CORN
Crop Observation and Recommendation Network

August 18-24, 2003
C.O.R.N. 2003-27

In This Issue:

A) Sclerotinia White Mold and SDS
B) Slower Than Normal Corn Development
C) Soybean Insect Update
D) Bean Leaf Beetle Pod Feeding
E) Weed Control Comments About Late-Planted Soybeans
F) Weed Control in Wheat Stubble
G) Winter Annual Weeds Have Arrived
H) Are Your Weeds Dead?
I) 2003 Ohio Wheat Performance Test Results

A) Sclerotinia white mold and SDS - the late season diseases have arrived early - Anne Dorrance CORN Questions

The unseasonably cool, wet weather have provided a perfect environment for several soybean pathogens this summer - and we are now beginning to see those effects. In addition, the heavy rains that fell across the area two weeks ago, put several patches of beans over the edge - as many of these have had repeated flooding events over the course of this summer. I think that this summer in particular - field locations where beans were planted into soil that was not quite fit or drainage is poor have really shown up. The challenge now is to sort out - what is the summation of repeated flooding and saturation from the diseases. The management for the future can be quite different - flooding injury can be corrected by fixing the drainage issues in a particular area of the field. The diseases can be corrected by choosing varieties that have resistance to those diseases.

Sclerotinia stem rot-This is caused by a fungus and appears as standing dead plants in the field. When you walk into the field, you will see a fluffy white growth on the stems and sometimes you can see hard black bodies called sclerotia. The infections occurred about 3 weeks ago, spores of this fungus land on blossoms and begin to colonize. In the past three years, the environment has been very dry and the fungus did not develop. In more common white mold years-infections will begin - but will sit dormant until the cooler weather in the fall - so we will only begin to see yield reduction when there is approximately 10 to 20% of the plants with disease. I have been asked if fungicides should be sprayed at this time. Absolutely not. The infections took place quite a while ago-the only new infections that will occur are from infected plants falling onto healthy plants and lodge in the field. Spraying will only increase the lodging. Economically, the cost of the fungicide and application will not result in a yield benefit to pay for this cost. In addition, the days to harvest interval is fast approaching.

SDS/Brown stem rot/Pythium-Rhizoc-Fusarium-flooding-we have an interesting situation this year as many fields have plants with early yellowing and interveinal browning with some yellowing bordering the brown areas. We are recovering a number of pathogens and opportunistic fungi from the crowns of these plants. The environment this year is textbook perfect for SDS and Brown stem rot, but we have also had repeated flooding events for this complex of Pythium, Rhizoctonia, and Fusarium root rot to continue to colonize these plants throughout the entire growing season. This has been an unusual summer. How to separate all of this. Dig the plants up, do not pull them up. Peal the clay off of the roots. (I've found that the soil doesn't shake off this year). Look at the secondary roots, if they are cream or white and have no cortical tissue left - or it can be easily pulled off in your hand - this is flooding injury. The soil may also smell "dead". To prevent this in the future - correcting drainage should alleviate this. If the roots are rotted, black (including the center of the root or stele) and the crown is a grayish color when you cut it open, but the pith is white - SDS is the most likely culprit. Look for SCN. In the presence of SCN, symptoms are more vibrant and death comes a bit earlier. Management again, is improving drainage and following a management scheme for SCN. For brown stem rot, the roots are in better shape, but the pith will be brown - on plants where symptoms are "new" the pith is not degraded but brown. I have seen this misdiagnosed on several occasions where the pith is degraded but it is not brown stem rot. Pith degradation can also occur from this root rot-flooding injury complex.

Pictures of all of these diseases can be found at the website: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/ In summary, the plants are now showing the symptoms of the type of growing season it was this year.

B) Will slower than normal corn development increase likelihood of frost injury and slow grain drying? - Peter Thomison and Allen Geyer CORN Questions

Cooler than normal conditions have slowed the development of the corn crop this summer. Moreover, due to excessive rainfall many corn fields were planted late (well into June) or replanted. According to the Ohio Agricultural Statistics Service, as of Aug. 17, 2003, corn is 52 percent in dough and 4 percent in dent compared with 66 and 17 percent for the five year average, respectively. The later than normal maturation of the corn crop has led to questions about the potential for frost damage and whether more fuel will be needed to dry the crop.

