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Black Root Rot Suspected in Louisiana Soybean

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Assistant Professor, Field Crop Pathology, Macon Ridge Research Station

Over the past two weeks, I have received many phone calls and conducted numerous field visits concerning black root rot of soybean. The suspected causal agent is Thielaviopsis basicola, which has primarily been described as a seedling disease of cotton. In 2009, the disease was described as a disease of vegetative soybean in Arkansas (http://www.apsnet.org/publications/plantdisease/2010/September/Pages/94_9_1168.1.aspx) and has been mentioned as an issue in Mississippi over the past several years (http://www.mississippi-crops.com/2014/08/01/soybean-disease-update-august-1-2014/). Information concerning late-season (R5-R6) symptoms and epidemiology of black root rot is limited.

During pod fill foliar symptoms of black root rot become obvious in soybean fields (below, Plate 1).

BRR1

These symptoms are easily noticed from the turnrow, and upon closer inspection, interveinal chlorosis is evident with leaf veins remaining green (Plates 2 & 3). Inspection below the canopy in the center of the affected area will usually reveal one or several plants that died earlier in the season (Plate 4).

Plate 2. Interveinal chlorosis caused by black root rot.
Plate 2. Interveinal chlorosis caused by black root rot.
Plate 3. Interveinal chlorosis.
Plate 3. Interveinal chlorosis.
Plate 4. Soybean plant death caused by BRR.
Plate 4. Soybean plant death caused by BRR.

Apparently, these dead plants go unnoticed because the death occurred during vegetative or early reproductive stages, and adjacent plants quickly covered them. Surviving, infected plants adjacent to the dead plants will be stunted and displaying these foliar symptoms (Plate 5).

Plate 5. Dead plants (left), stunted plants (center), and healthy plants (right) from a field affected by black root rot.
Plate 5. Dead plants (left), stunted plants (center), and healthy plants (right) from a field affected by black root rot.

Affected plants may snap-off at the soil line when pulled. When plants are excised, roots are black in color (below, Plate 6).

OLYMPUS DIGITAL CAMERA

Splitting stems near the crown will reveal white fungal growth in the center of the stem (below, Plate 7). Additionally, infected black plant stems from the previous season are often observed near infected roots.

OLYMPUS DIGITAL CAMERA

We have isolated what appears to be Thielaviopsis basicola from diseased roots using a selective medium, and are currently working to confirm identity and pathogenicity. The effects of fungicide seed treatments and in-furrow sprays are unknown. Varietal susceptibilities are currently unknown; however, the official variety trial at Dean Lee Research Station is significantly affected by black root rot and will be rated in an attempt to identify sources of resistance. Additionally, greenhouse screenings may be conducted this winter to corroborate rating information.

This fungus has a broad host range and survives in the soil for long periods of time. Apparently, conditions have been optimal for disease development this year. Incidence in most fields has been <1%; however, in some fields that have been planted to soybean continuously for several years and in a minimum/no till program, incidence has been as high as 10%. This does not necessarily translate to a 10% loss, as affected plants will have the ability to produce some seed depending on disease severity. Anecdotal evidence indicates that rotation to corn will lessen disease incidence. Other diseases/conditions that we have seen this year that may be confused with black root rot include: red crown rot, sudden death syndrome, and triazole burn (Plates 8, 9, 10, and 11).

Plate 8. Foliar symptoms of sudden death syndrome and/or red crown rot.
Plate 8. Foliar symptoms of sudden death syndrome and/or red crown rot.

 Plate 9. Red crown rot fruiting structures on soybean.

Plate 9. Red crown rot fruiting structures on soybean.

Plate 10. Whitish to bluish spore masses produced by the fungus that causes sudden death syndrome.
Plate 10. Whitish to bluish spore masses produced by the fungus that causes sudden death syndrome.

 

 Plate 11. Triazole fungicide burn.

Plate 11. Triazole fungicide burn.

