In This Issue:
- Yellow and Off Color Corn: A Cause for Concern?
- Potential for Nitrogen Loss After All the Rain
- Slug Update
- Cutworm In Corn
- Will There Be Root Rots?
- Weed Management Issues in Soybeans
- Section 18s and Soybean Rust
- Powdery Mildew is Still on the Move in Spite of the Cold Temperatures
- The Risk of Wheat Scab Remains Low in Ohio
- Alfalfa Insects
- Estimating Soybean Stands
- Armyworm in Corn and Wheat - May 24 Update
Authors: Peter Thomison, Robert Mullen
The recent period of cool, rainy weather has slowed growth of early planted corn. Corn seedlings often turn yellow (due to low nitrogen uptake and/or limited chlorophyll synthesis) or purple (reduced root development) under cool, wet conditions. Some hybrids are more likely to increase anthocyanin (purple pigment) content when plants are cool. Yellowing or purpling of corn plants at this stage of development generally has little or no effect on later crop performance or yield potential. If it's induced by environmental conditions, the yellow or purple appearance should change to a healthy green after a few sunny days with temperatures above 70 degrees F. If plants remain yellow, then closer inspection and assessment is needed to determine if yellowing is caused by nutrient deficiency or some other factor. Given our weather conditions this spring, nitrogen deficiency may be something to look for early this growing season (see next article on nitrogen).
Authors: Robert Mullen, Edwin Lentz, Peter Thomison
The big question for those who applied early-spring nitrogen is – how much nitrogen is still around after all the rainfall? The answer is “it depends”. It depends upon 1) when the nitrogen was applied, 2) the form of nitrogen applied, 3) soil temperature, and 4) soil moisture. There are two primary nitrogen loss pathways – leaching and denitrification, and the pathway for nitrogen loss is different under different soil and climatic conditions. Coarser textured soils (sandy) are more susceptible to leaching and fine textured soils (clayey) are more prone to denitrification losses of nitrogen, but leaching can and does occur on poorly drained ground especially soil that has improved drainage. Leaching occurs when nitrate forms of nitrogen are present in soil solution and precipitation greatly exceeds evapotranspiration resulting in downward movement of soil water. Because nitrate moves freely with soil water it goes down through the profile and enters tile drainage or it goes deeper into the soil profile. Denitrification occurs when soils are waterlogged or saturated for extended periods of time and soil temperature is adequate for microbial activity. Since there is a deficit of oxygen in the soil environment, obligate and facultative anaerobes will seek out nitrate and strip off oxygen(s) resulting in the formation of nitric or nitrous oxide gas which can leave the soil system.
Soil temperature is key in determining which loss pathway to be concerned about because leaching will be more prevalent when soil temperature is low (< 50 ºF), and denitrification is more prevalent when soil temperature is high (> 50 ºF). Both require nitrate to be present in the soil, so the first consideration is how much nitrate is in the soil? Nitrate-nitrogen is formed when soil ammonium is oxidized under adequate soil temperature and soil moisture conditions, and this reaction is called nitrification. Being the result of a biological reaction, the warmer the soil temperature the faster the formation of nitrate. Because it is an oxidation reaction, abundant oxygen must be present to allow the reaction to occur (saturated soil conditions inhibit nitrification).
