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Late Season Flood/Storm Events in Louisiana Soybeans

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Over the next few days, producers across the state will begin to assess damages to soybeans brought upon by tropical system Harvey.  Unfortunately, there is no cookie cutter answer to how a system like this will affect every grower.  The main distinction of how varying situations will need to be assessed is the growth stage of the soybeans at the time the event occurred.

The lack of available oxygen for plant processes is the main concern in flooded fields. Oxygen is required for many essential plant processes including respiration, water uptake, root growth, and nodulation.  When flood water covers a field, the oxygen concentration drops quickly and can be depleted in as little as 24 hours.  However, depending on additional factors, soybeans can survive flooded conditions for up to 96 hours.

Temperature: Higher temperatures (ambient and water) will accelerate plant respiration, leading to a depletion of oxygen sooner than cool temperatures with cloudy weather.

Water movement: Even moderate water movement can increase aerification and allow oxygen to the plant roots.

Soil type: Flooding is potentially worse on poorly drained clay soils due to the reduction in hydraulic conductivity (the speed at which water can move through and out of the soil) compared to coarse soils.

According to research conducted in Baton Rouge in the late 1990s, the most sensitive growth stages of soybeans to flood stress are the early reproductive stages of R3 to R5 with yield reductions as high as 93% and 67%, respectively, when flood water remained for seven days (Linkemer et al., 1998).  The lack of oxygen associated with flood waters reduces the plants ability to develop additional plant material due to a reduction of photosynthesis and respiration.  At R3, the loss of yield is caused by a reduction in both the number of pods and seed size while the yield reduction at R5 is attributed mainly to seed size.   The same study showed little loss in yield for soybeans flooded after R6 as this rapid seed fill stage is believed to be protected against temporary stresses (Linkemer et al., 1998; Westgate et al., 1989).

R5 Soybeans in standing water. Soybeans are most sensetive to flooding at growth stages R3 to R5. Todd Spivey

R8 soybeans in standing water. Todd Spivey

 

The yield losses discussed in these studies however, only refer to direct reductions of seed number and size by the plant.  The studies presented do not account for yield and quality reductions caused by outside factors associated with these type of weather events.  Late season flooding followed by warm conditions can become conducive to several fungal diseases such as aerial blight, anthracnose, pod and stem blight, and soybean rust.  It is important producers continue to scout fields for an increase in disease incidence in the coming days.

Consideration should also be given to the possibility of seed rot and seed sprouting.  Sprouting can occur in seed that have previously dried down to below 50% moisture before experiencing extremely wet weather.  Additionally, ease of harvest can be reduced with soybeans that received an application of gramoxone just prior to the storm.  As the leaves desiccate and are removed from the plant the stem can still imbibe water.  With no leaves to aid in moving the water out of the stem, the stems will not dry down and producers can see an increase in green stem incidence in many fields.   

 

Linkemer, G, J.E. Board, and M.E. Musgrave. 1998. Waterlogging effects on growth and yield components of late-planted soybean. Crop Sci. 38:1576-1584.

Westgate, M.E., J.R. Schussler, D.C. Reicosky, and M.L. Brenner. 1989. Effect of water deficits on seed development in soybean. II. Conservation of seed growth rate. Plant Physiol. 91:980-985.

Use of Harvest Aids in Louisiana Soybeans

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Todd Spivey, Sebe Brown, Josh Copes, Donnie Miller, Boyd Padgett

Over the last several weeks, we have received numerous calls about soybean harvest aid timing, products, and general recommendations.  The use of harvest aids in Louisiana soybeans is a common practice, with timely applications improving seed quality and harvest efficiency while potentially resulting in a soybean harvest 10 to 14 days earlier when compared to non-treated beans.

Timing

If the goal of harvest aid use in soybeans is to promote early harvest and improved harvest efficiency, harvest aids must be applied as timely as possible.  Once seed have separated from the white membrane inside the pod, they have reached physiological maturity and will no longer increase in size.  Any use of a harvest aid prior to the majority of seed reaching physiological maturity will result in a loss in yield.  Table 1 gives the paraquat label requirements for harvest aid application timing in soybean. Research conducted in Louisiana by Dr. Jim Griffin and Joey Boudreaux established that a harvest aid application could be made to soybean without yield penalty as long as soybeans are at reproductive growth stage R6.5 (physiological maturity). They provided a list of procedures to help determine when harvest aids can be safely applied to soybeans:

  1. Begin to scout fields for harvest aid timing when leaves begin to yellow
  2. Collect pods from the top four nodes of the plant at multiple, random locations within a field
  3. Open soybeans from pod, they should shell easily, and look for soybean separation from the white membrane
  4. If soybean separation from the white membrane has occurred for all pods collected, the seed has reached maximum dry weight and harvest aid application can be made without yield penalty

Plant appearance at growth stage R6.5 will vary by variety so close attention should be made to pods collected from the field and if seed have separated from the white membrane (Griffin and Boudreaux 2011 Louisiana Agriculture magazine Vol. 54, No. 2, Spring 2011).

