Sclerotinia stem blight in Soybeans

August 26, 2014 in Soybean, Soybean Disease Management, Soybean diseases

We have had a few reports of sclerotinia stem blight (white mold) on soybeans grown in the mid-Atlantic. This is a cool season disease that we see almost every year, but only to a very small degree. Often you will see it in high yield environments and in shaded areas of the field, such as along wood edges. The fungus overwinters as small, pebble-like structures in the soil which germinate to directly infect plants or produce a mushroom-like structure that can produce millions of spores over several days.  Sclerotinia spores are wimpy and require dead/dying tissues to germinate and take hold.  This is why we see the fungus cause issues during flowering- spores will land on decaying flower petals and grow into the flower and eventually the stem.  Over time lesions with distinct margins develop and more of the black pebble-like structures may be observed in or on the stem.  Affected plants may wilt or mature prematurely.

Sclerotinia stem blight.  Note the black structures on the stem and sharp lesion edges.  Photo: C. Whaley.

Sclerotinia stem blight. Note the black structures on the stem and sharp lesion edges. Photo: C. Whaley.

 

If you notice Sclerotinia stem blight (white mold) in your field it is likely there to stay. The best management practice at this point in full season beans is to schedule infested fields to be harvested last to minimize spread of the pathogen from field to field on farm equipment.  Hot weather is not favorable for this disease and only a few days of hot dry weather are needed to burn out Sclerotinia.

 
In soybeans planted into fields with a known history of Sclerotinia stem blight, and when weather is cool and wet, applications targeting the R1-R2 stage are the most efficacious.   Example fungicides for SSB suppression include labeled group 1’s (Topsin), labeled group 3’s (Proline, Domark, Topguard), Fluazinam (Omega); labeled group 7’s (Endura) and labeled group 11’s (Aproach). Often fungicide applications are not justified in Delaware or Maryland but they can be beneficial under some circumstances. As usual, preventative applications are most efficacious. For more information on fungicide ratings, please see the NCERA 212/218 fungicide efficacy tables click this link: Soybean Fungicide efficacy table_FINALx

Identifying Rusts in Corn

August 18, 2014 in Corn, Corn Disease Management

Over the past week several individuals reported the presence of rusts in Delaware corn fields.  Indeed, rust is present on our corn trials located in Georgetown, DE.  However, this was not common rust (CR), but Southern rust (SR).  How can you tell the difference between the two?  1) SR tends to sporulate mostly on the upper leaf surface; CR can be found on both upper and lower leaf surfaces.  2) Pustules of SR are orange to light brown in color and generally round ; CR are brick red or dark brown and often elongated or oval.  3) SR often is found on leaves in patches; CR tends to be evenly distributed on a leaf. 4) Common rust does best between 61-77 degrees F; Southern rust does best between 77 and 82 degrees F.  It is not uncommon to see both rusts on the same leaf.

 

southern rust

Southern rust on the ear leaf of a corn plant. Pustules often are only on the upper surface, are round, orange, and grouped in patches.

SouthernandCommon

Southern rust (Left) vs common rust (Upper right) on the upper surface of a corn leaf. Photo courtesy of A. Robertson, Iowa State University with permission.

SR needs to blow in from the south, often Latin America, every year.  This season it arrived sooner than usual and was a concern early in the southern states.  It was first detected in Virginia roughly a week ago, and I suspect it moved in to Delaware and the surrounding region soon thereafter.  Typically these diseases start in one or few “epicenters” in a field where the environment is slightly more favorable for disease development.  Over time and given the right environment, the disease may spread through and between fields.  Therefore, it is easy to miss the start of a rust outbreak if fields are not thoroughly scouted.

Our hybrids likely do not have much resistance against SR, so some growers may be concerned with the appearance of this pathogen in our fields.  First, if your corn is at or near or past dent you are likely at the point where within season management is not really an option and likely not economical, particularly at $3.60 a bushel.  Currently I do not recommend fungicides for southern rust, especially if you are at or near R5 and have already applied a fungicide around VT/R1.  Late planted fields that have not received a fungicide application might see some benefit, particularly if the field is irrigated.  Before making spray decisions carefully scout your fields to see if southern rust may be present and if any other issues may warrant your attention.

