2015 UW Extension Pest Management Update Meeting Series

General Agronomy

Be sure to get the latest field crop pest management updates, by attending the 2015 PMU Meetings!

Damon Smith, Extension Plant Pathology Specialist

Mark your calendars as the UW Extension’s Pest Management Update meetings are just around the corner (November 9-19). This year’s program will follow the new format established in the 2014 series, with more interaction between presenters and the audience, and participation by Bryan Jensen and Dan Heider with the University of Wisconsin Integrated Pest and Crop Management Program.

We will focus the entire morning (10-noon) on integrated pest management updates by crop (corn, soybean, alfalfa, and small grains). This session will be streamlined to focus on new pesticide registrations, pest updates, and highlight important issues from 2015. After lunch, topics will be more focused on specific updates and diagnostic training. These topics will include:

  • Herbicide resistance update and identification
  • Managing corn rootworms
  • Soybean stem disease identification

These diagnostic and focused trainings were a big hit in 2014 so don’t miss out in 2015!

The full schedule with dates, meeting locations, topics and registration contact information are in the link below. Please register with the host agent at least 1 week prior to the meeting at the location you wish to attend.

Note that due to low turnout in past years, the Arlington location has been dropped from the rotation in 2015. There will only be 7 locations to attend the update meetings, rather than 8 locations as in previous years. Be sure to look at the 2015 schedule included with this article when selecting your preferred date and location.

Please attend the meeting location at which you registered. Each meeting in the series is a separate county-based event and host agents cannot interchange registrant fees or meal counts.

Four hours of CCA CEU pest management credits are requested and available at each location.

The speakers will be extension specialists Mark Renz, weed scientist, perennial cropping systems; Dan Heider, IPM outreach specialist, Bryan Jensen, entomologist, and Damon Smith, field crop plant pathologist.

2015 Pest Management Update Topics:

  • Integrated Pest Management Updates in corn, soybeans, alfalfa, and small grains: Update on new products and/or use of existing products as well as brief highlights of the 2015 pest situations in each crop.
  • Herbicide resistance update and identification: Dan Heider and Mark Renz discuss the herbicide resistant weed situation in Wisconsin and how to identify problematic situations.
  • Managing corn rootworms: Bryan Jensen will take you through identifying corn rootworm problems and how to manage them in field corn.
  • Soybean stem disease identification: Damon Smith will discuss the 2015 soybean stem disease situation in Wisconsin. He will offer tips on how to identify and manage the various stem diseases that cause problems in Wisconsin.

Check out the full meeting schedule at this link.

What Should You Know about Corn and Soybean Diseases as You Prepare for Harvest?

Damon L. Smith – Extension Field Crops Pathologist, University of Wisconsin

As the fall is approaching and crop harvest plans are being made, it is important to continue to assess disease issues in corn and soybean. These assessments aren’t being made in order to make plans for in-field management, but to improve the quality of grain that is harvested and allow for some educated decision-making for 2016.

Some Diseases to Consider in Corn at Harvest

Figure 1. Gibberella stalk rot on corn. Severe stalk rot on the left and less severe stalk rot on the right.

Figure 1. Gibberella stalk rot on corn. Severe stalk rot on the left and less severe stalk rot on the right.

Now is the best time to begin scouting corn for stalk rot issues and also fungal ear rot potential. Diseases such as Anthracnose stalk rot and Gibberella stalk rot are becoming apparent in corn.  Inspect the stalks integrity on the outside.  Be sure to squeeze the outside of the stalk to gauge the potential severity of the rot on the inside of the stalk.  Cut a few stalks from diverse areas of the field to see how rotted stalks might be. In figure 1, the stalk on the left has a severe case of Gibberella stalk rot, while the stalk on the right is far less rotted.  Fields that had high levels of norther corn leaf blight (NCLB) this season, are going to be more prone to stalk rot due to the added stress of the foliar disease. The more severely rotted stalks are, the more likely they will lodge.  Therefore timely harvest is important. Growers should target harvesting of fields with severe stalk rot before fields that have less stalk rot, in order to minimize harvest losses due to lodging.

Figure 2. Diplodia ear rot.

Figure 2. Diplodia ear rot.

Ear rots can also be an issue at harvest time. Fusarium ear rot, Gibberella ear rot, and Diplodia ear rot (Fig. 2) are just a few that can damage corn in Wisconsin. It will be critical to check fields in the next several weeks in order to make decisions on what fields to harvest first.  Harvest priority should be placed on fields with a high level of ear rot.  As corn stands late into the fall, certain ear rot fungi can continue to grow, damage ears, and cause increases in mycotoxins in grain. The quicker these fields dry and can be harvested, the more likely the losses due to ear rot and mycotoxin accumulation can be minimized.

Soybean White Mold Management at Harvest

Figure 3. Sclerotia of the white mold fungus inside a soybean stem.

Figure 3. Sclerotia of the white mold fungus inside a soybean stem.

