Late Season Corn Foliar Disease Update and Hail-Damaged Corn

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

Scouting by my team and phone calls from extension personnel, consultants, and farmers have made it evident that there are several foliar diseases of corn showing up  in this first part of August. Gray leaf spot (GLS), northern corn leaf blight (NCLB), and tar spot have all been found in various locations over the last week or so. It is becoming very late in the season to try to control GLS or NCLB. Current data on tar spot indicate it likely doesn’t need to be controlled. Thus, there is likely not much to do at this point, but to document which fields have which diseases. This can help in fall scouting to make harvest decisions, as fields with higher levels of leaf disease may not have experienced any yield loss, but might have stalk integrity issues, which could lead to lodging. Determining which fields might be more prone to lodging can help establish harvest order to minimize any losses due to severely lodged plants. Below is more information about each foliar disease.

Gray leaf spot (GLS)

Figure 1. Gray leaf spot on a corn hybrid. Photo Courtesy of Craig Grau, University of Wisconsin-Madison

Gray leaf spot is cause by a fungus named Cercospora zeae-maydis. During times of very warm temperature and high humidity (greater than 90%), GLS can increase rapidly on susceptible hybrids. In fields with large amounts of corn residue (e.g. corn-on-corn rotation, minimal tillage, etc.) GLS may be more prominent due to higher levels of inoculum. Symptoms start as small narrow, blocky lesions that might be tan in the center and have a darker margin (Fig. 1). Lesion can increase in size and number and will typically move from lower leaves to upper leaves. Yield loss is most prominent when lesions reach the ear leaves either 2 weeks before tasseling or two weeks after tasseling. Currently, in Wisconsin, we have seen few fields where lesions have reached the ear leaves prior to brown silk. However, in a small number of fields planted to a susceptible hybrid, there has been rapid increase to the ear leaves prior to tassel. In those fields a fungicide application may result in adequate yield protection to cover the cost of fungicide application. See my previous article on how to make the decision to spray fungicide on corn.

Northern Corn Leaf Blight (NCLB)

Figure 2. Northern corn leaf blight on corn.

Northern corn leaf blight is caused by the fungus Setosphaeria turcica. The fungus is most active when wet weather coincides with temperatures between 65 F and 80 F. During these conditions, the fungus will readily make microscopic spores (called conidia) inside the symptomatic areas of leaves and those spores will get splashed onto more leaves. Therefore, the disease typically moves from the lower canopy, up the corn plant as the season progresses. When temperatures get above 80 F and it is dry, growth and spread of the fungus slows dramatically. This is why little NCLB was observed in July, but is showing up now. It is all about the temperature at which the fungus likes to grow. Lesions initiate as cigar-shaped lesions on lower leaves. When conditions are conducive lesions can expand and increase, moving rapidly up the plant (Fig. 2). Occasionally a gray-to-black fuzzy growth is evident in the center of lesions. This growth is sporulation of the fungus. Like GLS, yield loss is greatest when lesions reach the ear leaf either two weeks before or two weeks after tasseling. Again, consult my previous article on how to make the decision to spray fungicide on corn.

Tar Spot

Tar spot is a relatively new disease in the U.S. and Wisconsin. It is caused by a fungus called Phyllachora maydis. Tar spot causes small tar-like spots on the surface of corn leaves. For great information about tar spot and what it looks like, consult this Purdue Extension fact sheet. Tar spot was first found in the U.S. in 2015. In 2016 and 2017, tar spot was identified in Green, Iowa, Grant, and Lafayette counties in Wisconsin. In 2018 confirmations have been made in these same general areas. In Latin America Phyllachora maydis can be found in a complex with another fungus called Monographella maydis. In areas where the complex occurs significant yield loss has been described. However, in the U.S. Monographella maydis has not been found in complex with Phyllachora maydis. Furthermore, Phyllachora maydis is not known to cause yield loss on corn in the U.S. While it can be a striking disease, fungicide applications are not recommended for tar spot in the U.S. Much more work is needed to characterize this pathogen and understand the disease. We are working with Dr. Nathan Kleczewski at the University of Illinois to improve our understanding of this pathogen in the U.S. If you would like to confirm tar spot on corn, or provide samples for research purposes, you can send samples to the University of Wisconsin Plant Disease Diagnostic Clinic.