It is difficult to discuss the influence of timing without considering the form of nitrogen, so they will both be discussed together. Application of anhydrous ammonia is the form of nitrogen least susceptible to nitrogen loss because it is all in the ammonia form. This means two things – 1) all bacteria responsible for nitrification are killed in the ammonia saturation zone, and 2) the ammonia must be converted to nitrate to be susceptible to loss. Because the bacteria is controlled (at least in the ammonia saturation zone) ammonium (form of ammonia in the presence of water) remains in the soil for a longer period of time, but eventually the bacteria recover and begin their work of nitrification. Remember, nitrification is a microbial process that requires adequate soil temperatures, so low soil temperatures (around 50 ºF) do not promote nitrification. Fortunately, soil temperatures have been fairly mild this spring especially in the Northern half of the state. If anhydrous ammonia was applied as the nitrogen source, look for the number of days when soil temperature was above 50 ºF to get an idea of the nitrification rate. Generally speaking, soil temperatures around 60 ºF for three weeks results in about 50% of the total ammonium present in a soil being converted to nitrate. In the Northern half of the state we have not hit the 60 ºF mark too often and we have just barely been above 50 ºF, so nitrate formation should have been relatively slow (at least less than 40%). Thus nitrate loss from application of anhydrous should be smaller in the North. Central and Southern parts of Ohio have seen higher soil temperatures, so if anhydrous was applied a considerable amount of nitrate was formed, but do not assume all of the nitrate-nitrogen was lost especially on fine textured soils. Leaching may have occurred, but denitrification would have been slowed (at least initially) because soil temperatures were around 55 ºF. How much of the actual nitrogen was lost? This can not be pinpointed, but crops should be watched early to look for signs of nitrogen stress. Dry urea products would be considered similar to anhydrous ammonia when considering nitrification rates (a little quicker), but any form of nitrogen fertilizer that contained nitrate is at serious risk for loss after the several inches of rain that fell last week. In those fields be especially vigilant in looking for nitrogen stress.
There are a couple of tools that you can utilize to help you decide if you need additional nitrogen. One is the presidedress soil nitrate test (PSNT). This tool is primarily recommended when manure is applied in the fall, but it may help you make a decision on whether or not to apply additional fertilizer nitrogen. PSNT values above 25 ppm are less likely to respond to additional fertilize nitrogen. The other option is establishment of a nitrogen reference strip. This reference strip would give us a direct field comparison of where additional nitrogen was supplied and where it was not. A visual assessment would be adequate to determine if application of additional nitrogen is warranted. The rate does not have to excessive (about 50 pounds of nitrogen per acre), but it should be in a representative area of the field.
If you remember from last year we made available a simple question form to help you determine if you should consider more nitrogen. It is available at http://corn.osu.edu/story.php?setissueID=41&storyID=196. It may be a bit early for this article, but we are priming you for the next couple of weeks.
Authors: Ron Hammond, Bruce Eisley
We are now getting reports on slug feeding on field crops. These reports began early last week from fields in southeast OH along the river, and later in the week from fields near Columbus in Fairfield County. We are now seeing more feeding in more northern counties this week. The feeding we are seeing is following the relative size of the juvenile slugs. While sampling in Fairfield County southeast of Columbus, we saw slugs about 0.0275 gm in size, which is when feeding becomes obvious. As these slugs continue to grow, we expect to see much more feeding in the coming weeks.
Thus, it is critical that no-till growers begin their monitoring for slug injury. A concern that we have is that the cool and wet weather is resulting in crops that are not growing at a normal pace. Thus, corn and soybean will be in a more susceptible stage of growth (that is, smaller plants) for a longer period of time. When determining the need for control, take special note of the newer leaf tissue, either the inner most leaves in the corn whorl, or the unifoliates or new first triofoliates on soybeans. If this new leaf tissue is being severely fed on, treatment is perhaps warranted.
When treating, growers have two molluscicide toxins they can use. The first and most common are baits containing metaldehyde. There are three of these baits available, 1) Deadline MPs and 2) Orcal Snail and Slug Bait, which are similar materials that should be applied at 10 lb/acre. The other metaldehyde bait is 3) Metarex, a product from France that is made by a different technique than the other two baits. This different method produces bait which is smaller and more consistent in size with much less dust. Because of its easier flow-ability, Metarex can be applied at 7.5 lb/acre. However, pricing of all three materials on a per acre basis is probably close. The other toxin becoming more available is iron phosphate, which is in bait known as Sluggo. While this bait is a natural product and is considered “safer”, our thought is that the metaldehyde baits, when used as directed, are also very safe. Most problems with metaldehyde baits come from pets getting into open bags or from spills. When broadcast over fields, all the molluscicide baits are considered safe, including to the environment. Although we have not had as much experience with iron phosphate baits, they have been improve compared to years ago, and early indications are that if applied at the appropriate labeled rates with even coverage, they should do a good job. The availability of all these baits with the exception of Deadline MPs in OH is unknown, and can only be determined by checking with your dealers. Although we admit these treatments at the suggested rates are expensive, ranging from $16-18 per acre, we would point out this is still a better option compared to the amount of damage that can result, often a significant stand reduction or a yield loss greater than 20%. And compared to the potential cost of some of the stacked transgenic traits or some of the new fungicides for rust control, molluscicide baits might even be a bargain.