Table 1. Proper application timing of harvest aid in indeterminate and determinate soybean varieties.
Indeterminate Varieties 65% of pods have reached a mature brown color or seed moisture is less than 30%
Determinate Varieties Plants are mature; beans are fully developed, 50% of leaves have dropped and remaining leaves are yellowing.

Products

Producers have several harvest aid options, though the typical harvest aid application consists of paraquat with an additional nonionic surfactant.  With excessive morningglory pressure, growers might consider including carfentrazone (Aim) or saflufenacil (Sharpen) with paraquat to improve desiccation of vines and in situations with high grass pressure, a tank-mix of paraquat with sodium chlorate may be warranted to improve the desiccation of grassy weeds prior to harvest.  Questions have also been received in regards to the use of sodium chlorate to aid in drift reduction of paraquat applications made by air.  The LSU AgCenter has no data to support this claim and only recommends the use of these products together for improved desiccation of weeds and soybeans present in the field.

It is also imperative that producers consider the required preharvest interval (PHI) associated with each product label.  When using multiple products, the longest PHI must be adhered to.  Labeled rates and comments are presented below in the excerpt from the 2017 Louisiana Suggested Weed Management Guide.

Labeled rates and comments of soybean harvest aid products from the 2017 Louisiana Suggested Weed Management Guide

Redbanded Stink Bug Considerations

Producers should also continue monitoring redbanded stink bug (RBSB) populations and should not rule out the inclusion of an insecticide with the application of a harvest aid.  LSU AgCenter entomologists recommend the control of threshold populations of RBSB until the soybeans are out of the field.  This means that many producers could, and should, include an insecticide for the control of RBSB with their harvest aid application (sodium chlorate cannot be tank-mixed with any insecticide).  It is important to keep in mind the restrictions placed upon many of the products at this point in the season.  These restrictions may include total active ingredient restrictions and PHIs.  Acephate, a common recommendation for RBSB control, can only be applied up to 2 lb ai A-1 year-1 in Louisiana.  Other insecticides also have increased PHI such as the pre-mix product Endigo, with a PHI of 30 days.  It is important to read and adhere to the label of all labeled materials prior to use.  When label restrictions prevent the inclusion of an insecticide with the harvest aid, producers should not delay the harvest of soybean so that the seed can be removed from the field as quick as the label allows.

Redbanded Stink Bug Numbers Increasing in Soybeans

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Reports from the field indicate redbanded stink bug (RBSB) numbers are beginning to build in soybeans at the R5 development stage and beyond. Once RBSB colonize a field, native stink bugs often are forced out or are outcompeted, leaving only RBSB behind. The Louisiana threshold for RBSB is four insects per 25 sweeps. RBSB are strong fliers, and routine scouting is essential to detecting an influx of these insects. Furthermore, the presence of immatures signals that RBSBs are reproducing, meaning previously applied insecticidal controls may no longer be active. Recommended insecticides include pyrethroids, neonicotonoids and organophosphates.

The use of premix insecticides, including Endigo ZC and Leverage 360, may offer a degree of repellency not observed with other insecticides. Insecticide efficacy tests conducted at the Macon Ridge Research Station in Winnsboro demonstrated satisfactory control of RBSB while also having a possible added benefit of repellency. However, these insecticides perform best when populations of RBSB have not exceeded threshold. Once RBSB populations have exceeded threshold, the use of tank mixes of either acephate (0.75 to 1.0 pounds per acre) plus bifenthrin (6.4 ounces per acre) or Belay (4.0 ounces per acre) plus bifenthrin (4.0 ounces per acre) may be required to get them under control.

As with most insects, staying ahead of RBSB populations will make season-long control much easier while also reducing injury. Please contact your county agent or me for more information.

Bt Cotton Situation

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For the past two weeks, most of Louisiana has been in the midst of a very large bollworm moth flight. Our moth trap catches were averaging about 10 moths per day and moved to more than 100 late last week. I have received numerous phone calls on how the technology is holding up and what insecticide should be used to over-spray. Another issue to consider is how much these worms were pre-selected in Bt corn. My colleagues around the Midsouth and Texas have seen a very large number of worms coming through Bt corn and Louisiana is no exception. Further, LSU AgCenter entomologists discovered a change in susceptibility of bollworm to Cry1Ac and Cry2Ab. The resistance does not appear to be complete and some fitness costs may be associated. If these results are any indication of Louisiana’s bollworm population this year, we may experience more escapes in Bt cotton.