What about Spraying Fungicide After Hail Damage?

The best study on this subject was conducted by my colleagues at Iowa State University a couple years back. They found that for the most part application of fungicide after hail does not result in any benefits. Especially after the R2 growth stage. We also had an opportunity to look at a natural hail event in 2014 at Arlington. This happened around VT.  We were also unable to find a significant difference in treating with  a fungicide versus not treating after late season hail-damage. In addition, it isn’t likely that fungal infections will increase after hail. In fact in the Iowa State University study, they found a negative correlation between hail damage and fungal disease. Hail CAN increase Goss’s wilt risk. However, Goss’s wilt is caused by a bacterium. Thus, fungicide application does not work for this disease. For more information on Goss’s Wilt and how to manage it click here. In summary, given the current market prices and the fact that corn is generally through the silking period, fungicide application on hail-damaged corn is not needed.

Midseason Corn Disease Update

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

Northern Corn Leaf Blight symptoms on a corn leaf.

The Wisconsin Field Crops Pathology crew has scouted corn from the southern portion of Wisconsin, to as far north as Spooner. Overall, disease levels are low. We have run into northern corn leaf blight (NCLB) in fields in the southern and central portions of the state. In most cases incidence was in the 10% or less range, with severity in the 5-10% range on leaves below the ear leaf. We have also had several samples arrive in the diagnostic clinic and confirmed with NCLB. For more information on managing NCLB or other corn diseases in Wisconsin, see my previous post here.

Goss’s wilt has been confirmed in Grant Co. via the diagnostic clinic. Other samples have also been submitted that were suspected for Goss’s wilt. However, these turned out to be NCLB. For assistance in differentiating these two diseases, click here to view a PDF quick diagnostic guide.

Common rust remains super common. I have received several questions about spraying fungicide to control common rust. For field corn hybrids, no fungicide will be needed. In any specialty corn situations (inbreds for seed production, sweet corn, etc.) spraying for  common rust might need to be considered. Most field corn hybrids have excellent resistance to common rust and will yield well, despite finding some pustules on a corn plant.

Southern rust has not yet been found in Wisconsin. However, it has been reported very close to Wisconsin (http://ext.ipipe.org). You should continue to be diligent in scouting for this rust disease. Yield reductions can be substantial if the fungus moves in over the next several weeks. Fortunately, our weather systems have been moving into Wisconsin from Canada and Minnesota. This has likely slowed progress of the southern rust fungus from moving into Wisconsin. Click here to view a great new resource on southern rust by the Crop Protection Network.

 

In-Season Corn Disease Management Decisions – 2017

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

Tasseling has begun on field corn in the southern region of Wisconsin. With this, comes many questions about applying fungicide to control disease and preserve yield. What diseases are out there? What disease(s) should I focus on in-season? When should I spray? What should I spray? On top of these questions, we are also confronted with corn prices, which are less than ideal and create tight profit margins. So what should we consider for in-season disease management? Lets consider the diseases first, then the management decisions.

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.

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.

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

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. Incidentally, we did confirm our first NCLB lesions of the year in the diagnostic lab last week. So now is a good time to get out and scout!

Figure 3. Gray Leaf Spot lesions on a corn leaf.

Gray Leaf Spot (GLS): Gray leaf spot typically starts as small blocky or jagged, light tan spots. These can expand to become long, narrow, rectangular lesions (Fig. 3) that may have yellow or orange halos around them. Gray leaf spot is typically worse when temperatures are warm and humidity is frequently above 90%. Thus, in Wisconsin, this disease is generally more frequent in the southern and southeastern portion of the state. Conditions that favor GLS often do not favor NCLB. The GLS pathogen and NCLB pathogen have different temperature requirements. Yield loss from GLS will be the greatest if lesions develop on the ear-leaves right before and right after tasseling. Like NCLB, hybrids rated as susceptible will generally suffer greater yield reductions due to gray leaf spot. Management of GLS should focus on choosing hybrids with excellent resistance and managing corn residue. Corn residue allows the pathogen to overwinter.

