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September 2-7, 2003
C.O.R.N. 2003-29
In This Issue:
A) Hessian Fly in Volunteer Wheat
B) This Has Been a Tough Year for Soybeans
C) Evaluating Corn Hybrid Demonstration Plots
D) Controlling Dandelion with Fall Herbicide Applications
E) Control Weeds Before Wheat Emergence!
Reports of fields with large quantities of volunteer wheat are being received from some areas of the state, bringing up the question as how it might impact Hessian fly and other pests. Although the use of resistant varieties and planting after the fly-free date are the primary methods of controlling the Hessian fly, most states also recommend destroying volunteer wheat from nearby or adjacent fields to help eliminate fall egg-laying sites. Flies emerging in late-summer or early-fall will lay eggs on any available host plant, including this volunteer wheat. Controlling the volunteer wheat will help to keep spring Hessian fly populations down. Although tillage is appropriate, the use of herbicides is probably more cost effective.
Destroying volunteer wheat at least several weeks before planting the new wheat crop will also reduce the probability of diseases infecting the emerging wheat plants. Volunteer wheat can harbor virus diseases like barley yellow dwarf (aphid transmitted) and wheat streak mosaic (mite transmitted) that can serve as a source of the virus and insect vector. Several fungal diseases like leaf rust and Stagonospora leaf blotch can be present on volunteer wheat and wind and splashing rain can move spores to the emerging new crop in adjacent fields. Never plant wheat or spelt into the past seasons wheat stubble or volunteer wheat. Severe root diseases (take-all root rot) and foliar diseases generally significantly reduce yield.
The volunteer wheat must be controlled as soon as possible to be the most effective. If using herbicides to control the volunteer wheat, there are some things to consider from a weed management standpoint:
- volunteer wheat can be effectively controlled by glyphosate (at least 0.375 lb ae/A, 11 oz/A Roundup WeatherMax or 16 oz/A of 3.0 lb ae/gallon formulations), Select (8.0 oz/A), Assure II (8.0 oz/A), or Fusilade DX (16 oz/A).
- using Select, Assure II, or Fusilade DX will take out the volunteer wheat and other grass species and leave behind broadleaf weeds, especially winter annuals and cool-season perennials, that can be sprayed later to obtain more effective control.
- using glyphosate now to control volunteer wheat will reduce the effectiveness of controlling cool-season perennials and biennials and be too early to control most winter annual weeds. Despite this, it may still be better to apply glyphosate now to control the volunteer wheat and take what you get for cool-season perennial and biennial control. It certainly will be better than doing nothing, but not as effective as waiting until mid-October. Warm-season perennial weeds like hemp dogbane, johnsongrass, milkweeds, and bindweeds could be effectively controlled with this application as mid-September is a good time to be controlling warm-season perennials. To control all weeds in the field apply the appropriate amount of glyphosate for the toughest to control species.
If using tillage to control the volunteer wheat, scout the field later for winter annual weed germination. The tillage will create an ideal seedbed for winter annual weeds to become established.
Soybean fields in areas of Ohio that received higher than average rainfall are experiencing a number of problems as a result of all this rain. The symptoms in these fields include pockets of soybeans that are yellowing and dropping leaves or plants that are browning-up and dying in the field. The main culprits are the secondary effects caused by flooding injury as well as Phytophthora stem rot, Brown Stem rot, Sclerotinia Stem Rot, Diaporthe stem canker, Pythium-Rhizoc root rot complex and Sudden Death Syndrome (SDS). Soybean cyst nematode (SCN) is also causing some early senescing of soybeans in other areas of Ohio.
The challenge for producers and crop consultants is to diagnose the problems associated with these dying beans order to determine which management strategies should be put into place.
Sclerotinia stem rot is the easiest to diagnose - on the plants that are dying, the stems will have a white fluffy growth and black hard bodies can be found both on and within the stems. Choosing a variety with moderate levels of resistance will reduce losses to this pathogen. Soybean cyst nematode is also fairly easy to identify. When checking for SCN be sure to DIG plants from the soil and closely examine the roots. The cyst females look similar to tiny white pearls on the roots. Crop rotation is the most effective and consistent management procedure for this pest. Phytophthora stem rot is also fairly easy to identify. The classic symptom of Phytophthora stem rot is the continuous chocolate brown canker moving up from the soil to the plant. Choose a variety with a different Rps gene plus higher levels of partial resistance to manage Phytophthora stem rot. Diaporthe stem canker is identified by the presence of a girdling canker at the 3rd or 4rth node on the plant. Sometimes you can see tiny black dots or fruiting bodies in the canker.