Although some growers swear by the spraying of 28% N or other concoctions, our experience suggests highly variable control. It seems like that for every grower who says they work, we talk to 3-4 growers who says the do not. While we do not recommend these sprays, growers might choose to try them. If you do, use them at night with no wind when the slugs are climbing on the plants. Check the crop prior to spraying to make sure the slugs are out of the ground and climbing on the plants. As with any type of treatment, come back in a few days to a week to determine the level of control.
Authors: Ron Hammond, Bruce Eisley
We have been receiving reports of cutworm feeding injury and requests for thresholds necessary to warrant treatment. There are a number of factors that need to be addressed when making a decision to apply a rescue treatment. We will try to address some of these factors.
In many insects, the larger the insect the more damage they might cause. However, this is different in cutworm because when worms become a certain size, their feeding is almost complete and additional injury will not take place. Thus size of worms is very important. The worms that we were finding in central Ohio last week were from ¾ to 1 inch, which would suggest that additional feeding was likely. It is also important to remember that smaller worms are easier to kill and large worms (> 2”) are almost impossible to kill.
Plant size is also important because the larger the plants the less plants that will be damaged. For example, cutworms regardless of size will cut more 1-leaf stage plants than 6-leaf stage plants. Plants that we were observing in central Ohio last week were in the 1- to 2-leaf stages.
So taking all of these factors into consideration a simple guideline suggests that treatment might be considered when 3% or more of the plants are being cut and both the worms and plants are small. The object of any rescue treatment is to prevent any further cutting. A listing of insecticides labeled for cutworm control can be found at: http://entomology.osu.edu/ag/545/cicw.pdf.
Authors: Anne Dorrance, Pierce Paul, Dennis Mills
The vast majority of the corn and soybeans were planted before the past 10 days of rain. To our advantage soils in most of the state were very dry, with the exception of the southeastern corner, we needed the rain. In addition, for most of the state, the rains that fell were not heavy pounders, dropping an inch in 20 minutes or less. Having said this, prolonged wet spells and cool soil temperatures favor many soil borne pathogens, especially Pythium and Phytophthora. For our greenhouse and lab studies, we routinely bait these pathogens from soil, we accomplish this by drying the soil, bone dry – like much of the state before the rains fell. We then flood the soil overnight and then incubate it for 2 weeks prior to planting. It takes two weeks of incubation to really prime these pathogens. Based on this, combined with how dry the soils were in most of the fields in Ohio before the rains and the fact that soil temperatures were fairly cool throughout the rainy period, we don’t expect widespread root rots. However, if we get another heavy saturating rain, then the system will be set up for widespread problems. What we can expect to see is flooding damage. If the seed was planted a few days before the rains, did not germinate and had a seed treatment they might pull through. Where we expect to see the most problems, is where seeds germinated then water ponded over the top. When soils are flooded for more than 48 hours, carbon dioxide builds up and can kill the seedlings. Symptoms of flood injury on most plants are similar, the roots have a grayish color vs tan to brown caused by root rots. The other noticeable difference will be the smell when you walk into the field, it will stink if it is flooding injury. Over the next week as the soils begin to warm, we should finally see plants emerging.
Authors: Mark Loux
In our travels around the state last week, we saw a number of no-till soybean fields that had obviously not been treated with herbicide this season. Some of these had not yet been planted, while others had been planted (with Roundup Ready soybeans we assume) but the weeds were much more evident than the soybeans. The abundant populations of cressleaf groundsel in these fields added much color to the landscape. Some suggestions on weed management in these fields:
- One of the benefits of an April burndown in no-till soybeans is the prevention of seed production by winter annual weeds and dandelions. Allowing fields to go this long into the spring before herbicide application results in the production of millions (dare we say billions) of weed seeds per acre, which serve as the source for future weed infestations. At this point, it is too late to prevent most of the winter annual seed production for this year, but these types of situations make a good case for fall herbicide treatments. Fall herbicide treatments are effective for control of winter annual weeds, and can prevent much of the weedy mess that is evident in many fields now. Give serious consideration to fall treatments in fields that are not likely to receive an early-spring burndown treatment next year.