Physiological maturity (when kernels have obtained maximum dry weight and black layer has formed) occurs 7 to 8 weeks after silking. At physiological maturity (kernel moisture approximately 30-35%), frosts have little or no effect on the yield potential of the corn crop.

Table 1 shows the average number of calendar days and growing degree days (GDD) required to reach physiological maturity after silking. With average daily high and low temperatures of 80 and 60 degrees F, 20 GDD accumulate each day. At these temperatures, 65 days from silking to maturity are required (1300 divided by 20). Therefore, corn silking in early August would probably not be safe from major yield reductions due to frost until early Oct.

With "cooler" high and low temperatures of 75 and 55 degrees, only 15 GDD accumulate daily, requiring 87 days from early August silking to maturity. This would require warm, frost-free weather until mid October, which is past the average frost date for much of the state.

Table 1. Relationship between kernel growth stage and development. Source: National Corn Handbook. Chapter 40. Growing Season Characteristics and Requirements in the Corn Belt. R.E. Nield and J.E. Newman, 1986.

*Based on average daily high and low temperatures of 80 and 60 degrees F, respectively, during grainfill.

How many GDD can be expected from now until an average date of a killing frost? To answer this question, estimate the expected GDD accumulation from August 19 until the average frost date (50% probability) for different regions of the state (Table 2). These GDD expectations are based on 30-year historical normals reported by the Ohio Agricultural Statistics Service. The growing degree day accumulation was calculated using the 86/50 cutoff, base 50 method.

If you want to determine the "youngest stage of corn development" that can safely reach black layer before the average frost date at a given weather station, use the information in Table 2 on remaining GDD in conjunction with Table 1 which indicates GDD requirement to reach black layer at various stages of grain fill. Compare "GDD remaining" for the site with the GDD required to achieve black layer depending on the corn's developmental stage.

Table 2. Estimated GDDs remaining from August 19 to the first fall frost for Ohio.

If your corn is in the early dent stage as of August 19, will it be safe from frost? Table 1 indicates that it needs about 510 GDD to reach black layer from full dent stage and Table 2 indicates that all regions of the state will accumulate sufficient GDD to escape frost injury.

If your corn is in the late milk/early dough stage as of August 19, will it be safe from frost? According to the kernel development - GDD accumulation relationships indicated in Table 1, corn needs about 775 GDD to reach black layer. Table 2 indicates only two regions of the state (SW and SC Ohio) with that number of GDDs remaining until the 50% frost date. However, late planted corn has shown the ability to adjust its maturity requirements, and most of this adjustment occurs during the late kernel development stages. Corn planted in early June compared to early May requires 200 to 300 fewer GDDs to achieve physiological maturity. Also, during past growing seasons (such as 1992) when GDD accumulation was markedly less than normal, the corn crop achieved physiological maturity before the first frost occurred.

When GDD accumulation is less than normal due to late planting or a cool growing season, it may result in grain moisture content at black layer that is higher than normal. Also since black layer is likely to occur at a later calendar date (late September-early October), fewer drying days may be available. However, in several recent years when we have experienced later maturing corn crops, warm dry conditions in September have facilitated crop maturation and grain drying and minimized drying costs.

C) Soybean Insect Update - Hammond & Eisley CORN Questions

Numerous insects are occurring in soybean fields that still need watching. Soybean aphids continue to be a concern, especially in later planted fields that are still in the pod development stage. In earlier planted fields that are in the seed fill stage (R5-6), the potential for damage lessens. For those fields, treatment would probably not be warranted unless aphids numbered well over 1000 and the population is continuing to increase. This is probably not happening in most of those fields. Additionally, fields already with plants that are stunted, having yellow-margined leaves, covered with honeydew and sooty mold, should be checked carefully before taking further action. Much of the damage has probably already occurred, and most likely, aphid numbers have crashed. Do not make your treatment decisions on the appearance of injury or symptoms, but rather on the level of the aphid population. The majority of grower calls on aphids, however, are coming from fields planted later where crop development is not as far along. Soybeans still in the pod-development stages should be protected if aphids are reaching over 300 per plant and the population still is on the increase.

