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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.

Managing Southern Corn Rust in Louisiana

Managing Southern Corn Rust in Louisiana published on No Comments on Managing Southern Corn Rust in Louisiana

Primary Author:  Clayton Hollier, Plant Pathologist, Department of Plant Pathology and Crop Physiology
Boyd Padgett, Regional Director and Plant Pathologist, Central Region
Trey Price, Plant Pathologist, Macon Ridge Research Station

We have scouted several corn fields concerning reports of southern corn rust (SCR). These reports and field visits reveal an epidemic of southern rust across the state but in particular in the south central and central corn growing regions. Field surveys indicate that SCR has been reported in 25 of 64 parishes and might be in more that have yet to be surveyed. The LSU AgCenter does not recommend an automatic fungicide application to corn. However, when disease epidemics are progressing up to and including the soft dough stage, an application of a labeled fungicide can protect yield and quality and still be economically viable.

Figure 1. Southern corn rust. Spores are orange.
Figure 1. Southern corn rust. Spores are orange.

Ideally, fungicides should be applied prior to disease onset, but realistically, fungicides are usually applied at or just after onset. Therefore, individuals should make efforts to detect and treat diseases as early as possible to prevent losses to yield and quality. When it is determined an application is needed, a premix fungicide will offer wide spectrum activity (examples are: Headline AMP, Stratego YLD, Quilt, and Quilt Xcel). Follow label instructions for application timings, rates, etc. In most cases, a single application at tassel is justified when disease is present and active. The decision to apply a fungicide should be made on a field-by-field basis. The remainder of this newsletter will address disease identification and management considerations.

Southern corn rust can be found in Louisiana corn fields every year, but its impact on grain quality and yield is dependent on several variables. Although there is some level of resistance, it seems very low. The hybrids considered resistant to SCR in the past are no longer considered resistant, due to new race development found in Georgia four years ago. Therefore, the decision to manage SCR with fungicides should be based on a solid understanding of disease initiation and development. While this disease rarely develops to statewide damaging levels in most years, disease incidence and severity in individual fields may warrant a fungicide application. However, before applying a fungicide, several factors need to be considered. These include disease identification, environmental conditions favoring disease development, and the relationship between disease severity and yield loss.

Southern corn rust is caused by the fungus Puccinia polysora. Initial infections are caused by wind-blown spores. This is a warmer-season rust and, therefore, usually occurs late season and does not always have adequate time to affect yield. However, this rust is very aggressive and if disease epidemics initiate early (prior to or at tasseling), yields could be reduced.

Conditions favoring development include temperatures between 80 degreesF and 90 degrees F with high relative humidity or abundant rainfall. Once established, the spores can survive, spread and germinate at temperatures up to 104 degrees F.

Southern rust produces small circular to oval pustules and contains orange to light brown spores (Figure 1). Pustules are usually more abundant on the upper leaf surface and may also be found on the leaf sheath and husks when disease is severe.

Risk and Management
Risk to disease is influenced by several factors including genetic resistance, tillage practices, planting date, and environmental conditions. Later planted corn can also heighten risk to SCR.

The first line of defense for managing any corn disease should be selecting a disease-resistant hybrid. Since commercially available resistance is not an option, a fungicide may be needed. However, when SCR is not present a fungicide is not necessary. Another factor to consider is when SCR epidemics initiate relative to crop growth stage. The potential for yield loss is high when SCR develops prior to tasseling and conditions remain favorable for development during the growing season. When the rust initiates after tasseling, the potential for disease loss decreases.

The relationship of yield and defoliation can be found in Table 1 adapted from the National Crop Insurance Service’s Corn Loss Instruction.

Table 1. Estimated % corn grain yield loss due to defoliation at various growth stages.

scr tableAdapted from the National Crop Insurance Service’s Corn Loss Instruction to represent the leaf collar growth staging method. Included in the Mississippi State University, Grain Crops Update June 4, 2010, Erick Larson.