For more information on common vs Southern rust see this excellent factsheet by Dr. Kiersten Wise at Purdue University:  Common vs Southern Rust on corn

Below you will find the Corn Disease Working Group Fungicide Efficacy table for 2014.

CDWG Fungicide efficacy table_2014_FINAL

 

 

 

Nematode Assay Changes Effective as of September 1 2014

August 16, 2014 in Profitible soybean management, Soybean, Soybean Disease Management, Soybean diseases, Vegetables

The University of Delaware understands the importance of assaying soil samples for the presence of numerous plant-parasitic nematodes. While some nematode problems can be diagnosed in the field, laboratory assay of soil and roots is typically necessary to confirm field observations and is critical in determining crop rotations. Reading nematode samples requires highly specialized expertise, is time-consuming, and can be costly. The University of Delaware is proud to be able to continue to provide this important and unique service to the mid-Atlantic region. Please know that in order to continue the service, the University of Delaware will need to impose minimal fees to off-set increased costs as of September 1, 2014. These fees will allow us to cover escalating costs associated with labor and supplies. To contain further costs, the assay service is currently only accepting commercial vegetable and field crop samples. 

The Virginia Tech Nematode Assay laboratory has agreed to accept Delaware and Maryland Fruit and Ornamental nematode samples at the following address:

Nematode Assay Laboratory
115 Price Hall
Virginia Tech
Blacksburg, VA 24061-0331 Phone: (540)231-4650
Fax: (540)231-7477
Email: jon@vt.edu
Web site:  https://www.ppws.vt.edu/extension/nematode-laboratory/index.html (for sample submission fees, instructions, and forms.  Contact the service to determine out of state sample costs.)

 

The 2014-2016 UD Vegetable and Field Crop Nematode Assay Service price structure is as follows:

Assay Price as of September 1, 2014
Vegetable and Agronomic crops Juvenile assay $50 per sample
Soybean cyst nematode egg assay $30 per sample
3 day rush analysis Additional $20 per sample

A 10-14 day turnaround time will be required for most samples.  A 3-day-rush analysis can be conducted for an additional $20 per sample.

Sample submission instructions can be found here:

Soybean Cyst and Root Knot Nematodes in Soybean

August 14, 2014 in Soybean Disease Management

This year has thusfar been excellent for soybeans in the region. With outstanding crop growth comes large, lush canopies that require a substantial amount of water and nutrients to meet the physiological demands of the plant. When the water and nutrient requirements for beans are high and water availability is low, root related issues tend to become more pronounced. This has been the case in some dryland fields in the region where aboveground symptoms of soybean cyst (SCN) or root knot (RKN) nematode are starting to appear. These nematodes reduce translocation of water and nutrients from roots to foliage by comprising the root system in various ways.

What do these symptoms look like from afar? You might notice yellowing of foliage in portions of the field, particularly raised, well drained areas. In some cases you may notice plants are stunted to various degrees. To determine if SCN or RKN might be involved, use a shovel to dig up symptomatic plants. Carefully knock off as much soil as possible and examine the root system. SCN appears as small yellow or white lemon-shaped bumps on the roots. These bumps are much smaller than nodules formed by nitrogen fixing bacteria which also will be present on the root. If RKN is present you will notice larger, irregularly formed growths on the root system. The irregular shape and lack of a pink internal coloration further distinguish them from bacterial nodules. It’s always a good idea to look at healthy plants for a comparison whenever possible.

Downy Mildew on Soybean-Don’t Sweat It

July 31, 2014 in Soybean, Soybean Disease Management, Soybean diseases

The past two years Delaware has had a greater incidence than of soybean downy mildew usual.   In some fields incidence has been near 30%.   A few people have asked me about the seriousness of this disease and the potential impacts it might have on the soybean crop.

sdmupper

A soybean leaf with young and old downy mildew lesions. This leaf has less than 5% disease severity.