Like NCLB, fungicides can also be used to manage gray leaf spot. However, these should be applied as preventative applications. Thus using the same rule of thumb to make a spray decision for GLS, as for NCLB, can help you make the decision to spray fungicide. As for fungicide choice and timing, I will also consider that further below.

Figure 4. Brick-red Pustules of the common rust fungus on a corn leaf.

Common Rust: Symptoms of common rust can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 4). Typically these pustules are sparsely clustered on the leaf. They can also appear on other parts of the plant including the husks and stalks. Conditions that favor the development of common rust are periods of high humidity and nighttime temperatures that remain around 70F with moderate daytime temperatures. This fungus needs very little free moisture for infection to occur. Very hot and dry weather can slow or stop disease development.

Common rust is a extremely common (pun intended) and often results in little yield loss in Wisconsin. Most field corn hybrids planted in Wisconsin are very resistant to the disease. Management for common rust primarily focuses on using these resistant hybrids. Remember resistance is not immunity, so some pustule development can be observed even on the most resistant hybrids. Some inbred corn lines and specialty corn can be highly susceptible to common rust. Under these circumstances a fungicide may be necessary to control common rust. Most of the hybrids I have scouted this season have some pustules, however incidence and severity is relatively low. Therefore, a fungicide application to control common rust isn’t needed for most of these hybrids in Wisconsin. Residue management or rotation is typically not needed for this disease as inoculum (spores) have to be blown up on weather systems from the southern U.S.

Figure 5. 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. 5).  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 Disease(s) Should I Focus on In-Season? Based on the information above, the greatest emphasis for Wisconsin should be placed on controlling NCLB and GLS. Most hybrids planted in Wisconsin will be resistant to eyespot and common rust.

What Should I Spray, and When Should I Spray for Corn Foliar Diseases In Wisconsin? Fungicide should be used to preserve yield and reduce disease level. 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 2017 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 GLS. 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 over not treating across a range of environments. But in our current market, will this average gain cover the fungicide application? Today’s corn future price for September has a bushel of corn at $3.76.

Let’s Take a Closer Look at Corn Fungicide Return on Investment (ROI): While most of the early work on fungicide use in corn has focused on Headline® Fungicide, much of the industry has transitioned to using multi-mode-of-action products. These would be products mostly containing strobilurin (QoI) and triazole (DMI) fungicides in the same jug. Products such as Headline AMP® or Quilt Xcel® would fall into this category. These combination products have also been fairly consistent in response in my fungicide trials. You can find summaries of these trial results here. If we consider using Quit Xcel® at 10.5 fl oz or Headline AMP® at 10.0 fl oz, the list pricing of the product alone ranges from $15/acre (Quit Xcel®) to $22/acre (Headline AMP®). If the fungicide will be flown on with an aircraft, that cost will likely add nearly $15/acre to the application. Thus, fungicide plus application would range from $30/acre to $37/acre. If we can sell corn at $3.76 per bushel then we would need to preserve 8 bu/acre to nearly 10 bu/a in yield over not treating to break even! In a recent analysis of corn yield data where DMI+QOI products were applied at the tasseling period across the entire corn belt, the average yield preservation over not treating was 7.20 bu/a. This average projection is short of the 8 bu/a minimum we would need in the scenario above. However, the probability of preserving yield in the 8-10 bu/a range in this range is estimated to be 25% – 50%. This means that if we apply Quit Xcel® at 10.5 fl oz or Headline AMP® at 10.0 fl oz aerially, we will only break even 25% – 50% of the time with corn priced at $3.76 per bushel. If we can sell our corn for a better price or make the applications cheaper, then the odds will improve, but probably not climb above 70% even under the best case scenario. We do know that in Wisconsin, the odds of breaking even do improve if NCLB or GLS are active and increasing during the tasseling period. Get out there and scout!

So What About Fungicide Application Timing? We can investigate this questionover the U.S. corn belt using the same dataset. Applications focused on an early (V6) timing, a VT-R2 timing, or a combination of V6 plus a VT-R2 application. Let’s again focus on the QoI+DMI products. Based on observations across the corn belt the V6 timing averaged almost 3 bu/a of preserved yield over not treating. The VT application resulted in nearly 8 bu/a in preserved yield, while the two-pass program only offered a little over 8 bu/a. Clearly the higher average yield preservation occurs using a single application of fungicide at the VT-R2 timing. Wisconsin data has been consistent with this observation. Thus it is recommended that a single application of fungicide be used around the VT-R2 growth stages, when NCLB or GLS are active and increasing on or near the ear leaves.