In Wisconsin, the main disease to consider when making harvest plans in soybean is white mold. White mold is present in some soybean fields in the state and has caused considerable damage in a few of those fields. Remember that the white mold fungus not only causes stem blight and damage, but also causes the formation of sclerotia (fungal survival structures that look like rat droppings) on and in soybean stems (Fig. 3). These scelrotia serve as the primary source of fungal inoculum for the next soybean crop. They also get caught in combines during harvest. These sclerotia can then be spread in combines to other fields that might not be infested with the white mold fungus.  Therefore, it is important to harvest non-infested soybean fields first, followed by white mold-infested fields, to be sure the combine does not deposit any residual sclerotia in the non-infested fields.  If this is not an option and you must harvest white mold infested fields before non-infested fields, be sure to clean the combine thoroughly between fields.

For more information about white mold management in soybean you can click here and scroll down to “white mold” or watch a video by clicking here.

Identify Corn and Soybean Diseases Now to Make Decisions for 2016

While most of the focus during this time of season is on equipment and calibrating yield monitors, it is important to get an accurate diagnosis on any soybean and corn diseases you are seeing now.  This information will help this winter as you review variety and hybrid trials and make decisions about what you are going to plant in 2016. Have knowledge of the primary disease issues in your fields. This will allow you to choose varieties and hybrids with the best disease resistance package to combat those diseases. Finally, now is a great time to sample for soybean cyst nematode (SCN). For more information on sampling for SCN in Wisconsin, CLICK HERE.

Goss’s Wilt Confirmed in Wisconsin in 2015

Figure 1. Foliar symptoms of Goss's wilt on a corn leaf. Photo Credit: Larry Osborne, Bugwood.org.

Figure 1. Foliar symptoms of Goss’s wilt on a corn leaf. Photo Credit: Larry Osborne, Bugwood.org.

Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison

Goss’s wilt on field corn was confirmed for the first time in the 2015 season in Wisconsin this week in Grant County. Goss’s wilt has been confirmed in past years in Wisconsin, including the 2014 field season.

Symptoms and Signs

Goss’s wilt is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis. First visual symptoms usually appear as gray or yellow stripes on leaves that tend to follow the leaf veins (Fig. 1). Often “freckles”, or brown or green irregular spots, can be observed within the leaf lesions (Fig. 2). Freckles are an excellent diagnostic symptom to confirm Goss’s wilt. Vascular tissue (Fig. 3), husks, and kernels can sometimes take on an orange hue. Occasionally, bacterial ooze or dried ooze can be observed on symptomatic leaves. This disease is often confused with northern corn leaf blight (NCLB), which is a fungal disease. Earlier this season I wrote an article on differentiating between NCLB and Goss’s will. You can visit that article by CLICKING HERE.

Figure 2. "Freckles" on a corn leaf with Goss's wilt. Photo credit: Larry Osborne, Bugwood.org.

Figure 2. “Freckles” on a corn leaf with Goss’s wilt. Photo credit: Larry Osborne, Bugwood.org.

Factors that Cause Disease Development

The Goss’s wilt bacterium overwinters in old corn residue. The bacterium enters the plant through wounds or natural openings. Yield losses will depend on the susceptibility of the hybrid being grown. Factors that put corn fields at higher risk include:

  1. Reduced Tillage
  2. Continuous corn rotation
  3. Planting a susceptible hybrid
  4. Poor grassy weed control
  5. Hail, wind, or severe weather events causing injury on corn plants

Storms with hail and wind were prominent this past weekend in areas of Wisconsin. Fields with wind and hail damage should also be monitored closely for Goss’s wilt. This type of damage creates excellent entry points for the Goss’s wilt pathogen.

Figure 3. Orange vascular tissue of a corn plant with Goss's wilt. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org.

Figure 3. Orange vascular tissue of a corn plant with Goss’s wilt. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org.

Management

There is currently no research-based method of in-season management of Goss’s wilt. There are some foliar products being marketed for the control of Goss’s wilt, but university-based research has indicated that these products have little efficacy on Goss’s wilt in the field. Because this disease is caused by a bacterium, the application of fungicide WILL NOT control Goss’s wilt. Planting resistant hybrids in fields with a history of Goss’ wilt is recommended.  Residue management and crop rotations should also be implemented in at-risk fields. Some grassy weed hosts can be alternative hosts for the Goss’s wilt pathogen. Therefore, a sound weed management program in and around corn fields can be useful in managing Goss’s wilt. Colleagues at Purdue University have developed an excellent fact sheet covering alternative grassy weed hosts of Goss’s wilt. They also include some recommendations for controlling these weedy hosts. You can download a PDF version of the fact sheet by CLICKING HERE.

Additional Goss’s Wilt Information

University of Nebraska – http://pdc.unl.edu/agriculturecrops/corn/gosswilt

Purdue University – https://www.extension.purdue.edu/extmedia/bp/BP-81-W.pdf

References

Article modified from original version posted in 2014

L.E. Claflin. Goss’s Bacterial Wilt and Blight, in: Compendium of Corn Diseases, 3rd edition. Ed. D.G. White. APS Press.

Corn Diseases of 2015 and Should I Spray Fungicide?

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin

The phone has been ringing a lot lately and the primary questions are:

  • What corn diseases should I be concerned with this year in Wisconsin?
  • Should I spray a fungicide? And If so, what product and timing?

