In the past two weeks, instances of true armyworms (TAW) in wheat, corn and pastures have increased across the state. TAW are similar in appearance and size to fall armyworm (FAW). TAW possess a mottled brown head capsule (Figure 1) while FAW have an inverted “Y” on their head capsule. TAW develop into six instars, with larval development taking roughly 20 days and generational turnover occurring in 30 days. This insect is not well adapted to hot temperatures, and survival decreases significantly when air temperature is above 86 degrees F. TAW prefer grass hosts but will feed on broadleaves. TAW primarily feed at night, making observation during the day difficult. Larva consume 80% of the total foliage required for development in the last three to five days as larva. Larva congregate at the base of plants and on the soil surface to avoid midday temperatures. There are several natural enemies of TAW in Louisiana field crops. Predacious insects, parasitoids and pathogens occasionally will control TAW populations before a foliar overspray is required
TAW infesting Bt corn rarely causes economic injury, and Bt proteins available in field corn work very well controlling TAW. Non-Bt corn can experience significant injury from TAW, and fields should be scouted regularly to avoid defoliation. TAW can graze non-Bt corn to the ground; however, if the growing point is still beneath the soil (up to roughly V5), corn seedlings will recover quickly.
TAW can significantly injure wheat if worms are allowed to defoliate the flag leaf before soft dough or clip wheat heads at any stage. The LSU AgCenter threshold for TAW in wheat is when five worms per square foot are found and foliage loss is occurring.
In hayfields and pastures, TAW can cause significant injury to grass crops if left uncontrolled. TAW injury is identical to FAW, and routine scouting in the spring is recommended. The LSU AgCenter threshold is one worm per sweep.
Pyrethroid insecticides control TAW very well in corn, wheat and pastures. As a general rule, large worms are harder to control than small worms.
If you have any questions or concerns, please contact your local AgCenter agent for more information.
Many producers are having to replant corn due to poor stands. There are three main ways to remove a failed corn stand.
Use tillage equipment to physically remove the existing corn.
Apply 0.0469 lb clethodim/acre and wait six days before planting the second corn crop. That equals 6 ounces of a 1 lb/gal clethodim, 3 ounces of a 2 lb/gal clethodim or 2 ounces of a 3 lb/gal clethodim. Waiting six days before planting is critical to prevent injury.
Apply 0.625 lb paraquat/acre plus atrazine at 1 pint/A or diuron at 1 pint/A or metribuzin at 3 oz/A. Good coverage is essential. Even then, don’t expect outstanding control with this choice.
This article covers how to estimate the yield potential of field corn. Please contact Drs. Dan Fromme, cellphone: (318)-880-8079 office: (318) 427-4424 or Josh Copes, cellphone (318) 334-0401, office (318) 766-3769 for more information.
The 9th installment of Louisiana Rice Field Notes is now available. This is the second flood edition this week. This edition covers recommendations on how to proceed with harvest with all of the flood damaged rice, a very important proposed changed to the crop insurance “practical to replant” definition and the final planting dates (FPD) for rice, corn, sorghum, cotton and soybeans, and an important flood recovery meeting in Crowley tomorrow.
Trey Price, Field Crop Pathology, Macon Ridge Research Station
Boyd Padgett, Wheat Pathology, Dean Lee Research Station
At the time of this writing, most wheat in the state is at or past flowering with the exception of some later maturing varieties. We have seen issues with vernalization in a few entries in variety trials throughout the state. Simply, there was not enough cold weather to trigger reproductive development. Fusarium head blight (scab, Figures 1 & 2) has been of utmost concern to the few wheat producers we have this year. Conditions have been favorable for scab during flowering, and applications of Caramba or Prosaro using maximum rates and water volumes are recommended for management. The best control we can expect is 50%, and time will tell if applications were successful or not.
Other concerns this season have been stripe rust (Figure 3) and leaf rust (Figure 4). Conditions are currently favorable for both diseases; however, stripe rust activity is slowly decreasing and leaf rust activity is increasing rapidly. Most varieties are resistant to stripe, leaf, or both rusts, and fungicide applications are usually not necessary. In susceptible varieties, rusts are effectively and economically managed with triazole fungicide applications.