The three that are the hardest to differentiate are flooding injury, Phytophthora root rot, Pythium-Rhizoc Root rot complex, SDS and Brown Stem rot. All of these are associated with wet, compacted soils with poor drainage. The general symptoms are an early senescing of the plant, often with brown interveinal necrosis, and rotten roots.
Brown stem rot is often characterized by the chocolate-brown color of the pith. This has not been a good diagnostic feature for Ohio fields this year. This trait on known susceptible varieties is fairly poorly developed. To complicate matters a bit further - we have found all four of these diseases in the same area of a field and a few times in the same plant. The crown or base of the soybean stem is a slight gray color when SDS is the main pathogen and cysts are also present. In addition, 1 to 2 days following heavy rains, the SDS fungus may produce spores on the surface of the crowns. Again, dig plants up and look for the blue-green colored spores. Brown stem rot may or may not have the internal browning, but the outside of the stem has a "greasy-green" appearance.
For the root rotters (there are few if any roots -- plants are stunted and are dying) there really is no way at this time to differentiate among the pathogens causing root rot. The main management for all of these soybean pathogens however, is to improve soil drainage. All of these pathogens require free water to begin to colonize and infect the soybeans. A good overall management plan will help reduce losses in the future and keep soybean production profitable.
For more information on management of these soybean diseases in Ohio - consult the Ohio Field Crop Disease Team website at : www.oardc.ohio-state.edu/ohiofieldcropdisease or the North Central Soybean Plant Health Initiative Website at: http://www.planthealth.info
PLANT/EAR HEIGHT. Corn reaches it maximum plant height soon after tasseling occurs. Remember that although a big tall hybrid may have a lot of "eye appeal," it may also be more prone to stalk lodging in the fall. Unless your interest is primarily silage production, increasing plant height should not be a major concern. Generally later maturity hybrids are taller than earlier maturity hybrids. Big ears placed head high on a plant translate to a high center of gravity, predisposing a plant to potential lodging. The negative effects of stalk rot on stalk lodging in the fall may be worsened by high ear placement.
STALK SIZE. Generally speaking, a thicker stalk is preferable to a thinner one in terms of overall stalk strength and resistance to stalk lodging. As you inspect a test plot, you will see distinct differences among hybrids for stalk diameter. However, also check that the hybrids are planted at similar populations. As population increases stalk diameter generally decreases.
LEAF DISEASES. During the grain fill period, leaf diseases can cause serious yield reductions and predispose corn to stalk rot and lodging problems at maturity. The onset of leaf death shortly after pollination can be devastating to potential yield, since maximum photosynthetic leaf surface is needed to optimize grain yield. Hybrids can vary tremendously for their ability to resist infection by these diseases. Pay close attention during the 6 to 8 weeks after pollination for development of leaf diseases among hybrids in a test plot. Demonstration plots provide an excellent opportunity to compare differences among hybrids to disease problems that have only occurred on a localized basis. While gray leaf spot and Stewart's bacterial leaf blight have received considerable attention in recent years, check plots for other diseases to identify hybrids with resistance. Diseases that have been seen more often than normal this year include northern leaf blight and common corn rust.
STALK ROTS. Hybrids will likely differ widely when faced with strong stalk rot pressure. Begin checking plants in late August or about 6 weeks after pollination by pinching lower stalk internodes with your thumb and forefinger. Stalks that collapse easily are a sure indicator of stalk rot. Remember that hybrids with thicker stalks may be in plots having thin stands.
STALK LODGING/BREAKAGE. Perhaps as important as stalk rot resistance is the stalk strength characteristics of a hybrid. Sometimes, superior stalk strength will overcome the effects of stalk rot. If your variety plot is overcome with stalk rot in late August and early September, be certain to evaluate the stalk lodging resistance of the different hybrids. Demonstration plots also provide a good opportunity to evaluate another stalk related problem, green snap (a.k.a. brittle snap). Green snap damage in Ohio has usually been limited to localized areas where severe windstorms occur prior to pollination. Although green snap is not a major problem in Ohio, as it is in the western Corn Belt, there are differences in susceptibility among hybrids that growers may want to consider to avoid risks. Because damage from European corn borer (ECB) and western corn rootworm (WCRW) can be very localized, strip plot demonstrations may be one of the best ways to assess the advantages of Bt corns. This year damage from ECB is common in certain parts of the state and WCRW damage was magnified by strong winds that hit some fields repeatedly during late vegetative growth and early grain fill.