Biology

Downy mildew is an extremely common disease of soybean that occurs wherever soybean is grown.  Downy Mildew is caused by an oomycete, Peronospora manshurica.  Oomycetes are not Fungi, but are more closely related to brown algae in many aspects.  The commonality with the oomycete pathogens is that they are water-loving, so they become more problematic under consistently humid or wet conditions.    It overwinters in soybean residue as spores and can also enter fields through infested seedlots.  During periods of cool, humid weather the organism produces sporangia, which can be dispersed long distances to soybean leaves.  Younger leaves tend to be more susceptible than older leaves.  Under the right environmental conditions sporangia germinate and infect the foliage and produce pale green /yellow spots.  Often the underside of the leaf will be fuzzy and grey when viewed from below.  As the spots age the spots turn brown and may resemble symptoms of other foliar diseases.  Pods can also be affected without presenting any obvious outward symptoms.  Infection can result in pods or seed becoming encrusted in a white mass of the pathogen.  Crusted seeds often appear white, may crack, and can be lighter than unaffected seeds.  If favorable weather persists, the pathogen can continue to produce spores and spread.  Moderate, humid temperatures favor downy mildew, and temperatures above 86F stop disease progress and sporulation.  The downy mildew pathogen is highly diverse and over 33 races of the pathogen are known.

Impact and Management

Downy mildew seldom causes yield loss.  In rare cases reports of up to 8% yield loss have been reported in some states.  Variety plays a big role in susceptibility to downy mildew.  Therefore, you should take note of highly symptomatic varieties and consider switching varieties if downy mildew is a concern to you.  Unfortunately seed companies seldom provide downy mildew resistance ratings: this is a further indication of the rarity in which this disease significantly impacts the crop.  Rotation away from soybean for at least one year will help reduce primary inoculum and tillage, where practical, will further bury and destroy inoculum.  Fungicide applications specifically targeted for downy mildew are not recommended.  Furthermore, because downy mildew seldom appears at high levels, extension specialists do not have efficacy data for this pathogen.

Economics of fungicides in 2014 corn

July 20, 2014 in Corn, Corn Disease Management

Over the course of the week many people have asked if fungicides will be profitable in the 2014 corn crop.  The answer depends on several factors including: 1) potential for disease (hybrid resistance level, environment, presence or history of disease), and 2) economics (grain price, yield potential, and the cost of fungicide application).  I’ll try to keep the discussion as to the point as possible for clarity.

Potential for disease. The first thing you need to do when deciding if/when you should apply a fungicide to corn this year is to determine how much disease there is in the field. The most common disease in Delaware this year is Grey leaf spot. For Grey leaf spot and other residue-borne diseases, if you have greater than 5% severity on any of the three leaves below the ear in 50% or more plants in the field then the level of disease might require intervention. However, other factors will impact future development of disease including hybrid genetics and the environment. A “worst case environmental scenario” for a residue-borne disease such as Grey leaf spot is a no-till, irrigated field of continuous corn with a history of the disease. Table 1 provides some guidelines that may help you determine the risk level of your field based on hybrid resistance rating and irrigation practice.

 Table 1.  Hybrid resistance rating to Grey leaf spot, irrigation, and potential need for fungicide application in Delaware.  In this table disease is assumed to be present on 50% or more of plants at greater than 5% severity on the 3rd leaf below the ear leaf or above. In addition, the worst case scenario is assumed: a field with a history of GLS, no-till, corn after corn, and moderate temperatures. The lack of any of these factors will further reduce the likelihood of a fungicide benefiting the crop.

Hybrid Resistance Rating to GLS Irrigation practice Potential need for fungicide application (VT-R2)
Highly Resistant Irrigated Low-Medium
Unirrigated Very low*
Moderate Irrigated Medium
Unirrigated Low-Medium
Low resistance Irrigated High
Unirrigated Medium-High

*In general, dryland corn that is highly resistant to GLS does not require a fungicide application for management.