Other diseases of note have been Septoria leaf blotch (Figure 5) and bacterial streak (Figure 6). Septoria usually remains low in the canopy and does not escalate to damaging levels; however, if infections occur on the flag leaf or flag leaf -1, a fungicide application may be warranted. Most fungicides provide adequate control of Septoria leaf blotch. Bacterial streak cannot be reactively managed. Fungicides are not effective, of course, so variety selection in the fall is the primary management technique. LSU AgCenter scientists rate wheat varieties for multiple diseases at multiple locations in the state, and the results are available online (www.lsuagcenter.com), from your county agent, or your nearest research station. Bacterial streak and Septoria leaf blotch can be difficult to diagnose. Older Septoria lesions will have black spots (pycnidia) within lesions, while bacterial streak will not. Younger Septoria lesions may be indistinguishable from bacterial streak lesions; therefore, a quick diagnostic method can be used. First, cut an affected leaf section then submerge in water. Wait 5-10 minutes, and observe for bacterial streaming (Figure 7). This can be accomplished on the turn row with a pocket knife and bottled water.
It is no secret that this has been a tough year for corn so far. Soon after early planting, most producers received copious amounts of rainfall (particularly in NELA) over an extended period. Many fields were replanted because of flooding. On stands that withstood the flooding, the majority of field calls have involved corn plants that had poor nodal root development causing them to fall over (Figures 8 & 9) and stressing or breaking the mesocotyl (first true stem) in the process (Please see Dr. Dan Fromme’s post for more information on rootless corn syndrome (RCS) http://louisianacrops.com/category/crops/corn/). Most producers planted on the higher end of plant populations allowing tolerable losses due to RCS.
Interestingly, damping off (Rhizoctonia solani) was commonly observed in RCS situations where fields had been planted for at least one month (V3-V4). Over time, seed treatment efficacy declined, plants were stressed (particularly at the mesocotyl), and the pathogen took advantage of optimal environmental conditions. Classic damping off lesions were observed on the upper sections of mesocotyl (Figures 10 & 11), and the pathogen was subsequently isolated in the laboratory.
Since we have a significant number of corn acres that will be relatively late, foliar diseases, southern rust (Figures 12 & 13) in particular, will likely be a concern this year. Southern rust (SR) can be devastating if it develops early (tasseling or before) and conditions (warm, wet) are favorable for development. Scouting is key to managing this disease. Typically SR will develop low in the canopy and progress upward. Fungicides are effective on SR (Table 1). If the disease is present at or before tasseling, fungicide applications are warranted. Depending on disease severity and prevailing environmental conditions, applications could occasionally be warranted between tasseling and milk stage. Applications are rarely warranted after this stage, because the crop will usually “out-run” disease progression. Keep in mind that tasseling is the most vulnerable stage to foliar diseases. As plants mature, more defoliation can be tolerated as time goes by.
Northern corn leaf blight (NCLB) is an annual problem in Louisiana. In fact, we can probably drop the “northern” at this point. Scouting also is key to managing this disease. Similar to SR, if NCLB develops during late vegetative stages or near tasseling, a fungicide application may be advisable. Once the disease initiates, it will continue to progress for the remainder of the season. Hot and dry weather may slow NLCB progression somewhat, but with most of our acreage irrigated, temperature and moisture requirements for the pathogen are satisfied until black layer. Specific fungicide efficacy data on NCLB remains elusive; however, pooling of nationwide data indicates that fungicides are effective on NCLB (Table 1). Similar to SR, the further the crop is past tasseling, more defoliation can be tolerated.
For more information please do not hesitate to contact your local county agent, specialist, or nearest research station. Please visit our websites (www.lsuagcenter.com and www.louisianacrops.com) for the latest in field crop pathology.