 

 

 

 

Bacterial Blight (Angular Leaf Spot) Observed in Louisiana Cotton Fields

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Bacterial blight was once (prior to 1991) a major disease of cotton causing average annual losses of as much as 3.4%. In severe cases, losses ranged from 50 to 70%. From 1991 to 2000, average losses due to bacterial blight averaged 0.1%. Over the past few years, a resurgence of the disease has been noted in the Mid-South. Since 2009, the disease has been observed on many different varieties in many counties in Texas, Mississippi, Arkansas, and Missouri. This year the disease has been observed in three parishes to date. The causal agent of the disease is a bacterium, Xanthamonas campestris pv. malvacearum. This pathogen may affect all plant parts of cotton causing seedling disease and/or infection of leaves, vascular tissues, stems, petioles, bracts, and bolls. Foliar symptoms begin with small, water-soaked lesions on the underside of leaves that are “angular” because of leaf veins that restrict the movement of the bacteria. The lesions become visible on the upper surface of leaves and become necrotic (Figure 1), and the pathogen also may affect vascular systems in leaves resulting in purplish lesions that follow veins (Figure 2). Severe infections may result in defoliation. Stems and petioles also may be infected causing lodging, loss of branches, and/or leaf drop (Figures 3 and 4). Bolls also may be infected resulting in stained lint and possible transfer of the bacterium to seed.

Figure 1.  Angular lesions caused by bacterial blight.
Figure 1. Angular lesions caused by bacterial blight.

The pathogen may be seedborne, and in the past was successfully managed by acid-delinting and chemical treatments of seed. Infection occurs through natural openings or wounds to cotton plants. Survival of the pathogen on plant debris in the field may serve as initial inoculum the following cropping year. The bacterium is spread by wind, rain, insects, and equipment. Overhead irrigation or wind-driven rain will spread the bacterium during an active epidemic. Optimal conditions for disease development are high relative humidity (>85%) and temperatures ranging from 86-97°F.

Figure 2.  Bacterial blight lesions following leaf veins.
Figure 2. Bacterial blight lesions following leaf veins.
Figure 3. Angular leaf lesions infection of petiole, and leaf drop.
Figure 3. Angular leaf lesions infection of petiole, and leaf drop.
Figure 4.  Infection of stem and petioles causing leaf drop.
Figure 4. Infection of stem and petioles causing leaf drop.

Planting acid-delinted or chemically-treated seed will reduce the chances of infection. Sanitary measures to avoid spreading the bacterium should be used in fields where infection has occurred. Rotation to a non-host or planting resistant varieties also are management options. Turning under plant debris will help by reducing the number of bacteria that serve as primary inoculum at the beginning of the next growing season. Avoiding rank canopy growth will reduce leaf wetness periods and may help to reduce disease severity. Managing insect pests will likely lessen the spread of the pathogen. Do not stop irrigating, but do not over-irrigate. Water stress could be more detrimental than the disease, and cotton plants may compensate for foliage loss. Most importantly, there are varieties available that are resistant to bacterial blight. Below are some external sources with more information. Please contact your local parish agent, specialist, or nearest research station if you suspect bacterial blight or require additional information.

http://lubbock.tamu.edu/files/2011/11/2010Bacterial.pdf
http://www.mississippi-crops.com/2012/07/13/bacterial-blight-of-cotton-update-july-13-2012/
http://www.arkansas-crops.com/2011/07/20/alert-bacterial-blight-of-cotton-found-in-arkansas/

 

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.

Sorghum Midge and White Sugarcane Aphid Concerns

Sorghum Midge and White Sugarcane Aphid Concerns published on 3 Comments on Sorghum Midge and White Sugarcane Aphid Concerns

David Kerns and I have been receiving numerous phone calls this week about problems with applications of pyrethroids tank mixed with Transform for control of midge and white sugarcane aphid. The use of a pyrethroid for control of sorghum midge is a common practice in Louisiana; however, pyrethroids are very toxic to beneficial insects and are very likely to flare white sugarcane aphids in grain sorghum. Co-appliations of Transform and a pyrethroid have led to white sugarcane aphids recolonizing fields very rapidly and often resulting in poor control of aphids overall.

