Wisconsin Soybean and Corn Disease Update – August 2, 2021

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

Brian Mueller, Assistant Field Researcher, Department of Plant Pathology, University of Wisconsin-Madison

Camila Primieri Nicolli, Post-Doctoral Research Associate, Department of Plant Pathology, University of Wisconsin-Madison

It has been a while since I posted a corn and soybean disease update for Wisconsin in 2021, and that is because it has been reasonably quiet in the disease world up to this point. However, recent scouting and incoming reports indicate that this could change a bit as we move into August. Let’s take a look at what has happened so far this season and what to keep an eye out for over the next month or so.

The Soybean Situation

Figure 1. Phytophthora Root and stem rot of soybean

The Phytophthora Issue in Soybeans. During July we saw, and received reports, of some fields with Phytophthora root and stem rot of soybean (Fig. 1). I have received a lot of questions on why this happened. Are we seeing changes in the races of the Phytophthora organism in Wisconsin? My short answer is probably not. However, we have found a new species of Phytophthora in some fields that can affect soybean. The new species, Phytophthora sansomeana, can be found in mixed infections with P. sojae. Thus, even if we deployed the proper Rps resistances genes in our varieties, this “new” organisms might be causing some of the damage we observed this year.

We also are seeing fewer available soybean varieties with Rps 1K form of resistance. This form of resistance should be effective on about 99% of the fields in Wisconsin. Instead, we see varieties deployed that have the Rps 1c, Rps 1a, or no Rps gene indicated. Rps 1C is effective on about 75% of the acres in Wisconsin, just to give you some perspective. Thus, I don’t think this is necessarily an issue where we have seen race shifts in P. sojae, but a combination of issues where perhaps we aren’t deploying the correct resistance genes and we might have a new species of Phytophthora adding to the mix. You can learn more about managing Phytophthora root and stem rot of soybean in Wisconsin by clicking here. You will note that seed treatments can also be used to manage Phytophthora root and stem rot. You can click here to learn more about fungicide seed treatments and fungicide resistance in this group of organisms.

What’s Up with White Mold? For most of the state of Wisconsin, we are through the critical bloom time for infection by the white mold fungus. It is now really too late to make a fungicide application that is economically viable. However, scouting fields through August can help you determine what worked, what didn’t, and to figure out your harvest order. Remember, a great way to move the white mold fungus around is by contaminated combines at harvest. Start harvesting fields with no or low white mold incidence and work your way to those fields that look worse. Also consider cleaning combines between fields to limit movement of the fungal survival structures (a.k.a apothecia – the things that look like rat turds!) from one field to the next.

Look out for SDS. Now is also a good time to be scouting for sudden death syndrome (SDS) in soybeans. I’m not sure we will have a bunch of SDS this year in Wisconsin, but we will see pockets for sure. Knowing where you see it and what you did in 2021 can help with making variety and seed treatment decisions for 2022 and beyond. Remember that we do have decent partial resistance to SDS in many commercial varieties. Start here and choose the most resistant variety that fits your environment. Then consider layering a seed treatment (either Saltro or ILeVO) for improved management of SDS. You can learn more about seed treatment performance by studying the Fungicide Efficacy for Control of Soybean Seedling Diseases chart. You can also learn more about the performance of ILeVO in multi-state research trials by reading this report.

The Corn Situation

Corn in Wisconsin has been reasonably free of disease up to this point this season. However, we have noted a few foliar diseases beginning to pop up. We have observed gray leaf spot (GLS) becoming easy to find in most fields, while northern corn leaf blight (NCLB) is starting to show up in a handful of fields we have visited. We are also paying close attention to the tar spot and southern rust situations, I’ll expand on these below.