Table 1. Fungicide Efficacy for Control of Corn Diseases—April 2016
The Corn Disease Working Group (CDWG), which includes many members from the mid-South including several pathologists from Louisiana, has developed the following information on fungicide efficacy for control of major corn diseases in the United States. Efficacy ratings for each fungicide listed in the table were determined by field testing the materials over multiple years and locations by the members of the committee. Efficacy ratings are based upon level of disease control achieved by product, and are not necessarily reflective of yield increases obtained from product application. Efficacy depends upon proper application timing, rate, and application method to achieve optimum effectiveness of the fungicide as determined by labeled instructions and overall level of disease in the field at the time of application. Differences in efficacy among fungicide products were determined by direct comparisons among products in field tests and are based on a single application of the labeled rate as listed in the table.Table includes systemic fungicides available that have been tested over multiple years and locations. The table is not intended to be a list of all labeled products1. Efficacy categories: NR=Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent; NL = Not Labeled for use against this disease; U = Unknown efficacy or insufficient data to rank product
1Additional fungicides are labeled for disease on corn, including contact fungicides such as chlorothalonil. Certain fungicides may be available for diseases not listed in the table, including Gibberella and Fusarium ear rot. Applications of Proline 480 SC for use on ear rots requires a FIFRA Section 2(ee) and is only approved for use in Illinois, Indiana, Iowa, Louisiana, Maryland, Michigan, Mississippi, North Dakota, Ohio, Pennsylvania, and Virginia.
2Harvest restrictions are listed for field corn harvested for grain. Restrictions may vary for other types of corn (sweet, seed or popcorn, etc.), and corn for other uses such as forage or fodder.
Many products have specific use restrictions about the amount of active ingredient that can be applied within a period of time or the amount of sequential applications that can occur. Please read and follow all specific use restrictions prior to fungicide use. This information is provided only as a guide. It is the responsibility of the pesticide applicator by law to read and follow all current label directions. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Persons using such products assume responsibility for their use in accordance with current directions of the manufacturer. Members or participants in the CDWG assume no liability resulting from the use of these products.
Rootless corn has appeared in many fields during the early part of the 2016 growing season in Louisiana. Most are all of the nodal roots are missing in corn that is at the V1 to V2 stage. Existing nodal roots may be stubby and not anchored to the soil. Without an adequate root system, affected plants may be lodged, wilted, stunted, or even die (Figures 1 and 2).
Rootless corn phenomenon is currently attributed to the excessive amounts of rainfall which has eroded the soil near the plant base and/or the strong winds we have experienced breaking the upper roots and hindered the establishment of a strong root system.
Also, many fields currently have a hard packed soil surface from heavy rainfall which has hindered the establishments of the nodal root system penetrating into the soil. Other situations where rootless corn syndrome can occur are under hot and dry conditions on the soil surface, planting too shallow, compacted soils, and loose or cloddy soils.
These symptoms are unlike any associated with herbicide or insect feeding injury. Several sets of roots may not have formed below-ground, the crown may appear to be at or above the surface.
The important thing to remember is that roots will not develop in dry soil. They will not grow toward moisture. Plants can recover if rainfall is received which will soften the soil surface and promote nodal root development. Also, row cultivation may encourage root development if moist soil is thrown around the bases of the plant.
Cover crops can provide producers a variety of benefits from nutrient cycling and soil cover to nitrogen fixation and pollinator food sources. Cover crops come in many varieties including grasses, legumes and brassicas, however; cover crops maintain a “green bridge” throughout the fall and early spring that may facilitate the movement of pest insects into above and below ground plant structures.
Seedling corn, in Louisiana, is often adversely affected by many factors including excess moisture, cold temperatures and a complex of above/below ground insect pests. The complex of underground insects includes southern corn rootworm, wireworms and white grubs, while the above ground complex includes sugarcane beetles, chinch bugs and cutworms. Most of these insects require a food source that is present in fields for them to successfully overwinter and subsequently begin reproduction when temperatures begin to warm in the spring. The inherent benefits of cover crops often include the presence of large volumes of biomass and an abundant root structure that anchors soil or penetrates a hard pan. Yet, these attributes make cover crops an ideal source for the buildup of yield limiting insects.
