkweeds, common pokeweed, and bindweeds, application of glyphosate and/or 2,4-D in mid- to late September should provide the most effective control. Glyphosate should be applied at 1.125 to 1.5 lb ae/A (33 to 44 oz/A of WeatherMax and 48 to 64 oz/A of other glyphosate products) plus 2,4-D ester at 1-2 pt/A. Cool-season weeds such as Canada thistle, quackgrass, dandelion, and Canada bluegrass will be most effectively controlled with a glyphosate application after a light frost in mid- to late October. With cool-season species use 0.75 lb ae/A (22 oz/A WeatherMax and 32 oz of other glyphosate products) of glyphosate plus 2,4-D ester at 1 pt/A.

Control of volunteer wheat and perennial grasses, especially quackgrass, should help reduce the risk of wheat diseases in future wheat crops. To most effectively reduce wheat streak mosaic virus, rust, stagonospora blotch, and powdery mildew, the volunteer wheat should be controlled before September 1st. Control of quackgrass and other perennial grasses before September will reduce the future risk from take-all root rot. To selectively control the volunteer wheat and other grass species, an application of Select (8.0 oz/A) could be applied before August 15th, followed by glyphosate and/or other products later to control winter annuals and perennials. Glyphosate should effectively control volunteer wheat.

Purple deadnettle, field pennycress and shepherd's-purse have been documented as hosts of the soybean cyst nematode, and may have a role in increasing the nematode populations. Therefore control of these species may reduce soybean cyst nematode populations. The control of chickweed and other species may help reduce cutworm and slug populations in the spring. Where marestail is resistant to glyphosate and ALS inhibitors, be sure to include 2,4-D ester with the glyphosate to achieve adequate control.

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

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: Greg LaBarge        Web Editor: Nathan Watermeier

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