For more information on these topics or others, please contact your local extension agent, specialist, nearest research station, or visit www.lsuagcenter.com or www.louisianacrops.com.

Frogeye Leaf Spot Prevalent in Louisiana Soybean

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Over the past two weeks, many reports of frogeye leaf spot have been coming in from all soybean growing areas in the state. Overall disease severity in susceptible varieties has been light to moderate. The disease is caused by a fungus, Cercospora sojina, and has the potential to reduce yield by reducing leaf area and causing defoliation. Losses of up to 30% have been reported in the past. The disease may also cause discoloration of seed reducing seed quality. When scouting for frogeye, initial foliar symptoms are dark, water-soaked spots (1 to 5 mm) which later progress to lesions with gray to brown centers and reddish margins. Symptoms will be evident usually around R3, but may appear earlier or later. The disease may progress with more lesions developing, which may coalesce resulting in large necrotic areas on leaves. If infection is severe, frogeye may cause defoliation of soybeans. Young leaves are infected more readily than older leaves, and patterns of varying degrees of disease severity may be observed within canopy levels. Closer examination with a hand lens, or sometimes with the naked eye, will reveal gray to black conidiophores (reproductive structures) within the center of lesions. The disease is spread by windblown or rain-splashed conidia (spores) formed on the conidiophores. Conditions favorable for disease development have been prevalent in our current weather pattern of consistent rainfall, high humidity, and warm temperatures.

Figure 1.  Frogeye leaf spot lesions.
Figure 1. Frogeye leaf spot lesions.
Figure 2.  Coalescing frogeye leaf spot lesions (note the gray coloration near the centers of the lesions).
Figure 2. Coalescing frogeye leaf spot lesions (note the gray coloration near the centers of the lesions).
Figure 3.  A moderate infection of frogeye leaf spot.
Figure 3. A moderate infection of frogeye leaf spot.

Frogeye leaf spot may be managed by a number of methods. The first line of defense is planting a resistant variety and pathogen-free seed. Although our data is limited on varietal susceptibility, in 2013, we were able to rate soybean varieties for frogeye at Dean Lee Research Station in Alexandria. Results of those ratings are posted at: http://www.lsuagcenter.com/MCMS/RelatedFiles/%7B271517B6-5563-4FB9-BF4F-3D211119F027%7D/Dean-Lee-OVT.pdf. Another list from our friends in Mississippi and Tennessee is located at: http://www.mississippi-crops.com/wp-content/uploads/2013/07/2013-soybean-short-list-frogeye-responses.pdf. If your variety of interest was not included in these sources, please contact your seed representative for more information.

Sometimes a fungicide application may be warranted for management of frogeye leaf spot in susceptible varieties when disease severity is moderate to heavy and conditions favor disease development. One important consideration when making application decisions is the fact that strobilurin fungicide resistance is likely in this pathogen population, and has been confirmed in 9 parishes in Louisiana. Even if strobilurin resistance has not been confirmed in your parish and if strobilurin fungicides have been routinely applied in the area, it is likely that the majority of the pathogen population has become resistant. In some cases we have seen reduced efficacy of strobilurin fungicides (Aproach, Evito, Gem, Headline and Quadris) on frogeye leaf spot. In our trials in 2013 and others conducted throughout the United States, we have seen consistent reductions in disease severity when using triazole products such as Domark, Proline, and Topguard. Additionally, pre-mixes containing these triazoles have shown reductions in disease severity. Data is limited for Louisiana, and we have trials at several research stations examining fungicide efficacy for these products as well as many others not listed.

Other considerations should include application coverage as it relates to nozzle type and water volume. Fungicides usually require a minimum of 10 gallons/A by ground and 5 gallons/A by air. Hollow cone or flat fan nozzles are recommended to achieve optimum droplet size. When applying fungicides, rotate chemistries to avoid resistance issues and prolong the usefulness of products. Please do not hesitate to contact LSU AgCenter via your parish agent, specialist, or nearest research station for additional information.