Insecticide seed treatments (ISTs) are neonicotinoid based insecticides that coat the outer layer of the seed offering protection from below and above ground early season insect pests. The systemic nature of ISTs make these compounds water soluble and facilitate the vascular movement of the insecticide into the plant tissue. The value of ISTs in Louisiana varies among crops and environmental conditions, most agricultural commodities will usually not benefit from ISTs when planted under optimal environmental conditions (adequate soil temperature, optimal soil moisture and low pest pressure). However, insecticide seed treatments will typically produce an economic benefit when conditions are sub-optimal including very late or early planting, reduced tillage field arrangements, double cropping systems (soybeans behind wheat), pests that are present every year and consecutive plantings (i.e., corn behind corn). In addition to the above mentioned situations, data from the LSU AgCenter’s Macon Ridge Research Station confirmed the need of an IST when corn is planted behind cover crops (Figure 1). A statistically significant increase in yield was observed in corn treated with Poncho 500 IST in Berseen Clover, Crimson Clover and Hairy Vetch while a significantly lower yield was measured in corn planted behind Tillage Radishes treated with the IST (Figure 1). No fungicide seed treatment was used in this study. The measurable difference in yield may be due to the presence of below ground insects that also produced a notable decrease in vigor (Figure 2). Unfortunately for producers, there are no rescue treatments available for below ground insect injury in corn or any other agriculturally managed crop in Louisiana. Therefore, the use of an IST can help safely and effectively control below above and below ground insect pests in corn planted behind cover crops.
Aside from the use of ISTs, there are other management practices that can be done to minimize the effects of pest insects, from cover crops, on corn. Burning down cover crops in a timely fashion (6 weeks before planting) will provide enough time for available biomass above the soil to dessicate and force any harbored insects off of the plants. Yet, this timing may not allow enough time for below ground insects to cycle out or succumb to a lack of forage. Earlier burn down timings and the use of minimum tillage may allow enough time for insects to cycle out or be physically removed or destroyed with implements. If you elect to destroy your cover crops earlier than intended, check with your local NRCS representative or LSU AgCenter county agent to ensure enough time has passed that your preplant intentions are met (ie. Nitrogen fixation, nutrient cycling, etc.).
The use of ISTs is a best management practice recommended by the LSU AgCenter and will help ensure your crop is protected from yield limiting insects. The use of ISTs is highly recommended if you choose to plant corn behind cover crops particularly Berseen Clover, Crimson Clover and Hairy Vetch. If you have any questions or concerns please contact your local LSU AgCenter extension service.
Since writing the article yesterday that discussed estimating nitrogen (N) losses in corn following flooding events (http://louisianacrops.com/2016/03/18/corn-with-all-the-flooding-how-much-nitrogen-have-you-lost/), many new questions have been raised by producers. They are concerned that young corn will need to have the N replaced quickly after the flooding waters drain off fields in order to replace lost N and maximize the yield potential of the corn. This is a valid concern especially if little N is left in the soil after the floods recede or if N has not been applied at all and the corn is reaching the rapid vegetative growth stages. For example, a 200-bushel-per-acre corn crop will take up approximately 200 pounds of N during the growth and development of the crop. Seedling corn, prior to the V6 stage of development (6th leaf collar) will take up very little N, approximately 5% of its seasonal need. However, from V6 to VT (tasseling) corn will take up approximately 60% of its seasonal needs, or about 120 pounds of N for a 200-bushel corn crop. If that N is not available during that rapid growth period (approximately 30 days), then yield losses are certain to occur.