Results from our Bt technology tests and reports from the field indicate that Widestrike cottons (including 499, 312 and 333) are experiencing large amounts of injury. Our small plot work at the Macon Ridge Research Station in Winnsboro is averaging 10 percent fruit injury in Widestrike (WS) and 6 percent in Widestrike 3 (WS3). Based on our work we conducted with the mid-South entomology group last year, we validated a 6 percent fruit injury threshold in Bt cotton. Therefore, WS3 is better than WS, but both technologies would need to be over-sprayed to preserve yield in this situation.

Furthermore, Bollgard 2 (BG2) and Twinlink (TL) have a more robust Bt package than WS. However, I have seen these technologies fail under severe pressure. As of this week, reports from the field and results from our trial work indicate BG2 is still performing well — but this can change quickly. TwinLink’s performance has been inconsistent, with a number escapes being reported. This seems to be dependent on the environment and insect pressure. Keep in mind that stress can negatively affect Bt expression in cotton. Stressed plants may not express a high enough level of toxin to control bollworms.

Independent of environmental factors, if bollworm escapes are detected, a rescue spray may be warranted. The use of pyrethroids is strongly discouraged. Louisiana bollworm populations have the highest level of pyrethroid resistance in the United States, and pyrethroid applications may not provide adequate control. They may even flare secondary pests such as spider mites. The LSU AgCenter recommends the diamide chemistry (Prevathon, Besiege) for control of bollworms in cotton. Beware that Besiege contains a pyrethroid and use may inadvertently flare secondary pests. Keep in mind that bollworms are cryptic feeders, and worms that have established in squares and bolls may not be controlled by diamides. If you have any questions or comments, please feel free to contact your county agent or me.

Louisiana Rice Notes #6

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(click to view)

This edition covers current crop progress and issues, recommended fungicide rates and timings, shortage of Sercadis, and stink bug BMP’s.

Thrips Management in Cotton

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With the abnormally warm winter and spring, cotton planting in Louisiana has gotten off to an early start. In Louisiana, and across most of cotton states, thrips are considered the number one early season insect pest. The species we encounter greater than 85% of the time is tobacco thrips with western flower thrips typically comprising the other 15%.

Thrips control options are limited to seed treatments, in-furrow applications and foliar sprays. Over the past few years, control of tobacco thrips with thiamethoxam (Avicta, Cruiser, etc) has been declining and resistance has been confirmed through bioassays. As a result, we have switched almost exclusively to imidacloprid products (Aeris, Gaucho, Acceleron F1) and no longer recommend thiamethoxam (alone) as a seed treatment in cotton. Aeris treated seed contains imidacloprid + thiodicarb and performs very well in our thrips trials and in the field. The use of imidacloprid alone is another option; however, it may not perform as well as Aeris or imidacloprid + an acephate overtreatment. Overtreatment with acephate is an economical option that has demonstrated increased thrips control when applied on top of imidacloprid. Acephate alone controls thrips but the residual is significantly shorter than currently used products and increases the likelihood of foliar follow up applications.

The use of in-furrow applications of imidacloprid and AgLogic 15G are also options that work well for controlling thrips and other early season insects in cotton. AgLogic 15G is an aldicarb based replacement for Temik that is available in either gypsum or corn cob grit formulations with performance very similar to Temik when used at the appropriate rate.

Finally, foliar rescue treatments are utilized when seed treatments have played out. Foliar treatments should be made when immature thrips are present and/or when large numbers of adults are present and damage is occurring. The presence of immature thrips often signifies that the insecticide seed treatment has lost its efficacy. Avoid spraying solely based on plant injury since the damage has already occurred. Below are some considerations when deciding what foliar insecticide to use.

Dimethoate:

Positives: Relatively inexpensive, good efficacy at high rates, less likely to flare spider mites and aphids than acephate

Negatives: Ineffective towards western flower thrips, less effective than acephate or bidrin when applied at lower rates

Acephate

Positives: Relatively inexpensive, effective towards western flower thrips

Negatives: May flare spider mites and aphids if present, may be weaker against tobacco thrips in certain circumstances

Bidrin

Positives: Effective, less likely to flare spider mites and aphids than acephate

Negatives: Less flexibility with applications early season

Radiant

Positives: Effective, least likely to flare spider mites and aphids

Negatives:  More expensive, requires adjuvant

Insecticide choice depends on a number of factors such as cost, impact on secondary pests and spectrum of thrips species present. If a foliar thrips treatment is justified, do not wait for a herbicide application and only spray when necessary to avoid flaring spider mites and aphids.