2014 Northeast Research Station Pest Management and Crop Production Field Day

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NERS Field Day Flyer
NERS Field Day Flyer

Wheat update and transitioning into double-crop soybeans

Wheat update and transitioning into double-crop soybeans published on No Comments on Wheat update and transitioning into double-crop soybeans

By:

Josh Lofton, LSU AgCenter, Wheat Specialist and Field Crop Agronomist

Steve Harrison, LSU AgCenter, Small Grain Breeder

 

It’s the time of year where we will start to see combines rolling through wheat fields around the state. In fact, while we have been one to two weeks behind normal growth for much of the season, recent dry weather across the state has led to a rapid dry-down and, as a result, many locations in south Louisiana have already begun harvesting. Initial reports from these regions indicate yields have been very promising, with little noticeable influence from this last winter. The majority of north Louisiana has yet to begin harvest, with the majority of fields five to 15 days from beginning harvest. By the look of the crop, it can be expected that yield will potentially be as promising as those being seen in south Louisiana.

As the wheat crop is beginning to wind down, many producers across the state will begin to look toward the successive crop. For a majority of these wheat acres, this will be double-crop soybeans. One of the most important stages in double-crop production systems is the transition from the winter crop to the summer crop. To optimize the soybean yields, a quick and efficient transition is critical. One aspect that will influence this transition is wheat residue management. While most managers will want to manage the residue in a similar manner between years and locations, it is essential that residue management decisions be made individually. The two most commonly used management options for wheat residue management are no-tillage into standing wheat residue and no-tillage into burned wheat stubble. While burning of wheat residue has received substantial negative publicity in recent years, there are conditions and circumstances where it has the potential to be the best management decision. When deciding which management practice to use for each individual system, managers should look at the current growing conditions and previous typical conditions. The recent dry conditions have aided in wheat dry-down. However, it may have detrimental impacts on the successive soybean crop. While these dry conditions will be detrimental in different residue management conditions, these conditions are amplified when the wheat residue is burned ahead of the soybean planting. Therefore, if soils are dry, caution should be used if wheat residue burning is intended. Additionally, while the chance of precipitation in the latter part of the week is always in flux, if wet conditions return, this will also influence management. No-tillage into standing wet wheat stubble can negatively influence planting practices and could result in delayed or poor soybean stands. However, wet wheat stubble will burn inconsistently and potentially, again, negatively influence stand and emergence.

While we have a potentially very productive wheat crop finishing, producers and managers in double-crop systems must already be thinking toward the successive soybean crop. While these double-crop systems can be very productive and profitable, increased management is essential, and residue management is one of the most critical issues.

Potassium Deficiencies in Corn and Soybean

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This spring has brought us cool weather, wet/dry spells, and now potassium deficiencies. We have mainly been hearing about corn and soybean potassium deficiencies on the Macon Ridge. However, this does not mean that they are not showing up throughout our alluvial soils.

Potassium is very important for water use efficiency in crops. This means that irrigation on the fields showing deficiency symptoms will need to be managed diligently. There are a few reasons that symptoms could be showing up: 1) Dry soils, 2) Compaction, 3) Low soil test K, and 4) Reduced early-season root growth. Potassium cannot be taken up efficiently in the soil without the presence of water, which can cause a deficiency to appear even if adequate potassium was applied to the soil. Compaction can also cause a potassium deficiency to appear because the roots are not able to find enough potassium in the un-compacted soil. If a soil sample is collected and the results show that the soil is low in potassium, then the recommended amount of potassium should be applied after the current cropping season. Lastly, due to some of the weather that was experienced this spring, there could have been reduced root growth in spots affected by standing water or cold weather. This reduced root growth creates a smaller zone of potassium available to the plants.

K zone in corn

 

Deficiency symptoms in corn and soybeans are denoted by edge of leaf necrosis.