As the flood waters recede, many producers have questions about the best way to replenish the lost fertilizer N quickly to avoid yield losses. Most soils in north Louisiana are too wet to apply N with ground rigs, so that removes the potential to band or knife-in liquid N fertilizers. The only application method currently available to supply a large rate of N is to surface broadcast granular N fertilizer sources such as ammonium sulfate (21-0-0-24) and urea (46-0-0). Urea is less expensive than ammonium sulfate per pound of N; however, it also has the potential to volatilize. The volatility potential of ammonium sulfate is minimal. Volatilization is the conversion of ammonium (a solid) to
ammonia (a gas), which will be lost to the atmosphere. Trials evaluating urea volatility potential of N fertilizers have been conducted at the H. Rouse Caffey Rice Research Station over the past several years. Trials have shown that 30% or more of the N from urea surface broadcast on a dry soil can be lost by ammonia volatilization in a 10-day period of time. Of course, rates of volatilization can vary considerably across soil types. Typically, higher pH soils will have a greater N volatility potential as compared to acidic soil. Treating urea with a urease inhibitor with the active ingredient NBPT or NPPT will protect the urea from volatilization for a given amount of time, depending on soil properties and environmental conditions. In the same studies mentioned above, urease inhibitors greatly reduced ammonia volatility for a 10-day period when applied onto a dry soil. Incorporation of urea into the soil by irrigation or rainfall will minimize volatilization losses. Therefore, if incorporating N into the soil by irrigation or rainfall soon after application is not a possibility, then a urease inhibitor is recommended.
Surface soil moisture will have a tremendous effect on the volatility potential of urea. Typically, urea applied on a dry soil will begin to slowly volatilize initially, but the rate of volatilization really takes off around three days after application. When urea is applied onto a soil that is saturated (without standing water), the rate of volatilization will begin much quicker, and the potential for volatilization losses over a given period of time will be much higher. Figure 1 above, which came from a volatilization trial conducted at the Rice Research Station in 2014, illustrates this phenomenon.
Addition of a urease inhibitor will help in reducing the amount of volatilization losses when urea is applied on a soil without standing water. However, the protection time is generally cut in half. Therefore, the 10-day protection time may only be five days if applied on a moist soil. If the urease inhibitor treated urea is dropped into standing water, the NBPT will not give you any protection.
With all that said, if the soil is too wet for ground equipment and your corn needs N as soon as possible, then you must choose between applying granular ammonium sulfate and urea by air. If urea is your fertilizer of choice, then it is best to wait to apply the N (from an N-efficiency standpoint) when the surface of the soil becomes dry. Applications of urea treated with a urease inhibitor will enable applications onto a soil with a wet soil surface without significant N losses for a few days. Expect large losses of N from urea if it is not treated with a urease inhibitor and it is applied onto a soil with a moist soil surface. Expect even greater losses of N from urea if it is applied into standing water.
The rainfall events that occurred last week have caused tremendous damage to northeast Louisiana farm land due to flooding. In addition to the water received from the rainfall events, some farms were inundated with even more water as nearby bayous and rivers overflowed a few days later. Many of the hardest hit farming operations are still trying to save equipment, livestock, and assets from the rising flood waters. Some farming operations situated on higher ground, such as the Macon Ridge, may have been spared from the flooding but were still affected from the excessive rainfall nonetheless. One of the most commonly asked questions I have been receiving lately is: How much of my nitrogen (N) fertilizer have I lost in my corn crop? Unfortunately, the question is not that straight forward to answer. You really have to consider several factors. What N source did you use? Was it surface broadcast, was it incorporated in after application, was it banded in the soil (knifed-in), or was it dribbled on the soil surface? What was the soil moisture like at the time of application? How long after the fertilizer application did the flooding conditions occur? Was a urease or nitrification inhibitor used? Just to name a few.
Let’s begin by considering the N fertilizer source. First, we need to remember that nitrate-N is not stable under flooded, anaerobic (no oxygen) conditions and that it will be converted to a gas and lost very quickly by a process called denitrification. Ammonium-N, on the other hand, is stable under flooded, anaerobic conditions and will not be lost. Many farmers in northeast Louisiana use granular urea (46-0-0) or liquid UAN (urea ammonium nitrate; 32-0-0) as their N fertilizer source. The granular urea will be converted into the ammonium form after application through a process called hydrolysis. Therefore, if the flooding occurred quickly after urea application, most of the N will not be lost because it will be in the urea or ammonium form and will remain stable under flooded conditions. UAN on the other hand is made up of both urea and ammonium nitrate. The nitrate portion of UAN is approximately 25% while urea and ammonium-N make up the other 75%. Therefore, we would expect 25% of the N that is in the nitrate form to be lost very quickly after flooding.