What About Silage Corn and Ear Rot? When it comes to ear rot control and reducing the accumulation of mycotoxins in grain or silage corn, fungicide application should be made when white silks are out. Spores of fungicide that generally cause mycotoxin issues in the grain portion of corn will infect the plant through silks. Thus, apply fungicides during silking or with 5 days after silking starts, can be beneficial. Note though that if the goal is to target mycotoxin production and reduce deoxynivalenol (DON) accumulation in the grain portion of the plant, DMI only products should be used. Like winter wheat, the application of QoI containing fungicides can increase DON accumulation in the grain portion of corn plants. Some work has been done using Proline® to control Fusarium ear rot. This DMI only product has shown promise in reducing ear rot and DON accumulation in the grain portion of the corn plant and has a label for suppressing Fusarium ear rot in Wisconsin.

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 2017. Here is what you should consider:

1) Corn hybrid disease resistance score for NCLB and GLS – 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 weather is conducive for NCLB or GLS, 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 needed.

4) Consider your costs to apply a fungicide and the price you can sell your corn grain – Will you preserve enough yield 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. See fact sheet A3878 below for more information.

Other Resources

Video: Disease Management in Low-Margin Years (fast forward to 10:00 for corn information)

Fact Sheet: A4137 – Grain Management Considerations in Low-Margin Years

Fact Sheet: A3878 – Fungicide Resistance Management in Corn, Soybeans, and Wheat in Wisconsin

References

Munkvold, G.P. and White, D.G., editors. 2016. Compendium of Corn Diseases, Fourth Edition. 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.

Wise, K., Mueller, D., Sisson, A., Smith, D., Bradley, and Robertson, A., editors. 2016. A Farmer’s Guide to Corn Diseases. APS Press.

Wisconsin Late-Season Corn Disease Update

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

Figure 1. Anthracnose stalk rot symptoms in a cut corn stalk.

Figure 1. Anthracnose stalk rot symptoms in a cut corn stalk.

NCLB and Anthracnose Stalk Rot

As corn silage harvest has begun and the corn grain crop is finishing, there have been some disease issues of note in Wisconsin. Northern corn leaf blight (NCLB) activity has picked up quickly over the last several weeks. This is due to the fact that the weather has become much cooler and has remained wet. These conditions are favorable for the fungus. You will remember that NCLB was observed very early this season. See my previous post on this topic by clicking here.The hotter and dryer weather we saw mid-season was not only good for corn growth, but it kept the NCLB pressure minimal during the critical time of silking and pollination. As stated in the fourth edition of the “Compendium of Corn Diseases” (Carson, 2016) direct yield losses from NCLB are typically minimal if infection is moderate or delayed until 6 weeks after silking. Therefore, the expected direct yield loss from NCLB in Wisconsin in 2016 is expected to be low, due to its late onset.

Figure 2. Corn field with considerable lodging due to anthracnose stalk rot.

Figure 2. Corn field with considerable lodging due to anthracnose stalk rot.

Certainly, there are other issues to consider with this late onset of NCLB. Dry-down will be accelerated. If you have a considerable epidemic in silage corn, then it would be advisable to try to chop as quickly as possible or consider taking the crop as high-moisture corn. Another issue to consider is the fact that a late-season NCLB epidemic can increase the risk for stalk rot issues. We have observed earlier than typical onset of anthracnose stalk rot this season (Fig. 1). Anthracnose stalk rot has been observed in many fields, with a range in severity dependent on the hybrid resistance and field history. Fields in a corn-on-corn rotation, and/or no-tilled, and planted to a susceptible hybrid are at high risk of severe symptoms. We have observed several fields with significant lodging and wind damage where anthracnose stalk rot has advanced quickly (Fig. 2). In other fields lodging has been minimal, but some anthracnose stalk rot can be found.