HUSK COVERAGE/EAR ANGLE. Hybrids will vary for completeness of husk coverage on the ear as well as tightness of the husk leaves around the ear. Ears that protrude from the husk leaves are susceptible to insect and bird feeding. Husks that remain tight around the ear delay field drydown of the grain. Hybrids with upright ears often associated with short shanks may be more prone to ear and kernel rots that those ears that point down after maturity. Under certain environmental conditions, some hybrids are more prone to drop ears, a major problem if harvesting is delayed.
The following are some additional points to consider during your plot evaluations:
1. Field variability alone can easily account for differences of 10 to 50 bushels per acre. Be extremely wary of strip plots that are not replicated, or only have "check" or "tester" hybrids every 5 to 10 hybrids. The best test plots are replicated (with all hybrids replicated at least three times).
2. Don't put much stock in results from ONE LOCATION AND ONE YEAR, even if the trial is well run and reliable. Don't overemphasize results from ONE TYPE OF TRIAL. Use data and observations from university trials, local demonstration plots, and then your own on-farm trials to look for consistent trends.
3. Initial appearances can be deceiving, especially visual assessments! Use field days to make careful observations and ask questions, but reserve decisions concerning hybrid selection until you've seen performance results.
4.Walk into plots and check plant populations. Hybrids with large ears or two ears/plant may have thin stands.
5. Break ears in two to check relative kernel development of different hybrids. Use kernel milk line development to compare relative maturity of hybrids if hybrids have not yet reached black layer. Hybrids that look most healthy and green may be more immature than others. Don't confuse good late season plant health ("stay green") with late maturity.
6. Differences in standability will not show up until later in the season and/or until after a windstorm. Pinch or split the lower stalk to see whether the stalk pith is beginning to rot.
7. Visual observations of kernel set, ear-tip fill ("tip dieback"),
ear length, number of kernel rows and kernel depth, etc. may provide some approximate
basis for comparisons among hybrids but may not indicate much about actual yield
potential.
Common dandelion has become extremely problematic in many corn, soybean and wheat fields across Ohio over the past several years. Fall herbicide treatments are an effective tool for control of established dandelions, and are more effective than spring burndown treatments for reducing dandelion populations. In OSU research, we have been able to achieve near complete control of even relatively high dandelion populations with several fall herbicide treatments. Some suggestions/observations on fall herbicide treatments for dandelion control follow. The 2,4-D rates shown are based on a 4 lb/gallon formulation. Where a rate range is shown for glyphosate, the first number refers to the Roundup WeatherMax rate, and the second rate refers to Touchdown, Glyphomax, and other 4 lb/gal glyphosate products.
- Across all of the dandelion studies we have conducted in fields where soybeans will be planted, the most consistently effective dandelion control resulted from application of: glyphosate (22 oz/A of WeatherMax or 32 oz/A of other glyphosate products); a mixture of glyphosate (11 or 16 oz/A) plus 2,4-D (1 pt/A); or a mixture of Canopy XL (=> 2.5 oz/A) plus Express plus 2,4-D. These treatments can provide complete control of established dandelions, although this may not always occur. The Canopy XL treatment does have the advantage of controlling seedling dandelions the following spring (and low rates of Canopy XL are sometimes recommended with fall WeatherMax treatments for this reason).
- Where corn will be planted the following spring, effective dandelion treatments include the following: glyphosate (22 oz of WeatherMax or 32 oz of other glyphosate products; glyphosate (11 or 16 oz/A) plus 2,4-D (1 pt); 2,4-D alone (1 qt/A); Basis plus 2,4-D, or simazine plus 2,4-D (1 lb ai/A plus 1 qt/A).
- Application of 2,4-D alone (1 qt/A) is a low cost treatment for control of dandelions, but has typically provided about 20% less control compared to the treatments listed previously. In fields where dandelions and other winter annuals are the primary target of a fall herbicide treatment (but chickweed is not a problem), application of 2,4-D at the rate of 1 qt/A is an effective and inexpensive treatment.
- Treatments containing glyphosate should be applied with ammonium sulfate (and surfactant if required by the glyphosate label). Adding additional surfactant with cheap glyphosate formulations may improve weed control. Treatments that do not contain glyphosate should generally be applied with crop oil concentrate.
- Mixing glyphosate and/or 2,4-D with Scepter, Pursuit, Valor, or Sencor can result in reduced dandelion control, compared to application of glyphosate and/or 2,4-D alone.
- With all treatments, the effectiveness of control and reduction in the dandelion population will be dependent upon the population density. In low to moderate population densities, the treatments mentioned above are capable of controlling all or most of the dandelion present. This level of control may not be possible where dandelion population densities are extremely high.