Lets start with the first question. As far as foliar disease issue, I think we need to scout closely for northern corn leaf blight (NCLB) in Wisconsin. The Midwest is already seeing high levels of this disease and it is showing up in the lower canopy in corn fields in southern Wisconsin. Remember, that this disease can be easily confused with Goss’s wilt. Earlier this season I wrote a post about differentiating these two diseases. I encourage you to revisit that post as a refresher. In addition to NCLB, our scouting has revealed a second foliar disease present in the lower and mid-canopy of the corn crop. That second disease is eyespot. Lets talk about NCLB and eyespot in a little greater detail.

Figure 1. NCLB Lesions on a corn leaf

Figure 1. NCLB Lesions on a corn leaf

Northern Corn Leaf Blight (NCLB): The most diagnostic symptom of NCLB is the long, slender, cigar-shaped, gray-green to tan lesions that develop on leaves (Fig. 1).  Disease often begins on the lower leaves and works it way to the top leaves.  This disease is favored by cool, wet, rainy weather, which has seemed to dominate lately. Higher levels of disease might be expected in fields with a previous history of NCLB and/or fields that have been in continuous and no-till corn production. The pathogen over-winters in corn residue, therefore, the more residue on the soil surface the higher the risk for NCLB.  Management should focus on using resistant hybrids and residue management.  In-season management is available in the form of several fungicides that are labeled for NCLB. However, these fungicides should be applied at the early onset of the disease and only if the epidemic is expected to get worse.

Figure 2: A computer simulation of 5% NCLB severity on a corn leaf.

Figure 2: A computer simulation of 5% NCLB severity on a corn leaf.

While I hate talking about threshold levels for managing disease, it can be helpful in your decision making process to know what might be severe. While scouting look in the lower portion of the canopy. If some symptoms are present in the lower canopy, make a visual estimation of how frequent (percentage of plants with lesions) NCLB is in a particular area and how severe (how much leaf area is covered by NCLB lesions.  The lower leaves aren’t responsible for much yield accumulation in corn, but spores produced in NCLB lesions on these leaves can be splashed up to the ear leaves where disease can be very impactful. So by scouting the lower canopy and getting an idea of how much disease is present, you can “predict” what might happen later on the ear leaves to make an informed spray decision. The other consideration you should make while scouting is the resistance rating that the hybrid has for NCLB. If it is rated as resistant, then NCLB severity might not be predicted to get very severe, while in  a susceptible hybrid, NCLB might be present on 50% or more of plants at high severity levels. Note however, that even if a hybrid is rated as resistant, it can still get some disease. Resistance isn’t immunity! If NCLB is present on on at least half the plants and severity is at least 5-10% and weather is forecast to be rainy and cool, a fungicide application will likely be needed to manage the disease. So what does 5% leaf severity look like? Figure 2 is a computer generated image that shows 5% of the corn leaf with NCLB lesions. You can use this image to train your brain to visually estimate how severe the disease might be on a particular leaf. As for fungicide choice and timing, I consider that further below.

Figure 3. Eyespot symptoms on a corn leaf.

Figure 3. Eyespot symptoms on a corn leaf.

Eyespot: Eyespot typically first develops as very small pen-tipped sized lesions that appear water-soaked.  As the lesions mature they become larger (¼ inch in diameter) become tan in the center and have a yellow halo (Fig. 3).  Lesions can be numerous and spread from the lower leaves to upper leaves. In severe cases, lesions may grow together and can cause defoliation and/or yield reduction. Eyespot is also favored by cool, wet, and frequently rainy conditions.  No-till and continuous corn production systems can also increase the risk for eyespot, as the pathogen is borne on corn residue on the soil surface.  Management should focus on the use of resistant hybrids and residue management.  In-season management is available in the form of fungicides. Severity has to reach high levels (>50%) before this disease begins to impact yield. I often have eyespot present in my corn trials each year as we plant into continuous corn and use no-till. However, we typically do not see yield reductions from this disease even in non-sprayed plots. When scouting, note the disease and keep track of the severity. Again, fungicides should be applied early in the epidemic and may not be cost effective for this disease alone.

What fungicide should I spray and should I spray at all? My question is what are you trying to do? Control a disease or simply boost yield? Fungicide should be used as a tool to control a disease and preserve yield. There is no silver bullet fungicide out there for all corn diseases. However, there are many products which work well on a range of diseases. The Corn Fungicide Efficacy table lists products that have been rigorously evaluated in university research trials across the country. You can see there are several products listed that perform well on both NCLB and eyespot. So obviously, if a disease is present and you are trying to control the disease, you might expect more return on your investment, compared to simply spraying fungicide and hoping that there might be a yield increase.

Paul et al. (2011) conducted research to investigate the return on investment (ROI) of using fungicide at low and elevated levels of disease. Data from 14 states between 2002 and 2009 were used in the analysis. They looked at 4 formulations of fungicide products across all of these trials. I won’t go into detail about all products, but will focus on one here, pyraclostrobin. This is the active ingredient in Headline® Fungicide. In all, 172 trials were evaluated in the analysis and Paul et al. found that on average there was a 4.08 bu/acre increase in corn grain yield when pyraclostrobin was used. So there does appear to be some increase in yield with the use of fungicide, but in our current market, will this average gain cover the fungicide application?

Figure 4. Break-even scenarios for corn when foliar fungicide was applied.

Figure 4. Break-even scenarios for corn when foliar fungicide was applied.