- In the fields that have not yet been planted, most effective control of existing weeds will occur with tillage or a high rate of glyphosate (at least 1.5 lbs ae/A). Where time before allows, include 2,4-D ester at the rate of 0.5 lbs ai/A. We question whether a tillage tool that is designed for final seedbed preparation will effectively remove deep-rooted weeds. Where the field needs to be tilled, consider applying a pre-tillage application of glyphosate to ensure that existing weeds are completely controlled. Wait at least two days between herbicide application and tillage.
- In no-till Roundup Ready soybean fields where the soybeans have emerged and no herbicide has been applied, apply 1.5 lbs ae of glyphosate per acre as soon as field conditions permit. Scout fields in about two weeks, and make a second POST glyphosate application as needed to control later-emerging weeds or to complete control of weeds that survived the first application.
- Be cautious about mixing glyphosate with other herbicides in those fields that are a weedy mess, especially PRE herbicides that have contact activity on foliage. For example, we know that Valor can occasionally antagonize glyphosate activity, but usually does not when applied early in the season where 2,4-D is a component of the mixture. However, at this time of the season when weeds are much larger and 2,4-D ester cannot be used because planting is imminent, Valor is more likely to reduce the activity of glyphosate. Metribuzin can cause the same type of problem. Use true ammonium sulfate (liquid or spray-grade dry) in the spray mixture when going after large weeds, instead of an ammonium sulfate replacement product.
- The combination of glyphosate with chlorimuron (SynchronyXP, Classic) or cloransulam (FirstRate, Amplify) can improve control of some weeds, and should not cause antagonism problems.
- Keep in mind that a burndown treatment or POST glyphosate treatment applied now controls a significant percentage of this year’s summer annual weeds such as lambsquarters and giant ragweed, so resist the temptation to use low rates just to save a few dollars. For example, the model for giant ragweed emergence developed by OSU weed scientists Brian Schutte, Emilie Regnier, and Kent Harrison, https://agcrops.osu.edu/weeds/research/2006ragweedemergence.php, shows that about 70% of this season’s predicted giant ragweed emergence has already occurred. So, using a high enough rate of glyphosate to ensure control of the large giant ragweed present in a field now is an effective management strategy. Just be sure to scout and make the second glyphosate application to control the other 30% of the giant ragweed population that is yet to come.
- Glyphosate is most consistently effective on lambsquarters, velvetleaf, morningglory, and waterhemp when these weeds are less than 6 inches tall. OSU field research on various lambsquarters populations has shown that the 0.75 lb ae/A rate of glyphosate that many growers use in every situation is not high enough for consistent control of even small lambsquarters. More consistent control of this weed results from use of 1.1 lbs ae/A on plants less than 6 inches tall, and 1.5 lbs ae/a on larger plants, and following with a second POST application. The same strategy should be used on giant ragweed, except that giant ragweed plants are more likely to be 6 to 12 inches tall at the time of the first application, when the other weeds in the field are less than 6 inches tall.
- University research on weed-crop interference has shown that weeds need to be removed before they exceed 6 to 8 inches in height, in order to avoid yield loss from early-season weed interference. These same studies show that the yield of Roundup Ready soybeans is maximized by starting weed-free at the time of planting, and using one the following strategies: 1) apply PRE (residual) herbicides at planting and follow with a POST glyphosate treatment; or 2) make two POST glyphosate applications – the first when weeds are no more than 6 to 8 inches tall, and the second about 3 weeks later (see also https://agcrops.osu.edu/weeds/documents/PRENeeds06.pdfand https://agcrops.osu.edu/weeds/documents/Soybean_Mgmt_Guide.pdf) The problem with the second strategy is that many growers fail to make the first POST application when weeds are 6 inches tall. The value of the first strategy is that the timing of the POST application is less critical, since the PRE herbicides have already controlled many weeds in the field. The first strategy is also more effective for control of weed populations that have developed or may be developing resistance to glyphosate, and reducing the onset of resistance.