- In our limited comparisons of various 2,4-D products, the 2,4-D rate appeared to be more important that the type of 2,4-D applied. When applied alone, the rate should be the equivalent of at least 1 lb ae per acre (1 qt of 4 lb/gallon products). In combinations with glyphosate or Canopy XL/Express, a 2,4-D rate of 1 pt/A appears to be adequate.
- We recommend adding 2,4-D to all fall herbicide treatments, including glyphosate, because a mixture of herbicides with several different sites of action should delay the development of herbicide-resistant weed populations. A 2,4-D rate of 1 pt/A should be adequate in mixtures with glyphosate, Basis, or CanopyXL/Express. 2,4-D will also help control a number of winter annual and perennial broadleaf weeds. Mixing 2,4-D with glyphosate can reduce control of Canada thistle and some perennial grasses, compared to glyphosate alone, but this type of antagonism should not be a problem with other weeds.
- One of the most common questions we receive about fall dandelion control involves the timing of application - can herbicides be applied too early or too late in fall? We do not have enough information to exactly define when herbicides should be applied, but anywhere between about late September and mid November can be effective. If possible, wait until after a light frost to apply herbicides, but try to apply when dandelions are still mostly green and actively growing (before a hard freeze causes plants to become mostly purple). We view fall herbicide treatments as the most effective tool for control of established dandelions, and an application too early or too late that results in slightly reduced dandelion control is still worth the cost and effort.
Many producers underestimate the value of a preplant/preemergence glyphosate application in no-tillage wheat. Many of the weeds that compete with wheat in late fall and early spring, including dandelions, purple deadnettle, chickweed, and mustards, have emerged by late September and can be controlled with glyphosate before wheat emergence.
In the last few years winter annual grasses such as downy brome, cheat, and annual bluegrass and perennial grasses such as quackgrass and tall fescue are becoming more prevalent in winter wheat. This is due to the drastic increase in no-tillage wheat. Besides harvest problems and competition weedy grass species can be a host to wheat diseases, especially take-all root rot.
An application of glyphosate before wheat emergence is a more effective treatment for dandelion and winter annuals, compared to the herbicides that can be applied broadcast to wheat in late fall or early spring. This is especially true for grass weeds in wheat since there is only one herbicide available in Ohio. In addition, a dense population of winter annuals or dandelions may have already suppressed wheat growth by the time a spring or fall treatment can be applied.
Dandelion is especially problematic because preplant tillage and most wheat herbicides have limited effectiveness on this weed. Stinger, Curtail, Express, Harmony Xtra, and 2,4-D have limited activity on dandelion in the spring, but may not be effective enough in dense dandelion populations. Dandelion should be controlled with tillage or glyphosate prior to wheat emergence, since options after emergence are less effective. Tillage must be thorough enough to completely disrupt dandelion plants, and this may not be accomplished with one pass of an implement designed primarily for seedbed preparation. Dandelions will reestablish following a tillage operation that does not completely disrupt their taproots. Higher glyphosate rates will be required for dandelion control compared to winter annual weeds. We suggest the following glyphosate rates for dandelion: Roundup WeatherMax - 22 oz per acre; Touchdown, Glyphomax, and other glyphosate products - 32 oz per acre. Include ammonium sulfate in glyphosate treatments, and the appropriate amount of surfactant if specified by the product label.
Glyphosate applied at 0.375 lb ae/A (Roundup WeatherMax at 11 oz/A and most other formulations at 16 oz/A) should control most winter annual grasses, however, greater than 0.75 lb ae/A of glyphosate (Roundup WeatherMax at 22 oz/A and most other formulations at 32 oz/A) should be used to control perennial grass species. Maverick (0.67 oz/A) can be applied in the fall after wheat has emerged to control most Bromus species and suppress most mustard species. However only corn or soybean should be planted the spring following wheat harvest, as the product has a long soil residual. Weed control with Maverick is poorer in no-tillage compared to conventional tillage.
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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.
State Specialists: Pat Lipps, Anne Dorrance & Dennis Mills (Plant Pathology), Mark Loux, Jeff Stachler, and Tony Dobbels (Weed Science), Ron Hammond and Bruce Eisley (Entomology), and Peter Thomison (Corn Production); District Specialist: Ed Lentz (Agronomy); Extension Agents: Howard Siegrist (Licking), Harold Watters (Miami), Glen Arnold (Putnam), Roger Bender (Shelby), Jim Lopshire (Paulding), Dusty Sonnenberg (Henry), Steve Foster (Darke), Barry Ward (Champaign), Steve Prochaska (Crawford), and Andy Kleinschmidt (Van Wert)Editor: Andy Kleinschmidt Web Editor: Nathan Watermeier
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