Now let’s consider the amount of time that the fertilizer N was applied prior to flooding. Ammonium can be converted to nitrate under aerobic (oxygen present) conditions by a process called nitrification. The longer ammonium fertilizer was applied prior to flooding, the more of the ammonium will be converted to nitrate through a process called nitrification. The rate of nitrification depends on several factors including soil type, presence of nitrifying bacteria, and the temperature of the soil. Research has shown a wide range of nitrification rates based on the aforementioned soil properties, so for our purposes let’s assume that the average nitrification rate under cooler soil temperatures (<65oF) would be about 2.5% per day and under warmer soil temperatures the average rate would be at least 4.5% per day.
Another thing we have to consider is whether or not a urease inhibitor or a nitrification inhibitor was applied on the granular fertilizer or mixed in with the liquid N fertilizer. A urease inhibitor (active ingredient NBPT or NPPT) will temporarily delay the conversion of urea to ammonium form (urea hydrolysis) and will therefore temporarily delay volatilization losses (conversion of ammonium-N to ammonia, a gas) and subsequent nitrification reactions (conversion of ammonium to nitrate). Nitrification inhibitors (active ingredients nitrapyrin or DCD) will temporarily delay only the nitrification process. Urease inhibitors and nitrification inhibitors will not last forever. Research at the H. Rouse Caffey Rice Research Station has shown that urease inhibitors will typically give you protection for approximately 10 days when applied on a dry soil, approximately five days when applied on a moist soil, and will provide no protection when applied into standing water. Recent research out of northeast Louisiana indicated that nitrification inhibitors can give you protection 10-30 days, depending on the environmental and soil factors involved.
Using the N source, nitrification rate, and the addition of a urease or nitrification inhibitor, we can begin to estimate how much N will be lost under various scenarios.
Scenario 1. A corn farmer on the Macon Ridge knifed-in UAN at a rate of 200 pounds of N just after planting. The UAN was not treated with a urease or nitrification inhibitor. Saturated soil conditions occurred 12 days after planting and lasted for 2 days. The soil temperature was <65oF. How much N was lost?
Well, we know right away that 25% of the UAN is in the nitrate form and will be lost under saturated, anaerobic conditions. We also know that we have had 12 days for nitrification (conversion of ammonium to nitrate) to occur in a cool soil and that a nitrification or urease inhibitor was not used, so the nitrification rate will be about 2.5% per day.
Therefore, the N loses from denitrification = (0.25 * 200 lb N/A) + (12 days * 0.025 * 200 lb N/A) = 80 pounds of N per acre lost.
Scenario 2: Granular urea (46-0-0) was treated with a urease inhibitor and was applied on moist ground 3 days after planting at a rate of 50 pounds N per acre. A one-half inch rain occurred 2-days later that incorporated the urea into the soil, minimizing volatilization (ammonia gas) losses from the urea. Flooding conditions occurred 15 days later and the field stayed under water for four days. The soil temperature was <65oF. How much N was lost?
This time all of the N is in the urea form to begin with and was protected from volatilization losses prior to the small incorporating rain. However, the urease inhibitor probably only kept the fertilizer in the urea form for approximately 5 days before it was converted into the ammonium form. Nitrification (conversion of ammonium to nitrate) could then occur for about 10 days.
Therefore, the loss of N from denitrification = 10 days * 0.025 * 50 lb N/A = 12.5 pounds of N per acre lost. In addition, I would expect that the corn would not be able to survive this situation and would have to be replanted.
Remember, these N loss estimates are not perfect. There are a lot of factors involved in estimating N loses that were not covered and it could be argued that the rates of nitrification or the time that a urease or nitrification inhibitor could differ depending on the research you use as your source. However, it is a good starting point and should help growers in northeast Louisiana determine how much N will be available in the soil once saturated soils dry and flood waters recede.