How bad is tar spot? The Tarspotter app has been running at moderate to high risk of tar spot increase over the last couple of weeks in Wisconsin. Our scouting has confirmed that tar spot is present in at least 5 counties so far in Wisconsin (Fig. 2). All but Grant County show tar spot to be easy to find, but it is present in the lower canopy at low severity. In Grant County, we had to hunt a long time to find 2 spots in a research field on a known susceptible. These observations align with Tarspotter as it indicated just moderate risk in the southwest quadrant of Wisconsin, with high risk from south central to the north. If you plan on spraying a fungicide to manage tar spot, we recommend that this be done soon, prior to the R3/R4 growth stage. The goal here is to protect the leaves from the ear leaf up from continued increase by the fungus. If you would like to learn more about tar spot check out the new web book published by the Crop Protection Network.

Figure 2. County-level confirmations of tar spot in the U.S. as of August 2, 2021.

Continue to scout for tar spot and let us know what you are finding. We are now accepting good pictures of tar spot to confirm its presence in counties where we have observed it in years past, in Wisconsin. In counites that tar spot has never been confirmed, we would like to get a physical sample to verify (Fig 2). Feel free to reach out to me if you do find tar spot or any other disease of corn or soybean for that matter.

Figure 3. County-level confirmations of southern rust on corn in the U.S. as of August 2, 2021.

Has southern rust hit Wisconsin yet? The short answer is NO, not that we can find. We have scouted and asked several folks in our network, and nobody has observed and lab-confirmed southern rust in Wisconsin. However, figure 3 show county-level confirmations of southern rust of corn in the U.S. Based on this map, I would not be surprised if southern rust is confirmed in the next week or so in Wisconsin. Like tar spot, fungicides can be applied up to the R3/R4 growth stage with some benefits. Spraying after R4 will not yield economic returns. To learn more about managing southern rust of corn, check out the electronic fact sheet from the Crop Protection Network.

Keep an eye on the soybean and corn disease situation and scout, scout, scout. Let us know what you are finding!

Corn Southern Rust Update – August 19, 2017

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

Counties confirmed to have southern rust on corn – 8.18.17

Southern rust of corn has been confirmed in Wisconsin by the University of Wisconsin-Madison Plant Disease Diagnostics Clinic. The sample came in from Kenosha Co. on August 18, 2017. This find is not entirely surprising considering the rapid movement of the southern rust pathogen from the southern U.S. to the northern U.S. this season. The latest southern rust map can be found at http://ext.ipipe.org.

For more information on southern rust, please see my previous post on the subject. You also should visit the Crop Protection Network Southern Rust information page where you can download a brand new fact sheet about southern rust on corn and management of the disease. Note that late planted corn will be more vulnerable to yield loss from the disease. Corn that is still silking (R1 growth stage) to milk (R3 growth stage) is vulnerable to yield loss by southern rust. Corn that is at the R4 (dough) growth stage or later is not as vulnerable and will likely not respond to a fungicide application. Even if corn is at a vulnerable growth stage, remember that we also have to have conducive weather for the pathogen. Extremely dry weather is not conducive for disease. High humidity and temperatures in the 80s favor disease increase.

Continue to scout and growth-stage your corn. If you find evidence of what you think is southern rust, I would encourage you to send it to the Plant Disease Diagnostics Clinic for confirmation.

Midseason Corn Disease Update

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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.

 

Corn and Southern Rust

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

Figure 1. Corn Southern Rust Observations as of July 21, 2017 (Map from ext.ipipe.org)

If you are like me, you have been paying attention to reports from the southern U.S. indicating that southern rust of corn is making its way further north again this year. You can follow current southern rust updates on the iPiPE site. The latest reports place southern rust in central portions of Iowa and Illinois (Fig. 1), which means farmers in Wisconsin need to start paying attention to this issue. Scouting over the next several weeks is going to be critical for making in-season management decisions for this disease. Yield reductions in Wisconsin will be greatest if southern rust moves in prior to the “milk” (R3) growth stage in corn. Lets take a closer look at southern rust and its less damaging relative, common rust.