Management of anthracnose stalk rot is multi-faceted. First, choose hybrids with best resistance available. Hybrids that also have good resistance to foliar diseases will also offer an advantage when managing stalk rot, as foliar disease can stress corn plants and lead to increased risk of anthracnose stalk rot. Cultural practices such as crop rotation and tillage to manage surface residue can also help. Other practices that reduce plant stress such as balanced fertilization, proper planting population, providing suitable drainage, and using well adapted hybrids for your location will reduce the risk of anthracnose stalk rot. Finally, in fields were stalk rot is an issue, harvest as early as possible to avoid yield losses from lodging.

Fungicides are not recommended for managing anthracnose stalk rot. Attempts to use fungicides to manage anthracnose stalk rot often result in high variability and little translation to a yield advantage. In 2015 we conducted a corn fungicide trial where anthracnose stalk rot was detected at harvest. While higher levels of stalk rot were observed, and some treatments did lead to a significant reduction in stalk rot severity, no differences in lodging or yield were identified among the treatments. To view results of this 2015 trial, click here and scroll down to pages 2 and 3.

Bacterial leaf streak – A new disease of corn in the U.S.

Bacterial leaf streak (BLS) of corn has recently been reported for the first time on corn in the U.S. The first reports were in Nebraska with subsequent reports coming in from other states in the U.S. corn belt including Iowa, Illinois, Colorado, and Kansas. Efforts are underway in Wisconsin to monitor for the disease. As of this writing, BLS has not been found in Wisconsin. However, survey and scouting efforts are continuing, to monitor for this disease.

Bacterial leaf streak is caused by a bacterium named Xanthomonas vasicola pv. vasculorum. Very little is understood about this disease on corn, as it is so new. This pathogen presents no risk to humans or animals and there is little evidence to suggest that it will have an adverse effect on corn yield and quality. You can click here to read the USDA APHIS Statement on Xanthomonas vasicola pv. vasculorum. To learn more about the disease and to watch a video by Dr. Tamra Jackson-Ziems at the University of Nebraska CLICK HERE. Helpful information and hints on initially diagnosing BLS can be found HERE.

If you suspect that you have BLS in your corn crop in Wisconsin, leaf samples of corn plants can be sent in a sealed plastic bag with NO added moisture to the University of Wisconsin Plant Disease Diagnostic Clinic (PDDC). Information about the clinic and how to send samples can be found by CLICKING HERE.

References

Esker, P. 2016. Anthracnose stalk rot: in “Farmers Guide to Corn Diseases” Edited by: K. Wise, D. Mueller, A. Sisson, D. Smith, C. Bradley, and A. Robertson. APS PRESS.

M.L. Carson. 2016. Northern Corn Leaf Blight: in “Compendium of Corn Diseases, Fourth Edition.” Edited by: G.P. Munkvold and D.G. White. APS PRESS.

 

Wisconsin Corn Disease Update – July 27, 2016

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

Northern Corn Leaf Blight

Over the last week concerns have been increasing over corn diseases as we are at the critical time to make fungicide application decisions. See my previous post about the early onset of northern corn leaf blight (NCLB) in Wisconsin in 2016 by CLICKING HERE. While NCLB can be observed in many corn fields in the state, it can be difficult to find. The hot weather this year has managed to keep that disease in check. While now is a good time to scout and make spray decisions, remember that it would take 50% or more of plants in the field with 10% or more of the ear leaves covered with lesions of NCLB prior to the milk growth stage before significant yield loss will occur.

Goss’s Wilt

Just this week, the University of Wisconsin-Madison Plant Disease Diagnostic Clinic (PDDC) positively confirmed the first Goss’s wilt sample of 2016. This sample came from Grant Co. Other samples have arrived, but no definitive confirmation has been made in other counties in the state. For information on Goss’s Wilt you can visit my previous posting from 2015 by CLICKING HERE.

Rusts

Figure 1. 2016 Southern Rust Advancement in The U.S. as of July 27. Red highlights indicate counties where southern rust has been confirmed.

Figure 1. 2016 Southern Rust Advancement in The U.S. as of July 27. Red highlights indicate counties where southern rust has been confirmed.