Therefore, automatic insecticide applications for midge should be avoided, and applications should only be made if midge are present.  The Louisiana threshold for midge in sorghum is at 25 – 30% bloom, treat for one or more midge per head.  If midge and sugarcane aphids are present, tank mixed applications of chlorpyrifos and Transform will offer good midge control while also reducing the risk of flaring aphids. Chlorpyrifos may not be quite as effective as a pyrethroid for sorghum midge and large populations may require a second application 3 – 4 days later.  Transform tank mixed with Dimethoate is another option for midge and aphid control; however, producers should be prepared to follow up with a dedicated midge application 3 – 4 days later.

Also, pyrethroid applications for the headworm complex in grain sorghum are strongly discouraged. Pyrethroid resistance is very common in sorghum webworm and corn earworm in Louisiana, and insecticides such as Belt or Prevathon should be used for headworms. These chemistries are Lepidopteran specific and will not harm beneficial insects or flare sugarcane aphids.

 

Sugarcane Aphids in Sorghum

Sugarcane Aphid Numbers Increasing in Grain Sorghum

Sugarcane Aphid Numbers Increasing in Grain Sorghum published on 1 Comment on Sugarcane Aphid Numbers Increasing in Grain Sorghum

Infestations of sugarcane aphids in boot to heading grain sorghum are increasing in Louisiana. Many of these populations start off small and exponentially increase in a span of 5 to 7 days. Pyrethroid applications for midge control can reduce natural enemy numbers allowing sugarcane aphids to reach damaging numbers faster.  Honey dew produced by sugarcane aphid feeding will give the crop a glossy appearance and large accumulations will often result in sooty mold growth and harvesting issues later season.

Sugarcane Aphid Damage to Pre-boot Sorghum
Sugarcane Aphid Damage to Pre-boot Sorghum

Sugarcane aphids are difficult to control with currently labelled insecticides; however, Louisiana was granted a section18

Sugarcane Aphids in Sorghum
Sugarcane Aphids in Sorghum

emergency exemption for the use of Transform 50WG for the 2014 production season.  Transform applications should be initiated before grain sorghum becomes heavily infested and producers in Texas are making applications at 30% infested plants with 100 to 250 aphids per leaf present. Use lower aphid numbers with increasing stress due to plant water deficit. This treatment threshold appears to be working for Texas growers; however, these recommendations are not supported by university research due to the recent introduction of this pest to grain sorghum in the United States. Transform applications of 1 oz/acre should be used on medium to high sugarcane aphid populations with the largest gallonage per acre (GPA) feasible for applicators (5 GPA by air or 20 GPA by ground). If 1 ounce applications of Transform are not providing adequate control the rate should be increased to 1.5 oz/acre.

2014 Northeast Research Station Pest Management and Crop Production Field Day

2014 Northeast Research Station Pest Management and Crop Production Field Day published on No Comments on 2014 Northeast Research Station Pest Management and Crop Production Field Day
NERS Field Day Flyer
NERS Field Day Flyer

Transform Granted Section 18 for Control of Sugarcane Aphid in Louisiana Sorghum (Forage, Grain or Stover)

Transform Granted Section 18 for Control of Sugarcane Aphid in Louisiana Sorghum (Forage, Grain or Stover) published on No Comments on Transform Granted Section 18 for Control of Sugarcane Aphid in Louisiana Sorghum (Forage, Grain or Stover)

Transform WG Sec 18 Label for Sorghum                                                     Approval Letter With Effective/Expiration Use Dates

Please follow the link above to access the section 18 label.  The link to the approval letter outlines the effective and expiration dates for the use of Transform in sorghum, as well as specifics regarding number of applications and maximum acreage treated in Louisiana.

If you have any questions or concerns about sugarcane aphids or use of Transform in Sorghum 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)

Aerial 24C Labels for Capreno and Halex GT for Louisiana

Aerial 24C Labels for Capreno and Halex GT for Louisiana published on No Comments on Aerial 24C Labels for Capreno and Halex GT for Louisiana

Capreno Aerial Label in LA (12′-17′)              Halex GT Aerial Label in LA (14′)

Please follow the links above to view the specific labels.

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