Numerous defoliators continue to cause significant injury, including Mexican bean beetle in east central Ohio and grasshoppers throughout the state. As the soybeans enter the pod fill stage, the level of defoliation needed before treatment is needed rises to 25-30%. However, be aware that grasshoppers will also feed on the pods and destroy seeds. Growers should monitor any pod feeding closely. As with bean leaf beetles, treatment would be warranted if pod feeding goes over 10% and grasshoppers are still active. Bean leaf beetles, which will be entering their pod feeding stage, are addressed in the next article.

D) Bean Leaf Beetle Pod Feeding - Hammond & Eisley CORN Questions

Bean leaf beetle (BLB) populations are larger this summer in Ohio than we have seen in a few years. The concern with the bean leaf beetle will soon be shifting to pod feeding as we move into late August and early September. Pod injury could become more evident as the second BLB generation emerges and green pods become more attractive to feeding than foliage.

When pod injury occurs on 10-15% of pods, seed injury will become evident and yield losses are possible. If the pod injury occurs during periods of wet conditions that enable infection by seed diseases, the development of moldy bean seeds may lead to a loss in seed quality. The assessment of a field infestation depends on (1) determination of the current level of pod injury, (2) the abundance of adult BLB activity using a sweep net, (3) consideration of weather factors that may enable infection of the damaged pods by disease agents, and (4) the amount of time remaining before total leaf drop and dispersal of a BLB population from the field.

Growers should be sampling their fields weekly for the remainder of the summer. Hopefully visits have been made the past month to determine the relative size of the first generation to help with treatment decisions. Those fields that had large populations in the past few weeks should be monitored closely. The percentage pod injury is determined by randomly counting the total number of pods and those with feeding injury on about twenty plants. Adult BLB population should be estimated by taking 10 sweeps with a sweep net at three to four locations in a field.

Rescue treatment to prevent excessive development of seed damage may be warranted when pod injury exceeds 10%, fresh feeding scars predominate, and adult BLB are still present and actively feeding. Less than five BLB per sweep are unlikely to cause significant injury. Five to ten BLB per sweep indicates a potential problem possibly warranting rescue treatment. More than ten BLB per sweep will result in significant injury especially if two or more weeks remain until leaf drop. When the foliage dries and drops, beetles exit the field. Thus, the time remaining for BLB feeding is a key factor in the occurrence of pod injury.

Growers also should be aware of the harvest interval with the insecticide that is used for BLB control. Many of the materials have waiting periods of between 45 and 60 days, which would probably limit their use for use at this time. Because fields with higher populations of BLB tend to be the earliest planted fields, treatment-harvest intervals are now a concern for control of this insect.

E) Weed Control Comments About Late-Planted Soybeans - Stachler CORN Questions

Due to the continuing rains in many parts of Ohio, many late-planted soybean fields have not been sprayed. This presents some concerns that are addressed below:

1. For non-Roundup Ready soybeans, choose herbicides that are gentle on the soybeans. Herbicide injury at this time of the season to small soybeans almost always reduces soybean yields. FirstRate and Pursuit are the safest, the remaining soybean herbicides are capable of producing yield-reducing injury when applied to small soybeans at this time of the season. Choose adjuvant combinations that reduce soybean injury and consider using lower herbicide rates when possible to reduce injury.

2. Weeds in small soybeans at this time of the season usually have less effect upon soybean yields than do early-planted soybeans. This occurs because fewer weeds emerge in late-planted soybeans and soybeans grow faster compared to early-planted soybeans. However, the cooler temperatures and above-normal precipitation has allowed more weeds to germinate in late-planted soybeans compared to most years, therefore yield reductions may be greater this year for late-planted soybeans.

3. For Roundup Ready soybeans, be sure to adjust the glyphosate rate for the larger weeds and the types of species at the time of application. For generic glyphosate products consider including additional surfactant at 0.25 %v/v when spraying large weeds. Use a good quality surfactant.