Figure 2. Southern rust pustules on a corn leaf. Photo credit: Department of Plant Pathology., North Carolina State University, Bugwood.org

Southern rust is caused by the fungus Puccinia polysora. Symptoms of southern rust are different from common rust in that they are typically smaller in size and are often a brighter orange color (Fig. 2). Pustules of southern rust also typically only develop on the upper surface and will be be more densely clustered. Favorable conditions for southern rust development include high humidity and temperatures around 80F. However, very little free moisture is need for infection to occur. Southern rust is typically a rare occurrence in Wisconsin. When it does occur, it is usually in the southern and south-western portions of the state, with epidemics initiating late in the season. With that said, southern rust did make it to southern Wisconsin in 2016. However, the arrival was well past R3 and yield reductions caused by southern rust were insignificant in Wisconsin. Spores of this fungus have to be blown up from tropical regions or from symptomatic fields in the southern U.S. The fungus can not overwinter in Wisconsin. While southern rust epidemics can be rare events in Wisconsin, the disease can be serious when it occurs. Therefore close monitoring of forecasts and scouting are needed to make timely in-season management decisions.

Management of Southern Rust

Traditionally resistance was used to manage southern rust. However, in 2008 a resistance-breaking race of the southern rust fungus was confirmed in Georgia. Thus most modern hybrids are considered susceptible to southern rust. Rotation and residue management have no effect on the occurrence of southern rust. The southern rust fungus has to have living corn tissue in order to survive and can not overwinter in Wisconsin. Fungicides are typically used to control southern rust in parts of the U.S. where this is a consistent problem. Efficacy ratings are available for fungicides against southern rust on the Corn Fungicide Efficacy Table. As I said previously, should southern rust make its way to Wisconsin prior to the “milk” (R3) growth stage in corn, it could cause yield reductions. Growers and consultants should scout carefully through the R3 growth stage and be sure to properly identify the type of rust observed. If you need assistance in identifying rust on corn, 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.

Figure 3. Brick-red Pustules of the common rust fungus on a corn leaf.

Common rust is caused by the fungus Puccinia sorghi and is extremely common in Wisconsin, but often results in little yield loss. Most field corn hybrids planted in Wisconsin are very resistant to the disease. Symptoms can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 3). Typically these pustules are sparsely clustered on the leaf. They can also appear on other parts of the plant including the husks and stalks. Management for common rust primarily focuses on using 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.

Other Useful Resources about Rusts on Corn

Purdue Extension Fact Sheet – Common and Southern Rusts of Corn

WisCONTEXT Article on Southern Rust

Ohio State University Article on How to Differentiate Common Rust from Southern Rust

Video by Dr. Tamra Jackson-Ziems of the University of Nebraska – Identifying Rust Diseases of Corn

References

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

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

Wisconsin Corn Southern Rust Update – August 26, 2016

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

Corn Southern Rust positive confirmation map, by state county

Corn Southern Rust positive confirmation map, by state county

Southern rust of corn has been confirmed for the first time this season by the UW Plant Disease Diagnostic Clinic (PDDC) in Wisconsin. The positive sample was submitted from Rock Co. along the Wisconsin/Illinois state line and confirmed on August 25, 2016. As previously predicted, southern rust did make it to Wisconsin this year, however, its arrival is late enough that it should have minimal impact on yield. We published an article on WisContext about some reasons why this occurrence was expected in 2016, you can click here to learn more.

Most corn in Wisconsin is at least well into the milk stage (R3) or dough (R4). Once corn reaches the milk stage (R3), risk of yield loss from this and other foliar pathogens begins to quickly decline. Thus spraying fungicide at this time of the season is not recommended. If you would like to learn more about telling the difference between the two types of rusts that occur on corn, or management of southern rust specifically, see my previous article by CLICKING HERE.

If you need assistance in identifying rust on corn, 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.

Wisconsin Corn Southern Rust Update – August 10, 2016

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Figure 1. Corn Southern Rust Map - August 10, 2016

Figure 1. Corn Southern Rust Map – August 10, 2016

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

Wisconsin corn continues to remain free of southern rust as of August 10, 2016. However, new confirmations have been reported in southern Indiana (Fig. 1). You should continue to scout the corn crop for southern rust. However, the window of risk is quickly passing for much of Wisconsin. Once corn reaches the milk stage (R3), risk of yield loss from this and other foliar pathogens begins to quickly decline. Thus, as long as corn remains free of southern rust for another week or so, we will be in good shape for the rest of the season. If you need assistance in identifying rust on corn, 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.