The suggested application rate for Headline® Fungicide is 6 to 12 fl oz/acre. My latest cost sheets indicate that at the 6 fl oz/acre rate, the cost of the product alone would be about $20/acre. Note that this does not include the custom applicator cost. This is a variable expense that would need to be added in to get an accurate ROI for your operation. Today we can estimate that we might sell corn grain somewhere between $4 and $5 per bushel. We can then use the cost of the fungicide product and the price of grain to figure out how many bushels of corn we need to make in the crop that would be treated with pyraclostrobin vs. non-treating. Figure 4 is a table with various corn prices along the vertical axis and fungicide costs per acre along the horizontal axis. The cells indicate the bushels of corn per acre needed to break even when using a fungicide at the corresponding cost and corn grain sale price. Using the above scenario, we see that with corn priced between $4 and $5 per acre and a fungicide application cost of $20/acre, we would need to gain 4-5 bushels per acre when using Headline® Fungicide in the current corn market. Obviously these calculations are for just one product, but you can do the same for your farm and fungicide program and use the table to figure out what break-even yield gain you will need to cover your costs.

What are the odds of getting that 4 to 5 bushel per acre yield gain when using Headline® Fungicide? Paul et al. went further and calculated the probability of return at various corn prices and fungicide costs. They did separate analyses for foliar disease severity less than 5% and greater than 5%. In our current corn market with around $4/bu corn prices and a cost of Headline® Fungicide at $20/acre, Paul et al. found that at low foliar disease levels (<5% severity) the odds of a positive ROI using the fungicide would be around 50%. The odds of a positive ROI improve if disease severity is greater than 5%. In their calculations with higher levels of disease (>5% severity), the odds of a positive ROI would be between 60% and 70%. The morale of this story is that if you are going to use fungicides on corn, they should be targeted toward fields that will have, or are at risk, for disease!

So what about fungicide application timing? Over the last several years corn pathologists in the U.S. corn belt have conducted fungicide application timing trials on corn for grain. Programs included various products, but applications focused on an early (V5-V8) timing, a VT-R2 timing, or a combination of V5-V8 plus a VT-R2 application. Over a 5 year period and nearly 1,500 observations, the average yield gain when using fungicide at V5-V8 alone was 1.4 bu/acre, while that at the VT-R2 timing was 4.4 bu/acre, and 4.7 bu/acre for the two pass program. In Wisconsin in 2013, the best gain in yield when using fungicide was at the VT application timing with almost 10 bu/acre over the non-treated. In 2014, we saw the opposite, with an average loss of grain yield at the VT timing of around 10 bu/acre. In Wisconsin, we see that yield gain in fungicide trials is highly variable and depends on the hybrid and weather for that particular season. You can check out results of the fungicide trials and the performance of various products over the last two years by visiting my Fungicide Test Summaries page and viewing the results in the 2013 and 2014 reports.

Finally, be aware that in some cases, application of fungicide in combination with nonionic surfactant (NIS) at growth stages between V8 and VT in hybrid field corn can result in a phenomenon known as arrested ear development. The damage is thought to be caused by the combination of NIS and fungicide and not by the fungicide alone. To learn more about this issue, you can CLICK HERE and download a fact sheet from Purdue Extension that covers the topic nicely. Considering that the best response out of a fungicide application seems to be between VT-R2, and the issues with fungicide plus NIS application between V8 and VT, I would suggest holding off for any fungicide applications until at least VT.

Summary

As we approach the critical time to make decisions about in-season disease management on corn, it is important to consider all factors at play while trying to determine if a fungicide is right for your corn operation in 2015. Here is what you should consider:

1) Corn hybrid disease resistance score – Resistant hybrids may not have high levels of disease which impact yield.

2) Get out of the truck and SCOUT, SCOUT, SCOUT – Consider how much disease and the level of severity of disease present in the lower canopy prior to tassel.

3) Consider weather conditions prior to, and during, the VT-R2 growth stages – if it is cool and wet, disease may continue to increase in corn and a fungicide application might be necessary. If it turns out to be hot and dry, disease development will stop and a fungicide application would not be recommended.

4) Consider your costs to apply a fungicide and the price you can sell your corn grain – Will you gain enough out of the fungicide application to cover its cost?

5) Hold off with making your fungicide application in Wisconsin until corn has reached the VT-R2 growth stages – The best foliar disease control and highest likelihood of a positive ROI will occur when fungicide is applied during this timing when high levels of disease are likely.

6) Be aware that every time you use a fungicide you are likely selecting for corn pathogen populations that will become resistant to a future fungicide application – Make sure your fungicide application is worth this long-term risk. To learn more about fungicide resistance, you can CLICK HERE to download a UW Extension fact sheet.

Other Resources

Wisconsin Field Crops Fungicide Information Page 

Diseases Showing up in Iowa Corn, 2015

UNL CropWatch: Worn Disease Update

References

White, D.G., editor. 2010. Compendium of Corn Diseases. APS Press.

Paul, P. A., Madden, L. V., Bradley, C. A., Robertson, A. E., Munkvold, G. P., Shaner, G., Wise, K. A., Malvick, D. K., Allen, T. W., Grybauskas, A., Vincelli, P., and Esker, P. 2011. Meta-analysis of yield response of hybrid field corn to foliar fungicides in the U.S. Corn Belt. Phytopathology 101:1122-1132.