F) Weed Control in Wheat Stubble - Stachler CORN Questions

The continuing rain showers have delayed the mowing of wheat stubble. As discussed four weeks ago in a newsletter article entitled "Wheat Stubble Friend or Foe?", mowing wheat stubble has many advantages. Now that little has been done some strategies may need to change.

• Weeds are getting larger with many entering the flower stage, especially grasses and some broadleaf weed species. If nothing is done soon to control summer annual species, large amounts of seed will be produced, causing more problems in the future.
• Scout wheat stubble fields for the presence of marestail (horseweed). If present, eliminate seed production of this species. We believe marestail seeds can travel up to 0.75 miles from where the seeds are produced and maybe farther. In Southwest Ohio, be sure to include 2,4-D ester with glyphosate in order to control glyphosate-resistant marestail. This is IMPORTANT because there are indications that the glyphosate-resistant marestail may also be in Butler, Fayette and Ross Counties. For the remainder of the state, if using glyphosate only, be sure to use high rates but strongly consider adding 2,4-D to achieve good control.
• Mowing wheat stubble now should greatly reduce the amount of summer annual weed seed production. Mowing now will not allow good regrowth of warm-season perennial weed species for good long-term control. Mowing now may still allow time for proper regrowth of cool-season perennial weed species, although there is no guarantee.
• Spaying wheat stubble fields now and not mowing or mowing later may be a more viable option since the field may be more fit to handle a sprayer compared to a tractor and mower. If choosing this method of control, remember to use high rates of glyphosate plus 2,4-D ester because of the large weeds and use high gallonage to achieve good spray coverage. This strategy will certainly not allow for proper control of warm-season perennials and may cause many of the cold-season perennials to not regrow properly for good long-term control. Depending upon the amount of winter annual germination and regrowth of cold-season perennials in the field, a second herbicide application may not be necessary.
• Many wheat stubble fields have lots of volunteer wheat emerging. Most years we get some germination early and then another flush later when significant rainfall occurs. Maybe this year most of it has germinated already, although time will tell. A herbicide application now would be a great way to get rid of the winter wheat, but scout the field for any later germination.
• Where tillage is planned it may be wise to make an herbicide application now to control the summer annual weeds since it may be a few weeks before tillage can be done. This may provide some good control of warm-season perennial weed species and for controlling the volunteer wheat, although an excellent seedbed will be made for winter annual weeds to germinate. The other option is to do the tillage now which will stop most summer annual weed seed production and spray later. With this option, the field will need to be level enough for the later herbicide application. Planning for the later herbicide application will be wise because the tillage operation will not kill all of the volunteer wheat or simple perennial weed species and will make a nice seedbed for additional volunteer wheat and winter annual weed species to germinate.

G) Winter Annual Weeds Have Arrived - Stachler CORN Questions

Common chickweed and purple deadnettle began emerging in central Ohio last week! With the excess moisture and poor crop canopies in many fields, winter annual weeds could be very numerous this fall.

H) Are Your Weeds Dead? - Stachler CORN Questions

If you have a Roundup Ready soybean field that has been completely clean and now there are some marestail (horseweed) plants starting to come above the soybean canopy, then it is possible those plants are glyphosate resistant. We have reports of the glyphosate-resistant marestail spreading to surrounding counties from where last years plants were found. If individual plants of any weed species from any herbicide program, especially ALS-inhibiting herbicides, are surviving and right next to them are dead plants of the same species, then it is possible those plants are resistant. If you believe you have herbicide-resistant weeds feel free to call Mark Loux or Jeff Stachler. Jeff Stachler can be reached at 614-292-1393.

I) 2003 Ohio Wheat Performance Test Results - Beuerlein, Lipps, Minyo CORN Questions

The OSU Wheat Performance Trials have been published and are also on-line; trials can be viewed at this website: http://www.oardc.ohio-state.edu/wheat2003/

The purpose of the Ohio Wheat Performance Trial is to evaluate wheat varieties, blends, brands, and breeding lines for yield, grain quality and other important performance characteristics. This information gives wheat producers comparative information for selecting the varieties best suited for their production system and market. Varieties differ in yield potential, winter hardiness, maturity, standability, disease and insect resistance, and other agronomic characteristics. Selection should be based on performance from multiple test sites and years.

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: Harold Watters        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.

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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.

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