Authors: Anne Dorrance
It will be too wet to work in the fields for a few more days, so I thought I would give you something to discuss at the coffee shops. First Section 18s fungicides. These labels are for emergencies. If soybean rust does get rolling, we needed a tool, none of the fungicides that were labeled for soybeans during 2003 had high levels of efficacy. The strobilurins if applied well before spores hit can slow soybean rust down, but it’s the triazoles that really stop this fungus. In addition, these Section 18 labels for soybean rust are also special, in that EPA gave them for a 3-year time period as part of a quarantine exemption. Soybean rust fits this criteria as a new invading pathogen. Most Section 18s require application every year. The spirit of a Section 18 label is that these are for emergencies – there is not supposed to be any marketing.
The sentinel plot network is more extensive in 2006 than it was in 2005. More sentinel plots were placed futher west in Louisiana and Texas. Good news from both Georgia and Texas last week, they have had no new infections, even in places where soybean rust overwintered. Temperatures in both regions are expected to rise which should continue to delay the build-up of soybean rust. Weather conditions in Texas have been especially hot and dry and not favorable for soybean rust, much like we had last year.
In Ohio, no soybean rust was identified. Scouting has started on a weekly basis. Sentinel plots in Licking, Madison, Henry and Wood counties range from unifoliate to V1 growth stage. No symptoms of brown spot or stand problems were reported.
Authors: Pierce Paul, Dennis Mills
In spite of the relative cool weather conditions, powdery mildew is still moving fairly rapidly in some locations. In some fields planted with susceptible varieties, the disease has reached threshold levels on the leaf below the flag leaf. Our research data suggest the best results from fungicide treatment of powdery mildew are achieved when applications are made between the flag leaf emergence and boot growth stages. The wheat is currently between early head emergence and flowering. However, since cooler temperatures have slowed the development of the crop, while the disease continues to spread to the upper leaves, a fungicide application may still be beneficial. The current pattern of disease and crop development may result in substantial damage to the flag leaf before grain fill is complete. Remember, the goal of fungicide application is to keep the upper leaves disease-free during grain fill. So, if a susceptible variety is planted and 2 to 3 pustules are detected on the leaf below the flag leaf, it is time to apply a fungicide. Both Tilt or PropiMax provide very good control of powdery mildew. Check labels for application timing restrictions.
Authors: Pierce Paul, Dennis Mills
Spore numbers are up. Data collected from spore samplers here in Wooster showed that the number of spores of Fusarium graminearum, the fungus which causes wheat scab, has increased following the recent rains. Moist, humid conditions are prevalent in most locations, so, even if the rains subside (as is predicted), spore numbers are still likely to increase over the next few days. The wheat is beginning to flower in some locations and we expect flowering to continue across the state over the next 7 to 10 days. Although the coincidence of flowering and high spore numbers will likely increase the risk of wheat scab over the next week or so, (the crop is most vulnerable to infection during flowering), infection of wheat heads by the fungus and subsequent development of the disease require warm (temperatures above 60 F), humid conditions. While we have had humid conditions, temperatures have been very low, and these cooler temperatures (especially night-time temperatures) will likely result in the risk of scab remaining low throughout the state. If humid conditions persist and temperatures increase later in the week (as is predicted), the risk of scab will likely increase, especially in areas where flowering is now occurring. This is the critical period for wheat scab development, so growers should continue to check the scab prediction center (http://www.wheatscab.psu.edu) for frequent updates.
Authors: Ron Hammond, Bruce Eisley
We have arrived at a turning point in alfalfa production, when first cuttings have been done or will be in the near future, and our concerns turn from alfalfa weevil to potato leafhoppers. Although weevils are not being reported in high numbers across the state, we have received a few reports of large populations. If you have recently had your first cutting, you should observe the regrowth for any signs of continue weevil feeding if they were present in your field. You should then begin your sampling for potato leafhopper. Leafhoppers have arrived in Ohio and will be moving to alfalfa shortly if not there already. For growers who have yet made their first cutting and alfalfa weevil is continuing to increase in size and feeding, we would recommend first determining if the alfalfa can be harvested rather than making an insecticide application. Recommendations call for consideration of harvesting when alfalfa is between 12 and 16 inches tall, and harvesting if weevils need control but the alfalfa is over 16 inches. We will have more information on potato leafhopper next week.
