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Wheat and Corn Pathology Update

Wheat and Corn Pathology Update published on No Comments on Wheat and Corn Pathology Update

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.

 

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/

Reports of Fusarium Head Blight of Wheat (Scab) Statewide

Reports of Fusarium Head Blight of Wheat (Scab) Statewide published on No Comments on Reports of Fusarium Head Blight of Wheat (Scab) Statewide

Trey Price, Field Crop Pathology, Macon Ridge Research Station
Boyd Padgett, Small Grain Pathology, Dean Lee Research and Extension Center

Over the past two weeks there have been multiple reports from producers and consultants throughout Louisiana of wheat scab, also known as Fusarium head blight (FHB). Reported incidences have ranged from 10 to 20 percent. 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/orangish 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).

 

Since 1996, outbreaks of FHB have been as variable as the weather. Outbreaks have been reported in the Great Plains, Central U. S., Mid-South, and Southeast with reported losses of up to 20% and up to 80% in isolated fields. Conditions favoring development are wet, warm weather during flowering. The fungus may infect wheat from flowering to harvest with the most devastating infections occurring during flowering. This infection timing creates hurdles for managing the disease.

The pathogen oversummers 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 of the earlier research showed that tebuconazole (Folicur and generics) may reduce incidence and severity of FHB. Later research shows that Prosaro (prothioconazole + tebuconazole), Proline (prothioconazole), and Caramba (metconazole) may be 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. Essentially we have a 5 day 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 is also 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.

The vast majority of fields in Louisiana are currently past the application window. Fungicide applications at this point would likely by off label and ineffective.

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 calls and observations at this point, FHB has been more prevalent this year compared to previous years. If we have similar weather conditions next year during flowering, expect to encounter FHB again in 2016. 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 next year.

For more information, please see the following resources:

www.scabusa.org
www.scabsmart.org
http://www.mississippi-crops.com/2015/04/24/wheat-disease-update-april-24-2015/
http://www.arkansas-crops.com/2015/04/23/update-management-Arkansas/#sthash.xSo0EAqt.dpuf

 

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

Bacterial Blight (Angular Leaf Spot) Observed in Louisiana Cotton Fields published on 1 Comment on Bacterial Blight (Angular Leaf Spot) Observed in Louisiana Cotton Fields

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.

2014 Northeast Research Station Pest Management and Crop Production Field Day

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