Time to Start Looking for Corn Diseases in Wisconsin

Figure 1. NCLB Lesions on a corn leaf

Figure 1. NCLB Lesions on a corn leaf

Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison

The 2014 field season was a bit of a challenge for corn growers in Wisconsin, to say the least. We had poor growing conditions, which made for a lot of challenges including diseases. On the top of that list in Wisconsin was Northern Corn Leaf blight (NCLB). A close second was Goss’s Wilt.  Already in 2015, states like Iowa and Nebraska have already reported both diseases on corn. This is among the earliest reports of both diseases in many years. In Wisconsin, we haven’t seen either of these yet, but given the weather patterns recently, I think it is only a matter of time.

For many folks, identification of these two diseases can be challenging. Many are confused by the subtleties of each disease “signature.” Diagnosis is critical in making your management decisions properly. Obviously, the best way to properly diagnose any plant disease problem is to send a sample to the Plant Disease Diagnostic Clinic. A sample can be sent by following their helpful sample guidelines, which can be found by clicking here. In addition to sending a diagnostic sample, there are some signs and symptoms that can be identified in the field, to help get you closer to diagnosing the right disease. Here are some helpful details for each disease.

Figure 2. Foliar symptoms of Goss's wilt on a corn leaf. Photo Credit: Larry Osborne, Bugwood.org.

Figure 2. Foliar symptoms of Goss’s wilt on a corn leaf. Photo Credit: Larry Osborne, Bugwood.org.

Northern Corn Leaf Blight (NCLB): NCLB is caused by a fungus called Exserohilum turcicum. The most diagnostic symptom of NCLB is the long, slender, cigar-shaped, gray-green to tan lesions that develop on leaves (Fig. 1).  Disease often begins on the lower leaves and works it way to the top leaves.  This disease is favored by cool, wet, rainy weather, which has seemed to dominate lately. Higher levels of disease might be expected in fields with a previous history of NCLB and/or fields that have been in continuous and no-till corn production. The pathogen over-winters in corn residue, therefore, the more residue on the soil surface the higher the risk for NCLB.  Management should focus on using resistant hybrids and residue management.  In-season management is available in the form of several fungicides that are labeled for NCLB. However, these fungicides should be applied at the early onset of the disease and only if the epidemic is expected to get worse. Often the best time to apply fungicides to field corn to maximize the benefits is near the VT/R1 growth stage. However, if NCLB is visible on leaves earlier than this time, a fungicide might be beneficial at those earlier stages. The only way to determine this is to scout frequently and keep an eye on the disease situation in your corn crop.

If you elect to control NCLB with fungicides, you might consider taking a look at my page on FUNGICIDE INFORMATION. This page talks about fungicide use in general and also includes the Corn Fungicide Efficacy Table. You will find products listed with good efficacy toward NCLB on this table.

Additional NCLB Information

Purdue University – https://www.extension.purdue.edu/extmedia/BP/BP-84-W.pdf

Iowa State University – http://www.extension.iastate.edu/CropNews/2014/0714Robertson.htm

Figure 3. "Freckles" on a corn leaf with Goss's wilt. Photo credit: Larry Osborne, Bugwood.org.

Figure 3. “Freckles” on a corn leaf with Goss’s wilt. Photo credit: Larry Osborne, Bugwood.org.

Goss’s Wilt: Goss’s wilt is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis. First visual symptoms usually appear as gray or yellow stripes on leaves that tend to follow the leaf veins (Fig. 2). Often “freckles”, or brown or green irregular spots, can be observed within the leaf lesions (Fig. 3). Freckles are an excellent diagnostic symptom to confirm Goss’s wilt. Vascular tissue, husks, and kernels can sometimes take on an orange hue. Occasionally, bacterial ooze or dried ooze can be observed on symptomatic leaves. Fungicides do not work for Goss’s wilt, because this is caused by a bacterium, not a fungus. Management is preventative for Goss’s wilt. Choose hybrids with the best possible resistance, manage excessive amounts of corn surface residue, and rotate crops. The longer the rotation between corn crops, the better. There are some foliar products being marketed for the control of Goss’s wilt, but no efficacy data are currently available.

Additional Goss’s Wilt Information

University of Nebraska – http://pdc.unl.edu/agriculturecrops/corn/gosswilt

Purdue University – https://www.extension.purdue.edu/extmedia/bp/BP-81-W.pdf

Corn Diagnostics Quick Guide: Many of you likely attended the 2014 Pest Management Update Series and obtained the corn diagnostics quick guide sheet to help differentiate between Goss’s wilt and NCLB. I have again attached it to this post for download as a PDF. This is a quick guide to help you differentiate the diseases in the field. Remember, the only way to definitively differentiate the diseases is to send a sample to the diagnostic clinic. Get out there and SCOUT, SCOUT, SCOUT!

 

Disease Considerations for Soybean and Corn Harvest

Damon L. Smith – Extension Field Crops Pathologist, University of Wisconsin

As the fall is approaching and crop harvest plans are being made, it is important to continue to assess disease issues in corn and soybean. These assessments aren’t being made in order to make plans for in-field management, but to potentially improve the quality of grain that is harvested.

Some Diseases to Consider in Corn at Harvest

Figure 1. Gibberella stalk rot on corn. Severe stalk rot on the left and less severe stalk rot on the right.

