Time to Start Looking for Corn Diseases in Wisconsin

/in , , , , /by
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

/in , , , , , , /by

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

/in , , , , , , , , , /by

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 here.  To 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

/in , , /by
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?

/in , , , , /by

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.