 

Economics.  With corn heading towards $4.00 per bushel, a greater yield benefit is needed to cover application costs. Table 2 provides examples of the bushel returns you would need to cover fungicide treatment (applicator cost + product) at different grain prices.

Table 2. The required bushel / acre yield increases required to pay for various fungicide application costs at 5 different grain prices.

Application Cost (per Acre) Grain Price (bu)
$3.50 $4.00 $4.50 $5.00 $7.00
$20 5.7 5 4.4 4 2.9
$25 7.1 6.3 5.6 5 3.6
$30 8.6 7.5 6.7 6 4.3

 

The likelihood that a fungicide will pay for itself is greatest in situations where disease potential is high, application costs are low, and grain prices are high. How often does the fungicide pay for itself? A 2011 paper published in the journal Phytopathology examined 187 studies of corn responses to fungicides conducted throughout the corn belt from 2002-2009.   The chances that the application costs of a fungicide would be covered by the yield return were estimated across a range of grain prices ($2-7 / bu) and application costs ($16-40 / acre). In a nutshell, the research showed that in over 85% of the grain / application cost combinations, there was a greater than 50% chance that the application of a fungicide would not pay for itself if there was less than 5% disease severity on the ear leaf between R4 and R6. Conversely, only 33% of the grain / application cost combinations did not pay when there was more than 5% disease severity on the ear leaf between R4 and R6.  Therefore, the greatest chance for a grower to break even or profit from a fungicide is when the potential for disease is high.

In addition, the study showed that although fungicide use in corn can certainly be beneficial, responses are also highly variable from location to location and year to year. For example, fungicide applications reduced corn yields in 26-48% of the studies included in the metaanalysis.

The take home message is that you will need a greater bu/A yield increase this year to cover the application cost. You are more likely to recover this cost in disease favorable environments (no till, irrigated, corn after corn, history of GLS or other common residue-borne diseases) when susceptible hybrids are planted.

Reference: P. A. Paul, L. V. Madden, C. A. Bradley, A. E. Robertson, G. P. Munkvold, G. Shaner, K. A. Wise, D. K. Malvick, T. W. Allen, A. Grybauskas, P. Vincelli, and P. Esker. 2011. Meta-Analysis of Yield Response of Hybrid Field Corn to Foliar Fungicides in the U.S. Corn Belt. Phytopathology 101:1122-1132.

 

Delaware Potato Disease Advisory # 10

July 18, 2014 in Potatoes, Vegetables

The 10th Delaware Potato Disease Advisory can be found here:  Potato Disease Advisory #10 July 18 2014

 

Thoughts on Half Rate Fungicide Use in Field Crops

July 12, 2014 in Corn, Corn Disease Management, Soybean, Soybean Disease Management

Research has shown that when applied at labeled rates, fungicides can and do significantly increase crop productivity in disease favorable environments.  Rates provided on labels are determined by chemical companies following years of research and huge financial investments.  Not only are the rates chosen to give the grower adequate control of listed pathogens, but they also reduce fungicide resistance risk.  By applying fungicides at levels lower than the rates listed on the label you not only significantly reduce any benefit in terms of disease suppression, but also increase the chance that pathogens with resistant characters will continue to grow and reproduce.  Once resistant individuals dominate a population disease control with that product and also products with active ingredients belonging to the same group lose efficacy.   In some cases, such as with the strobilurins, efficacy may be completely lost.  This is why products such as Quadris and Headline can no longer be used to control Frogeye Leaf spot in soybeans in many states in the South and Midwest.

I encourage you to follow the labels and ensure that these valuable disease management tools remain effective for years to come.  There are many products available that have excellent efficacy on fungal diseases in Corn and Soybean and these products vary in price.  I suggest selecting a product that fits your budget and can be applied at the labeled rate as opposed to purchasing a more expensive product and cutting the rate to make it fit your budget.       

Soybean Disease Update

July 4, 2014 in Profitible soybean management, Soybean, Soybean Disease Management, Soybean diseases

This last week I surveyed 20 full season soybean fields in Kent County, Delaware.  Most fields were around v5, with a few further along.  The following, fairly obvious diseases or disorders  were present in some fields.