Figure 1. Gibberella stalk rot on corn. Severe stalk rot on the left and less severe stalk rot on the right.

Now is the best time to begin scouting corn for stalk rot issues and also fungal ear rot potential. Diseases such as Anthracnose stalk rot and Gibberella stalk rot are becoming apparent in corn.  Inspect the stalks integrity on the outside.  Be sure to squeeze the outside of the stalk to gauge the potential severity of the rot on the inside of the stalk.  Cut a few stalks from diverse areas of the field to see how rotted stalks might be. In figure 1, the stalk on the left has a severe case of Gibberella stalk rot, while the stalk on the right is far less rotted.  The more severely rotted stalks are, the more likely they will lodge.  Therefore timely harvest is important. Growers should target harvest on fields with severe stalk rot before fields that have less stalk rot, in order to minimize harvest losses due to lodging.

Figure 2. Diplodia ear rot.

Figure 2. Diplodia ear rot.

Ear rots can also be an issue at harvest time. Fusarium ear rot, Gibberella ear rot, and Diplodia ear rot (Fig. 2) are just a few that can damage corn in Wisconsin. Ear rots are becoming evident in some corn I have scouted in the last week or so.  It will be critical to check fields in the next several weeks in order to make decisions on what fields to harvest first.  Harvest priority should be placed on fields with a high level of ear rot.  As corn stands late into the fall, certain ear rot fungi can continue to grow, damage ears, and cause increases in mycotoxins in grain. The quicker these fields dry and can be harvested, the more likely the losses due to ear rot and mycotoxin accumulation can be minimized.

Soybean Disease Considerations at Harvest

Figure 3. Sclerotia of the white mold fungus inside a soybean stem.

Figure 3. Sclerotia of the white mold fungus inside a soybean stem.

In Wisconsin, the main disease to consider when making harvest plans in soybean is white mold. White mold is present in some soybean fields in the state and has caused considerable damage in a few of those fields. Remember that the white mold fungus not only causes stem blight and damage, but also causes the formation of sclerotia (fungal survival structures that look like rat droppings) on and in soybean stems (Fig. 3). These scelrotia serve as the primary source of fungal inoculum for the next soybean crop. They also get caught in combines during harvest. These sclerotia can then be spread in combines to other fields that might not be infested with the white mold fungus.  Therefore, it is important to harvest non-infested soybean fields first, followed by white mold-infested fields, to be sure the combine does not deposit any residual sclerotia in the non-infested fields.  If this is not an option and you must harvest white mold infested fields before non-infested fields, be sure to clean the combine between fields.

For more information about white mold management in soybean you can click here and scroll down to “white mold” or watch a video by clicking here.

Wisconsin Corn and Soybean Disease Update – August 21, 2014

Damon L. Smith – Extension Field Crops Pathologist, University of Wisconsin

Figure 1. IPM Pipe Southern Corn Rust Advisory for August 21, 2014.

Figure 1. IPM Pipe Southern Corn Rust Advisory for August 21, 2014.

I have spent the last several days rating and scouting corn and soybeans in the southern tier of Wisconsin. There are a few active diseases out there to keep track of.

Field Corn

In field corn we have found a few fields with low levels of northern corn leaf blight (NCLB). Levels of NCLB seem to be a bit higher in southwestern Wisconsin. Severity on lower leaves in field corn was in the 10 – 15% range, with no damage apparent on ear leaves. Around the Arlington, WI area, NCLB is very limited with only a few lesions evident every 100 ft. or so.

Eyespot is becoming more evident in field corn.  In fields with corn debris from a previous crop, the severity levels are in the 25-30% range on lower leaves and 10-15% on ear leaves.

Low levels of common rust (less than 5%) can also be found on some field corn hybrids in Southern Wisconsin.

Southern rust has been reported as far north as east-central Nebraska. The southern rust epidemic is being monitored closely in the Midwest. No southern rust has been found or reported in Wisconsin (Fig. 1).

For more information about corn diseases in Wisconsin, see my previous article by clicking here.

Sweet Corn

Several fields with severe epidemics of NCLB on sweet corn have been reported.  These were late-planted fields. Sweet corn is generally more susceptible to NCLB than field corn. Common rust and eyespot can also be found at varying levels on sweet corn in the central and southern portion of Wisconsin.

In research plots at the Arlington Agricultural Research Station, sweet corn planted on June 25th is beginning to tassel. Levels of NCLB are currently low in this field, but common rust is increasing rapidly. Some leaves have 20-25% severity. Any late-planted and/or susceptible varieties of sweet corn should be monitored closely for foliar disease and any decision to spray fungicide should be made by the tasseling/R1 growth stage.

Soybean

The most widespread disease on soybean that we have observed is Septoria brown spot.  Overall levels of Septoria brown spot are low, and can mostly only be found on lower leaves, which is typical for this disease. In many cases a fungicide specifically for this disease is not warranted in Wisconsin, unless there are factors that might lead to increased levels of severity, including continuous soybean rotation, very susceptible varieties, or extremely conducive weather. Most soybean fields are past the R3 growth stage , when a fungicide application might be beneficial for control of foliar diseases. However, this disease should be monitored in fields that were planted late.