Wisconsin Corn Disease Update – July 27, 2016

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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!

Corn Rusts in Wisconsin

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

Figure 1. Brick-red Pustules of the common rust fungus on a corn leaf.

Figure 1. Brick-red Pustules of the common rust fungus on a corn leaf.

I have been receiving lots of questions over the last couple of weeks about rusts in corn. Folks are identifying pustules of common rust on field and silage corn in Wisconsin. Common rust is caused by the fungus Puccinia sorghi. Symptoms can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 1). 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. Thus, the weather conditions in Wisconsin over the last month have been somewhat conducive for this disease. However, very hot and dry weather can slow or stop disease development.

Management of Common Rust

Management for common rust primarily focuses on resistant hybrids. Most modern commercial hybrids have excellent resistance to common rust. 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. Fungicides with efficacy toward common rust can be found on the Corn Fungicide Efficacy Table. 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 2. Southern rust pustules on a corn leaf. Photo credit: Department of Plant Pathology., North Carolina State University, Bugwood.org

Figure 2. Southern rust pustules on a corn leaf. Photo credit: Department of Plant Pathology., North Carolina State University, Bugwood.org

A related rust that we need to pay close attention to this season is southern rust. Southern rust is caused by the fungus Puccinia polysora. Symptoms of southern rust are different from common rust in that they are typically smaller in size and are often a brighter orange color (Fig. 2). Pustules of southern rust also typically only develop on the upper surface and will be be more densely clustered. Favorable conditions for southern rust development are similar to those for common rust. high humidity and temperatures around 80F encourage disease development. However, very little free moisture is need for infection to occur. Southern rust is typically a rare occurrence in Wisconsin. When it does occur, it is usually in the southern and southern western portions of the state, with epidemics initiating late in the season. Spores of this fungus have to be blown up from tropical regions or from symptomatic fields in the southern U.S. The fungus can not overwinter in Wisconsin. While southern rust epidemics can be rare events in Wisconsin, the disease can be serious when it occurs. Therefore close monitoring of forecasts and scouting are needed to make timely in-seaosn management decision.

Figure 3. Corn Southern Rust Observations as of July 22, 2016

Figure 3. Corn Southern Rust Observations as of July 22, 2016

Currently the Corn Southern Rust iPIPE map is showing numerous confirmed cases of southern rust in the southern, southeastern U.S. and Kansas and Kentucky. No confirmed cases have been identified in Illinois, Iowa or Wisconsin. However, close attention should be paid to this disease in 2016 as the confirmed cases this year have been earlier than in the past. This could mean that conditions are ripe for movement of southern rust inoculum into Wisconsin.

Management of Southern Rust

Traditionally resistance was used to manage southern rust. However, in 2008 a resistance-breaking race of the southern rust fungus was confirmed in Georgia. Thus most modern hybrids are considered susceptible to southern rust. Rotation and residue management have no effect on the occurrence of southern rust. The southern rust fungus has to have living corn tissue in order to survive and can not overwinter in Wisconsin. Fungicides are typically used to control southern rust in parts of the U.S. where this is a consistent problem. Efficacy ratings are also available for fungicides against southern rust on the Corn Fungicide Efficacy Table. Should southern rust make its way to Wisconsin prior to the “milk” (R3) growth stage in corn, it could cause yield reductions. Growers and consultants should scout carefully through the R3 growth stage and be sure to properly identify the type of rust observed. If you need assistance in identifying rust on corn, 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.

Other Useful Resources about Rusts on Corn

Purdue Extension Fact Sheet – Common and Southern Rusts of Corn

2014 Crop Alert – University of Nebraska

Video by Dr. Tamra Jackson-Ziems of the University of Nebraska – Identifying Rust Diseases of Corn