 

1) Bacterial blight.  This is typically an early season disease that comes in from the residue.  The bacteria causing the disease enters via wounds and natural openings, produces a toxin within leaves.  Often leaves appear ragged, and yellow lesions form at the leaf edge.  The disease is most severe in young tissue so you may see it early in young plants or in the upper portions of the canopy in older plants.  Lesions eventually dry out and may fall from the plant, resulting in a “shredded” appearance.  This is a fairly common disease that does not require control.  It can be confused with other foliar diseases, particularly Brown spot.

bacterial blight close

Soybean showing early symptoms of bacterial blight. Note the bright yellow halo and angular lesions. Lesions may have a water soaked appearance. No fungal structures will be present in or on lesions. Photo by N. Kleczewski

2) Brown spot.  This is disease caused by Septoria glycines,  a fungus that is present in soybean residue.  The pathogen is spread to the foliage by splashing rain, and symptoms are most evident on very young plants or the lower portions of the canopy in older plants.  The fungus produces lesions of leaves that are small, brown, and irregular in shape.  This disease can be confused with bacterial blight.  Unlike bacterial blight, Brown spot hangs out in the lower parts of the canopy and is not often noticed in the upper canopy.  We tend to think that brown spot isn’t an issue, but in cases where defoliation of the lower canopy is severe (>25% of the canopy defoliates early ) you can see reductions in yield and seed quality.  How often this occurs in Delaware is not currently known.

3) Downy mildew.  Downy mildew isn’t caused by a bacterium nor a fungus.  Instead, it is caused by a fungal-like-organism (Pythium spp. and Phytophthora spp. are other members of this group).  Symptoms include yellow lesions on the top of the leaf that eventually turn grey with red margins.  On the underside of the leaf grey to white fuzzy masses can be seen on the opposite side of the lesions.  Sporangia often can be viewed with the aid of a hand lens.  This disease can be seed or residue borne, but also can spread long distances on air currents.  Downy mildew requires extended periods of wet weather or humid conditions, and does not do well above 80F.  This is why we tend to see it in fields with older plants and dense canopies.   Last season downy mildew was widespread in many soybean growing regions and was evident in many soybean fields in Delaware.  This disease has historically not been an issue, so control has not been warranted.  If pods are affected seed quality can be impacted.

DM on soy

A soybean leaf with typical small, yellow lesions that can be indicative of Downy Mildew. Fuzzy grey to white masses are often present on these lesions on the underside of the leaf. Sporangia can be viewed with the aid of a 10-20x hand lens. Photo by N. Kleczewski

4) Sun scald.  No this isn’t a disease, but some people may think they are seeing Cercospora leaf blight, which causes purple seed stain.  Cercospora leaf blight tends to occur later in the season, and symptoms will be more uniform across the leaf.  Leaves turn purple and eventually develop a leathery appearance.  Sunscald occurs when leaves flip over and exposed to strong sunlight.  In the case of sun scald, you will notice that often only the portion exposed to the sun is purple and there is a clear line of unaffected tissue that was shaded from the sun.  Upper tissues are commonly affected.  High temperatures or wind also can contribute to sun scald symptoms.  Sunscald is superficial and not an issue.

sunscaldbeans

Sunscald on soybeans occurs when leaves are flipped and exposed to strong sunlight. Often symptoms are apparent in the upper canopy. Photo by N. Kleczewski

 

 

Cucumber Downy Mildew Detected in Maryland

July 3, 2014 in Vegetables

Cucumber Downy Mildew has been detected in Maryland and confirmed by Kate Everts, Vegetable pathologist for UMD and UD.  Growers should begin preventative fungicide programs for CDM and diligently scout for disease symptoms on susceptible crops.  Fungicide recommendations can be found in the 2014 Commercial Vegetable Production guide at this link:  Commercial Vegetable Production Guide 2014.  For more information on the CDM forecast, field identification, and control recommendations, follow this link: CDM information

CDM

Characteristic Cucumber Downy Mildew symptoms on a leaf.

 

 

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