Downy mildew has also been observed on soybean in various areas from central to southern Wisconsin. Fungicide application for control of this disease has not proven beneficial in university research trials. Therefore, fungicide application is not recommended for this disease under most circumstances. In soybean fields that are irrigated, the frequency between irrigation events should be lengthened in order to reduce the levels of downy mildew. Warmer, dry weather will also further reduce the level of downy mildew.

Figure 2. Damage from white mold in a soybean field under irrigation.

Figure 2. Damage from white mold in a soybean field under irrigation.

Active white mold has been found in fields in the central and southern portions of Wisconsin. Severity levels vary greatly depending on the fields and level of previous infestation by the white mold fungus. We have observed levels ranging from a few plants in spotty areas of a field to widespread damage with plant mortality across the entire field. The latter case was in a field with a history of white mold and frequent overhead irrigation (Figure 2). Application of fungicide for control of white mold is not recommended after the R3 growth stage. However, fields should be scouted and damage noted to facilitate future planting and management decision in that field. Fields with white mold should be harvested after fields that do not have white mold. The black survival structures (sclerotia; resemble rat droppings) of the white mold fungus can be easily spread on combines from one field to the next. If harvesting white mold infested fields last is not feasible, care should be taken to thoroughly clean combine mechanisms where soybean trash and debris can be trapped, between fields. For more information about white mold and management of the disease, click hereTo watch a short video about white mold you can click here.

Other diseases such as brown stem rot, sudden death syndrome, and stem canker have been found at extremely low levels in soybean fields in Wisconsin this season. This situation should be monitored closely as soybeans approach the R6 and R7 growth stages. These two diseases may become more apparent at that time. Again, good record keeping of where these diseases are found can facilitate future management decisions for those fields.

Goss’s Wilt Confirmed for First Time in 2014 in Wisconsin

Figure 1. Foliar symptoms of Goss's wilt on a corn leaf. Photo Credit: Larry Osborne, Bugwood.org.

Figure 1. Foliar symptoms of Goss’s wilt on a corn leaf. Photo Credit: Larry Osborne, Bugwood.org.

Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison

This week the University of Wisconsin Plant Disease Diagnostic Clinic confirmed Goss’s wilt in dent corn from Grant County in Wisconsin. Goss’s wilt has been confirmed in past years in Wisconsin, but this is the first confirmation of the disease for the 2014 season.

Symptoms and Signs

Goss’s wilt is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis. First visual symptoms usually appear as gray or yellow stripes on leaves that tend to follow the leaf veins (Fig. 1). Often “freckles”, or brown or green irregular spots, can be observed within the leaf lesions (Fig. 2). Freckles are an excellent diagnostic symptom to confirm Goss’s wilt. Vascular tissue (Fig. 3), husks, and kernels can sometimes take on an orange hue. Occasionally, bacterial ooze or dried ooze can be observed on symptomatic leaves.

Factors that Cause Disease Development

Figure 2. "Freckles" on a corn leaf with Goss's wilt. Photo credit: Larry Osborne, Bugwood.org.

Figure 2. “Freckles” on a corn leaf with Goss’s wilt. Photo credit: Larry Osborne, Bugwood.org.

The Goss’s wilt bacterium overwinters in old corn residue. The bacterium enters the plant through wounds or natural openings. Yield losses will depend on the susceptibility of the hybrid being grown. Factors that put corn fields at higher risk include:

  1. Reduced Tillage
  2. Continuous corn rotation
  3. Planting a susceptible hybrid
  4. Hail, wind, or severe weather events causing injury on corn plants
Figure 3. Orange vascular tissue of a corn plant with Goss's wilt. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org.

Figure 3. Orange vascular tissue of a corn plant with Goss’s wilt. Photo credit: Howard F. Schwartz, Colorado State University, Bugwood.org.

Management

There is currently no research-based method of in-season management of Goss’s wilt. There are some foliar products being marketed for the control of Goss’s wilt, but no efficacy data are currently available. Because this disease is caused by a bacterium, the application of fungicide will not control the disease. Planting resistant hybrids in fields with a history of Goss’ wilt is recommended.  Residue management and crop rotations should also be implemented in at-risk fields.

Additional Goss’s Wilt Information

University of Nebraska – http://pdc.unl.edu/agriculturecrops/corn/gosswilt

Purdue University – https://www.extension.purdue.edu/extmedia/bp/BP-81-W.pdf

Reference

L.E. Claflin. Goss’s Bacterial Wilt and Blight, in: Compendium of Corn Diseases, 3rd edition. Ed. D.G. White. APS Press.

Tasseling Corn – Scout Now for Foliar Diseases and What About Fungicide?

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin

I have been riding through much of the southern tier of Wisconsin this week and am noticing quite a few corn fields that are beginning to tassel. This growth stage presents itself as a good time to scout for foliar diseases of corn and make decisions on in-season management for any diseases you might find.

As for which diseases might be important this year? I wish I had a crystal ball. However, if I had to make an educated guess, three come to mind: Northern corn leaf blight, Eyespot, and Anthracnose leaf blight.

Figure 1. NCLB symptoms on a corn leaf.

Figure 1. NCLB symptoms on a corn leaf.