Corn_PotassiumDeficiency

Currently, the LSU AgCenter soil fertility research group does not currently have data on the yield benefits of foliar-applied potassium. Furthermore, research throughout the region has shown little to no yield benefits for foliar potassium applications. Additionally, if yield benefits are seen, there is a chance that the cost of application will often outweigh any yield benefit.  Therefore, caution should be used prior to any in-season foliar potassium application.  If a foliar K is going to be added, follow the label carefully due to the chance of leaf burn.

 

If you have any questions, please contact:

Beatrix Haggard, Northeast Region Soil Specialist: (318) 498-2967

Josh Lofton, Agronomist: (318) 498-1934

Dan Fromme, Corn and Cotton Specialist: (318)880-8079

2014 Projected Commodity Costs and Returns for Louisiana

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Please see the link below for information on the 2014 projected commodity costs and returns for Louisiana.

Cotton, Soybean, Corn, Grain Sorghum and Wheat Production in Louisiana

Section 24c Granted for Acephate in Louisiana Soybeans

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This special local need label allows soybean producers to apply a maximum of  2lbs (ai/acre) of acephate  per season. The previous maximum was 1.5lbs (ai/acre)  per season.

For more information or if you have any questions or concerns, please contact:

Sebe Brown at 318-498-1283 (cell) or 318-435-2903 (office)

Dr. David Kerns at 318-439-4844 (cell) or 318-435-2157 (office)

Dr. Julien Beuzelin at 337-501-7087 (cell) or 318-473-6523 (office)

Dr. Jeff Davis at 225-747-0351 (cell) or 225-578-5618 (office)

 

2013 Macon Ridge Research Station Pest Management and Crop Production Field Day

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MRRS Flyer

Corn Earworms Increasing in Soybeans

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Recently, I have been receiving quite a few phone calls regarding corn earworms (CEW) moving into soybeans. Many of these populations were below the action threshold of 8 larvae (>1/2 in) per 25 sweeps and sporadically located across the northern half of the state.

Corn earworms are typically more attracted to soybeans from R2-R5 because adults are attracted to flowering beans to oviposit their eggs. Fields further along in maturity are less desirable for colonization by ear worms; however, fields should be scouted routinely so populations are not missed. Pheromone trap catches confirmed a relatively small flight, less than 50 adults, in traps at the Northeast and Macon Ridge Research Stations in the past week. Although this is not a large outbreak of corn earworms, larvae will consume flowers and small pods if populations are not kept in check. Fields that have had a previous application of a pyrethroid for stink bugs or three-cornered alfalfa hoppers will typically have greater numbers of earworms because of the elimination of natural enemies.

Pyrethroid susceptibility monitoring has indicated high levels of resistance, and pyrethroid applications may not provide effective control of these pests. However, new diamide chemistries – including Besiege, Prevathon and Belt – have demonstrated satisfactory control of these and other worm pests in soybeans.

Belt (AI: Flubendiamide) and Prevathon (AI: Chlorantraniliprole) only provide control of lepidopteran pests; Besiege (AI: Chlorantraniliprole + Lambda-cyhalothrin ) will control a broader spectrum of pests because of the inclusion of lambda-cyhalothrin. A tank mix of a pyrethroid and 0.5lbs of acephate may provide effective control on low numbers of CEW. Beware, only 1.5 lbs of acephate can be applied per acre per season in soybeans, and producers may want to save acephate for stink bug applications.

For more information or if you have any questions or concerns, please contact:

Sebe Brown at 318-498-1283 (cell) or 318-435-2903 (office)

Dr. David Kerns at 318-439-4844 (cell) or 318-435-2157 (office)

Dr. Julien Beuzelin at 337-501-7087 (cell) or 318-473-6523 (office)

Dr. Jeff Davis at 225-747-0351 (cell) or 225-578-5618 (office)

 

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