Southern rust continues to be a disease to scout for in Wisconsin. No positive confirmations have been made in Iowa, Illinois, or Wisconsin. However, the disease has been confirmed in parts of Nebraska (Fig. 1). We have scouted several fields of dent corn and also sweet corn. Only pustules of common rust have been observed in these fields. Conditions have been suitable for this disease over the last several weeks. Remember that rust pathogens have to be blown in from the south. The inoculum of the fungi that cause these diseases do not overwinter in Wisconsin. To learn more about the two types of rust that can affect corn in Wisconsin and how to manage them, CLICK HERE to visit my post from last week.

Remember to get out there and SCOUT, SCOUT, SCOUT!

Wisconsin Northern Corn Leaf Blight Update – June 29, 2016

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

Since the first 2016 confirmation of northern corn leaf blight (NCLB) in Wisconsin on June 16, we have received additional corn samples from other areas of the state in my laboratory and also in the University of Wisconsin Plant Disease Diagnostic Clinic. All confirmations have been made in the laboratory, confirming the presence of the pathogen. Figure 1 shows counties, highlighted in red, where corn samples originated and were confirmed with NCLB.

While it is unusually early to find NCLB at this incidence level in Wisconsin, I continue to urge you to remain patient. All samples that we have examined have had low severity (very few and/or small lesions present on a single leaf). In addition, most of the damage has been confirmed on lower leaves which do not contribute as much to yield as the ear leaves eventually will. As I mentioned in my previous post on June 16, Our economic analyses indicate that the likelihood of positive return on investment from a fungicide will be higher when the application is made as close to the tasseling period as possible. Considering that the weather this week is very dry and severity of NCLB has been relatively low, I would encourage growers to wait until closer to tasseling before making the decision to apply fungicide. If weather over the next week or two begins to turn continually wet, then this decision should be re-evaluated at that time. To learn more about NCLB and return on investment when using fungicide CLICK HERE. To watch a video about corn diseases in Wisconsin and fungicide use in corn, CLICK HERE. Remember to continue to SCOUT, SCOUT, SCOUT!

Northern Corn Leaf Blight Positively Identified in Wisconsin in 2016

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

I was hoping that I would write this article later in the year. But it has happened relatively early for us. We have positively confirmed northern corn leaf blight (NCLB) on field corn at about the V7-V8 growth stage near Janesville, WI this week. This is a bit early for us, however, not entirely surprising given the levels of residual inoculum from the fungus that causes NCLB left from 2015 in many fields and the cool wet conditions we have had this spring. The situation has been similar to that in Iowa. Dr. Alison Robertson also reported the first find of NCLB in southeast Iowa this week. While this find is relatively early for Wisconsin, I don’t think the world is going to come to an end. Patience will be key over the next couple of weeks. I’ll explain why folks should be cautious in making management decisions below.

Figure 1. Northern Corn Leaf Blight symptoms on a corn leaf.

Figure 1. Northern Corn Leaf Blight symptoms on a corn leaf.

What is the Organism that Causes Northern Corn Leaf Blight (NCLB)? 

A fungus called Setosphaeria turcica (synonym: Exserohilum turcicum) causes NCLB (Fig. 1). The fungus loves it wet and cool. The fungus is most active when wet weather coincides with temperatures between 65 F and 80 F. During these conditions, the fungus will readily make microscopic spores (called conidia) inside the symptomatic areas of leaves and those spores (Fig. 2) will get splashed onto more leaves. Therefore, the disease typically moves form the lower canopy, up the corn plant as the season progresses. When temperatures get above 80 F and it is dry, growth and spread of the fungus slows dramatically. Remember the disease triangle?  It takes three things for a plant disease to occur – susceptible plants, fungal inoculum present near those susceptible plants, and favorable weather. Early this season, all three legs of the triangle were present. We have lots of residual inoculum left from 2015, we have lots of corn planted again in many fields that had corn last year, and we had cool rainy conditions early on this season. However, as we think about the disease triangle moving forward, and look at forecasts over the next 7-10 days, weather is not going to be conducive for the NCLB fungus. Temperatures are forecasts to be above 80 F and there isn’t much rain in site. Without the weather component of the triangle, fungal growth, spread, and subsequent disease development will be halted.