Transform (Sulfoxaflor) Granted Section 18 for Use in Louisiana Cotton

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The EPA has granted a section 18 request for the use of Transform (sulfoxaflor) for 2016 Louisiana cotton production season. Please see the link below for information on conditions and restrictions outlined by the section 18 label.

Section 18 Authorization Letter for Transform in Louisiana Cotton

Wheat and Corn Pathology Update

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Wheat and Corn Pathology Update (4/15/2016)

Trey Price, Field Crop Pathology, Macon Ridge Research Station

Boyd Padgett, Wheat Pathology, Dean Lee Research Station

Wheat

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.

 

 

Figure 1 (Fusarium head blight).
Figure 1 (Fusarium head blight).

 

Figure 2 (scabby kernels below healthy ones).
Figure 2 (scabby kernels below healthy ones).

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.

 

Figure 3 (stripe rust).
Figure 3 (stripe rust).
Figure 4 (leaf rust).
Figure 4 (leaf rust).

 

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.

 

Figure 5 (Septoria leaf blotch).
Figure 5 (Septoria leaf blotch).

 

 

    

Figure 5 (Septoria leaf blotch). Figure 5 (Septoria leaf blotch). Figure 6 (bacterial streak).
Figure 6 (bacterial streak).

 

Figure 7 (bacterial streaming).
Figure 7 (bacterial streaming).


Corn

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.

 

 

 Figure 8 (plant death as a result of RCS).
Figure 8 (plant death as a result of RCS).

 

Figure 9 (normal vs. poor nodal root development).
Figure 9 (normal vs. poor nodal root development).

 

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.

 

 

 Figure 10 (damping off).
Figure 10 (damping off).

 

 Figure 10 (damping off).
Figure 10 (damping off).

 

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.

 

Figure 12 (southern rust on upper surface of leaf).
Figure 12 (southern rust on upper surface of leaf).

 

 Figure 13 (southern rust on leaf sheath).
Figure 13 (southern rust on leaf sheath).

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.

 

Soybean Insecticide Seed Treatment Decisions

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One of the most important decisions producers must make when planting soybeans in Louisiana is planting date. Soybeans have the utility to be planted in early March to late June. This wide variation in planting dates exposes seedling soybeans to a multitude of insect pests that affect both above and below ground plant structures.

Optimal seeding dates for each maturity group planted in Louisiana are:

  • Group III – April 15–May 10
  • Group IV – April 15–May 10
  • Group V – March 25–May 5
  • Group VI – March 25–April 30

Soybean seedlings possess an exceptional amount of vigor and can tolerate a substantial amount of insect injury during the seedling stage. However, early planted soybeans may also encounter greater amounts of environmental fluctuations that affect air and soil temperature. Cool conditions can negatively affect vigor and under the right conditions stall plant growth and development. The addition of insect injury, to the aforementioned  environmental conditions, increases stress the plant encounters resulting in loss of stand and yield potential. Therefore, the inclusion of an insecticide seed treatment (IST) provides growers a risk management tool when soybeans are planted early.  The primary insect pests of early planted soybeans are bean leaf beetles, wireworms and grape colaspis.

On the opposite end of the spectrum are soybeans planted late i.e. behind wheat or are late due to unforeseen circumstances such as inadequate or excessive soil moisture. These beans are more at risk for insect injury due to the potential for large insect populations to build in neighboring fields and generally more insects present in the environment. As a general rule with all agronomic crops, the later the crop the more insect pressure that will be encountered throughout the season.  This is particularly evident when soybeans are planted into wheat stubble. Wheat stubble is favorable for the development of threecornered alfalfa hoppers and thrips. Thus, an IST is a sound investment when soybeans are planted late.

However, soybeans planted in a timely manner that being within the recommended planting window, under optimal soil conditions and low pest densities will often not benefit from the addition of an IST.  Insecticide seed treatments typically produce the most benefits when environmental conditions are sub optimal as outlined in the prior paragraphs. With the current economic climate and many ag professionals looking at areas to cut inputs, justifying the use of an IST on soybeans when planted under optimal conditions becomes harder to support. Saving the cost of an IST can go to making a stink bug application later season that may provide a greater economic return.