Northern Corn Leaf Blight (NCLB): The most diagnostic symptom of NCLB is the long, slender, cigar-shaped, gray-green to tan lesions that develop on leaves (Fig. 1).  Disease often begins on the lower leaves and works it way to the top leaves.  This disease is favored by cool, wet, rainy weather, which has seemed to dominate lately. Higher levels of disease might be expected in fields with a previous history of NCLB and/or fields that have been in continuous and no-till corn production. The pathogen over-winters in corn residue, therefore, the more residue on the soil surface the higher the risk for NCLB.  Management should focus on using resistant hybrids and residue management.  In-season management is available in the form of several fungicides that are labeled for NCLB. However, these fungicides should be applied at the early onset of the disease and only if the epidemic is expected to get worse.

Figure 2. Eyespot symptoms on a corn leaf.

Figure 2. Eyespot symptoms on a corn leaf.

Eyespot: Eyespot typically first develops as very small pen-tipped sized lesions that appear water-soaked.  As the lesions mature they become larger (¼ inch in diameter) become tan in the center and have a yellow halo (Fig. 2).  Lesions can be numerous and spread from the lower leaves to upper leaves. In severe cases, lesions may grow together and can cause defoliation and/or yield reduction. Eyespot is also favored by cool, wet, and frequently rainy conditions.  No-till and continuous corn production systems can also increase the risk for eyespot, as the pathogen is borne on corn residue on the soil surface.  Management should focus on the use of resistant hybrids and residue management.  In-season management is available in the form of fungicides.  Again, fungicides should be applied early in the epidemic and may not be cost effective for this disease alone.

Figure 3. Anthracnose leaf blight symptoms on a corn leaf.

Figure 3. Anthracnose leaf blight symptoms on a corn leaf.

Anthracnose leaf blight (ALB): ALB symptoms include oval or elongated lesions that are brown in color and surrounded by a yellow or orange area (Fig. 3). Sometimes on older lesions, small black hair-like structures (setae) can be observed erupting from the leaf surface in the center of the lesions. In severe cases, ALB can result in leaf death that can affect yield. Again, the ALB pathogen overwinters on corn residue. Therefore fields in no-till and/or continuous corn production might be at higher risk for ALB. Long periods of rainy overcast and warm weather can favor ALB. Fields with poor soil fertility can also be at higher risk for ALB development.  Management should focus on selecting resistant hybrids and residue management. Some fungicides are labeled for management of ALB, but control and yield increase in response to applications have been inconsistent.

Over the last several years there has been a lot of interest in applying foliar fungicides on corn to protect or increase yield.  There are many products on the market and we tested several of these at various timings in 2013 on hybrid grain corn.  The results of that trial can be found by clicking here and scrolling to page 2. In this study we had very low levels of common rust. Yield was highly variable in the trial and only one product/timing resulted in a yield increase over the non-treated plots. This high level of variability and inconsistency in treatment has also been observed in trials conducted throughout the corn belt of the U.S. over the last several years.

In a recent summary of foliar fungicide trials on corn from 2010-2013, 985 site/trials were conducted. No single product was identified to be more effective than another in these trials, however disease ratings were not the focus. When timing of fungicide application was analyzed, the best time to apply a fungicide and expect some yield increase over the non-treated control was between the VT and R2 growth stages.  The average yield increase across all trials and years at the VT to R2 timing was 3.5 bushels per acre.

Figure 4. Break-even scenarios for corn foliar fungicide application costs.

Figure 4. Break-even scenarios for corn foliar fungicide application costs.

While there seems to be an overall positive response in yield with the application of fungicide, that increase is likely not high enough to recover the cost of application.  A quick review of fungicide prices and expected application costs reveals that to apply fungicide one time might cost around $28 USD. Figure 4 shows a table of various costs to apply fungicide along the top, corn prices along the left column, and the bushel advantage required by the fungicide application to break-even with the cost of fungicide application in the center.  The red box in figure 4 shows our 3.5-bushel average advantage that we saw across the region-wide trial. The arrow shows the corn price needed to recover the cost of one $28 fungicide application. This $8.00/bushel corn price is more than twice today’s average corn price!

The previous point on economics was made in the absence of disease on corn, however.  When might we expect more consistent yield benefit from a fungicide? The answer is in the situations where disease levels are high of course! These situations include the following factors:

  1. Hybrids susceptible to foliar disease are used in fields with a history of disease
  2. Continuous corn production systems
  3. No-till or reduced tillage systems
  4. Late-planted corn
  5. Where irrigation is used
  6. Weather conditions are favorable for disease development

If one or more of these factors are important in your field, then scouting during the tasseling period will be important.  Gauge the present levels of disease and look at the weather forecast to see if the epidemic might increase. Then make a consideration on if a fungicide application is needed in your field. Consider the economics of that application and also the fact that repeated application of fungicide can also promote fungicide resistance in some of the pathogens you might be targeting.  So spray responsibly.

For more information about fungicides and fungicide mode of action visit my fungicide information page by clicking here.

New Publication on Corn Disease Loss Estimates in the U.S. and Canada

A new publication on corn disease loss estimates in the U.S. and Ontario, Canada in 2012 has been developed.  The publication is a product of a team effort composed of plant pathologists from across the corn production belt of the U.S. and Canada and describes the predominant disease of corn in 2012 and the estimated loss as a result.  CLICK HERE TO DOWNLOAD A PDF OF THIS NEW PUBLICATION.