Figure 2. A photo-micrograph of spores produced by the NCLB fungus.

Figure 2. A photo-micrograph of spores produced by the NCLB fungus.

What should I do About Managing NCLB in 2016?

Farmers and consultants should start actively scouting corn fields in Wisconsin and keep track of disease and disease development. Remember, that while the disease is manifesting early, it is currently affecting leaves that will be in the lower canopy of the plant and are not responsible for a large portion of grain yield. 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 disease. 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

Figure 3: A computer simulation of 5% NCLB severity on a corn leaf.

Figure 3: A computer simulation of 5% NCLB severity on a corn leaf.

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 3 is a computer generated image that shows 5% of the corn leaf area 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.

When Should I Spray and What Should I Spray?

While it might seem tempting to spray fungicide now (between the V6-V8 growth Stages) because of NCLB, remember that the disease will likely slow due to the hot dry weather pattern we are about to encounter. I would encourage you to be patient and save your fungicide spray until as close to tassel (VT growth Stage) as you can. 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 (two fungicide applications). Over a 6 year period and well over 1,500 observations, the average yield gain when using fungicide at V5-V8 alone was 1.9 bu/acre, while that at the VT-R2 timing was 4.7 bu/acre, and 5.6 bu/acre for the two-pass program. Now consider the price of corn and the fungicide cost. Figure 4 is a breakeven table for fungicide cost compared to corn price. You will see that I have called out a couple of possibilities depending on the price of your fungicide. If we consider the price of corn grain to be somewhere between $3 and $4 and the cost of a fungicide to be in the $10 to $14 range, you can see from the table that you would need 2.5 to 4.7 bu/acre of additional corn grain in the treated fields, over not treating, to cover the cost of fungicide. Considering these numbers, and the nationwide average bushel return when using fungicide at various corn growth stages, you can see that the VT application timing for fungicide seems to make the most sense.

So what about fungicide application on corn in Wisconsin? We have compiled a 3-year dataset where we have looked at return on investment when using fungicide in fields where little disease was active (< 5% NCLB ear-leaf severity) or where diseases active (>5% NCLB ear-leaf severity). When NCLB was active, we found that there was a positive yield response when using fungicide about 74% of the time with an average yield gain of 5.4 bushels per acre (Figure 5). When disease activity was low, that positive yield response dropped to just 32% with little gain over zero bushels (Figure 5).

How about the return on investment in the current corn market? If we consider the current corn pricing and a fungicide cost of $10 to $14, Figure 6 shows that in Wisconsin a positive return on investment (ROI) occurs about 50-65% of the time when disease is active on ear-leaves (Figure 6). When disease is not active, the odds of positive ROI drop to just 12% – 20%. For a full discussion and explanation, I would encourage you to watch this video about corn disease and fungicide applications in Wisconsin.

Also 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.

For information on fungicide efficacy for NCLB you can consult fungicide efficacy trial data in Wisconsin BY CLICKING HERE. You can also consult the National Corn Disease Working Group fungicide efficacy table that was recently updated. The fungicide efficacy table can be found BY CLICKING HERE.

Summary

While it is earlier than normal to see NCLB in Wisconsin, I would encourage people to be patient in managing this disease with fungicide. Remember that conditions are going to be hot and dry over the next 1 – 2 weeks, which will dramatically slow the progress of NCLB. Also, 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 closer to VT. With the price of corn grain comparatively lower than in years past, one application of fungicide is about all that Wisconsin farmers can afford, therefore I would maximize that application and apply it as close to the VT growth stage as you can. As you approach that growth stage, continue scouting and consider if the disease is active. If it isn’t active and the weather pattern continues to be hot and dry, a fungicide application may not be needed at all.

Wisconsin Corn Disease Management Update Video

Corn Disease Management UpdateDamon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Now that the corn crop has been planted, you are probably thinking about disease and pest management once the crop emerges. After the 2015 field season, disease management is on everyones mind. To complicate this issue, corn prices are low. Is a fungicide application going to be worthwhile in this market? Watch this video to learn about potential corn disease issues in Wisconsin and what we have learned about using fungicide on field corn in the state.

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!