Authors: Dennis Mills
Soybeans are emerging now and evaluation of the stand is needed. Poor stands may occur at one time or another and producers need to be aware of the potential yield loss from those deficit stands and the cost of replanting. Soybeans have a high capacity to compensate for low plant populations and gaps in the row.
If a poor stand does occur producers should investigate.Was the poor stand the result of poor seed quality, cold wet soils, hot dry soils, planting too deep or shallow, soil crusting, herbicide injury, insect feeding, poor soil to seed contact, or disease infection? Determine if the cause can be corrected to avoid a similar situation.
One popular method to determine plant population is by using a hula hoop. This involves placing a circular measuring device such as a hula hoop on the ground and counting the plants contained within. Use the table below, listing various sizes of hoops, to determine the number of plants per acre.
Hula hoop method for determining drilled soybean populations
|InsideDiameter of Hoop|
|No. of Plants||30"||32"||34"||36"||38"|
Authors: Ron Hammond, Bruce Eisley
Having just received our first report of a serious armyworm problem in corn planted into a rye cover crop, it is time to begin scouting for possible economic situations. Field corn planted no-till into grassy habitats should be monitored closely for armyworm activity. Fields that may be at risk for significant armyworm infestations include corn planted no-till in rye cover crops and corn planted no-till into old hay fields. A severe infestation of armyworm can reduce stand when an infestation occurs in the pre-whorl stage and cause significant defoliation when corn is hit in the whorl stage. Total destruction of a field of no-till corn can occur if a severe infestation is allowed to develop without application of a rescue treatment.
Detection of foliar feeding injury by armyworm on 15 to 20% of a stand should be regarded as an indicator of a potential problem, and the field should be rechecked within a few days to determine whether the impact of defoliation is increasing and a rescue treatment may be warranted. In general, a severe infestation will impact almost 100% of a stand and defoliation of the plants will exceed 50%, stand height is being reduced, and some plants being eaten down to ground level. If defoliation remains less than 50% and the new growth exhibits minimal feeding injury, the stand will likely recover with minimal impact on yield. Rescue treatment in corn may be needed if stand infestation is greater than 50% and larvae are not mature.
Since armyworms are foliar feeders, they are relatively easy to control with most foliar treatments. A list of labeled insecticides for armyworm on corn is available at http://entomology.osu.edu/ag/545/cicaw.pdf. During the day, armyworm larvae will most likely be found seeking shelter in the whorl or possibly in the ground cover. In general, armyworm larvae will feed first on the lower leafs and then progress to the new growth - especially when corn approaches the early whorl stage.
Wheat fields should also be checked at this time for armyworm. Check several places in the field and determine the number and size of the worms present. Rescue treatment is recommended in wheat if counts average 6 or more worms per row foot and larva are in the early stages of development or if head cutting is occurring. Treatment of armyworm larvae reaching maturity will achieve limited results because the period of maximum feeding will have passed. A list of labeled insecticides for armyworm on wheat is available at http://ohioline.osu.edu/b545/pdf/b545.pdf on pages 20 and 21.
For additional information on armyworm in both crops, see our Field Guide, Bulletin 827, at: http://ohioline.osu.edu/b827/index.html.
State Specialists: Ann Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science), Peter Thomison, (Corn Production), Ron Hammond and Bruce Eisley (Entomology), Robert Mullen (Soil Fertility). Extension Educators: Howard Siegrist (Licking), Harold Watters (Champaign), Glen Arnold (Putnam), Roger Bender (Shelby), Steve Foster (Darke), Steve Bartels (Butler), Steve Prochaska (Crawford), Bruce Clevenger (Defiance), Gary Wilson (Hancock), Tammy Dobbels (Montgomery), Todd Mangen (Mercer), Ed Lense (Seneca), Alan Sundermeier (Wood), Mark Koenig (Sandusky), Jim Lopshire (Paulding) Greg LaBarge (Fulton) and Keith Diedrick (Wayne)