Outside of early or late planted soybeans are situations where ISTs are justifiable. These include weedy fields with incomplete burn down applications, reduced tillage field arrangements, fields with historically problematic early insect pests (wireworms and/or threecornered alfalfa hoppers) and continuous plantings of one crop.  Each field is unique and the use of ISTs as a blanket treatment over every acre may not be justifiable with $8 soybeans.

Conditions Favorable for Fusarium Head Blight (scab) in Wheat Again This Year

Conditions Favorable for Fusarium Head Blight (scab) in Wheat Again This Year published on No Comments on Conditions Favorable for Fusarium Head Blight (scab) in Wheat Again This Year

Trey Price, Field Crop Pathology, Macon Ridge Research Station;

Boyd Padgett, Central Region Director, Dean Lee Research and Extension Center

Last year Louisiana wheat was devastated by Fusarium head blight (scab) because of warm and wet weather conditions during flowering.  Weather conditions are currently favorable for wheat scab development statewide.  Most of the wheat in the state is at or very near flowering, which is the most susceptible stage to scab infections.  Wheat in southernmost production regions is already showing early signs of scab infection.

The disease is mainly caused by the fungus, Fusarium graminearum, which also causes ear, stalk, and root rots in corn. Symptoms of the disease will first appear 10 to 14 days after flowering as bleached heads which will be noticeable from the turn row (Photo 1).  This symptom is often mistaken with the appearance of maturing wheat.  Upon closer inspection, affected wheat heads will usually have infected kernels showing the characteristic bleached appearance with pinkish/salmon/light orange coloration along the glumes (Photo 2).  This coloration is millions of microscopic spores (reproductive structures) of the fungal pathogen.  There are usually healthy kernels along with the diseased kernels on the same head (Photo 3).  In extreme cases, however, the entire head may be infected.  At harvest, affected seed will be shriveled, off color, and much lighter than healthy kernels and are referred to as “tombstones” (Photo 4).

Photo 1. A view of a field heavily-infected with FHB.
Photo 1. A view of a field heavily-infected with FHB.
Photo 2. Closer view of a head infected with FHB. Note the salmon-colored fungal growth near the center.
Photo 2. Closer view of a head infected with FHB. Note the salmon-colored fungal growth near the center.
Photo 3. Wheat heads with FHB-affected and healthy kernels.
Photo 3. Wheat heads with FHB-affected and healthy kernels.
Photo 4. Diseased kernels (left) vs. relatively healthy kernels (right).
Photo 4. Diseased kernels (left) vs. relatively healthy kernels (right).

The pathogen over summers on corn, wheat, small grain residue, and other grasses.  With that in mind, there are some cultural practices that may aid in management:  crop rotation, tillage, mowing/shredding, or staggered planting/varietal maturity.  At harvest, combine fan speed may be increased to remove infected seed, which is lighter than healthy seed.  Additionally, seed cleaning equipment may help remove affected seed but may not be cost effective.  These cultural practices alone will not completely manage FHB.  An integrated approach is required to lessen the impact of FHB.

Triazole fungicides may be somewhat effective on FHB.  Some earlier research indicated that tebuconazole (Folicur and generics) may reduce incidence and severity of FHB.  Later research has shown that Prosaro (prothioconazole + tebuconazole), Proline (prothioconazole), and Caramba (metconazole) are most efficacious on FHB.  THESE APPLICATIONS WERE MADE UNDER IDEAL CONDITIONS WITH IDEAL TIMINGS AND THE MAXIMUM CONTROL WAS AROUND 50%.  AVERAGE CONTROL WAS ABOUT 40%.

Timing is critical.  We have a very short window during flowering to make an effective application for FHB.  The biggest problem is that ideal conditions (wet weather) for FHB infection are not ideal for making fungicide applications.  Head coverage also is critical.  Sprayers should be calibrated to deliver maximum water volume (minimum 15 GPA by ground, 5 GPA by air) and optimal droplet size (300 to 350 microns).  For ground sprayers, nozzles angled at 30° to the horizontal will maximize head coverage.  Some research has shown that dual nozzles angled in opposite directions will also increase head coverage.

It is common to see 2-3 years of epidemics of FHB followed by years with little to no disease.  Judging by the amount of scab we saw last year and current weather conditions, the probability is high for another severe epidemic.  An online (www.wheatscab.psu.edu) risk assessment tool that is based on temperature and relative humidity is available online, which has regional commentary that will help you to determine your risk at a given location.

For more information, please see the following resources:

www.scabsmart.org

www.scabusa.org

http://www.mississippi-crops.com/2015/04/24/wheat-disease-update-april-24-2015/

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