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!

Soybean Disease Considerations As the 2019 Harvest Approaches

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Damon L. Smith, Associate Professor and Extension Field Crops Pathology Specialist, University of Wisconsin-Madison

Shawn P. Conley, Professor and Extension Soybean and Small Grains Agronomy Specialist, University of Wisconsin-Madison 

As the days get shorter and the temperatures start to cool, soybean harvest is on everyone’s minds. As the crop is maturing and beginning the early stages of drydown, calls about diseases are starting to come in. In the southern third of the state most of the calls have centered on sudden death syndrome or SDS. To the north, most questions pertain to Sclerotinia stem rot or white mold. Below we will discuss SDS in some detail and provide a brief update on the white mold situation as well as elaborate on seed decay issues that we should pay attention to as we begin harvest.

Scout for Sudden Death Syndrome (SDS)

Figure 1. Symptoms of sudden death syndrome on soybeans

The first noticeable symptoms of SDS (Fig. 1) are chlorotic (i.e., yellow) blotches that form between the veins of soybean leaflets. These blotches expand into large, irregular, chlorotic patches (also between the veins), and this chlorotic tissue later dies and turns brown. Soon thereafter entire leaflets will die and shrivel. In severe cases, leaflets will drop off leaving the petioles attached. Taproots and below-ground portions of the stems of plants suffering from SDS, when split open, will exhibit a slightly tan to light brown discoloration of the vascular (i.e., water- conducting) tissue. The pith will remain white or cream-colored. In plants with advanced foliar symptoms of SDS, small, light blue patches will form on taproots and stems below the soil line. These patches are spore masses of the fungus that causes the disease.

Figure 2. Symptoms of BSR in soybean stems compared with a healthy soybean stem in the center.

Foliar symptoms of SDS can be confused with those of brown stem rot. However, in the case of brown stem rot (BSR), the pith of affected soybean plants will be brown (Fig. 2). In addition, roots and lower stems of plants suffering from BSR will not have light blue spore masses.

Once symptoms of SDS are evident, yield losses are inevitable. Yield losses can range from slight to 100%, depending on the soybean variety being grown, the plant growth stage at the time of infection and whether or not SCN is present in a field. If SDS occurs after reproductive stages (R5 or R6) impact on yield is usually less compared to the development of SDS at flowering, which can lead to substantial yield losses. When SCN is present, the combined damage from both diseases can be substantially more than the sum of the damage expected from the individual diseases.

SDS is caused by the soilborne fungus, Fusarium virguliforme (synonym: F. solani f. sp. glycines). F. virguliforme can overwinter freely in the soil, in crop residue, and in the cysts of SCN. The fungus infects soybean roots (by some reports as early as one week after crop emergence), and is generally restricted to roots as well as stems near the soil line. F. virguliforme does not invade leaves, flowers, pods or seeds, but does produce toxins in the roots that move to the leaves, causing SDS’s characteristic foliar symptoms.

SDS cannot be controlled once plants have been infected. Foliar fungicides have NO effect on the disease. Recently a new seed treatment has been identified that has efficacy against SDS. The active ingredient fluopyram can be found in the seed treatment iLeVo and is rated “very good” in multi-state trials. Other methods of control include using SDS-resistant varieties whenever possible in fields with a history of the disease; however, keep in mind that SDS-resistant varieties with maturity groups suitable for Wisconsin and other northern regions (groups I and II) can be limited. If SDS and SCN are both problems in the same field, planting an SCN-resistant soybean variety may also be beneficial in managing SDS. Do not delay planting soybeans to avoid symptoms of SDS.  In Wisconsin, it has been demonstrated that the benefits to yield when planting early outweigh the benefits of reduced SDS symptoms if planting is delayed. Improve soil drainage by using tillage practices that reduce compaction problems. Rotation, while useful in managing other soybean diseases, does not appear to significantly reduce the severity of SDS. Even after several years of continuous production of corn, F. virguliforme populations typically are not reduced substantially. Research from Iowa State University has shown that corn (especially corn kernels) can harbor the SDS pathogen.

For more information CLICK HERE to download a full color fact sheet on SDS. A short video on SDS can also be viewed by CLICKING HERE.

Know Where White Mold is in 2019

Figure 3. White Mold in a Soybean Field

Symptoms of white mold (Fig. 3) are becoming pretty apparent in some parts of central and northern Wisconsin. White fluffy growth (mycelium) is readily evident over the last week while weather has been humid and wet. Incidence in the northern half of the state is high in some fields. Fields in the northeast and northwest corridors of the state seem to be hardest hit, but severity is highly variable from one field to the next.  This is likely due to a combination of variety and micro-environments that can influence the disease. Most of the soybean crop is at the R6 growth stage, with some earlier maturing fields are almost through R7 or have made it to R8. Now the question is how much soybean yield might I lose from white mold?

Research has demonstrated that for every 10% increase in the number of plants that are infected with white mold at the R7 growth stage, you can expect between 2 to 5 bushels of yield loss. Thus, fields with low levels (say 3% incidence) will likely experience no detectable yield loss while fields with 20% incidence could lose as much as 10 bushels per acre.

What should I do if I see white mold in my soybean field now?

The first step is to get out and survey your fields for white mold. It is a good idea to determine how much white mold you have in your fields, so you can make some educated harvest decisions. One way to move white mold from one field to the next is via combines. You could clean your combine between each field, but this can be time consuming. So by determining which fields have no white mold and which fields have the most white mold, you can develop a logical harvest order by beginning your harvest on fields with no white mold and working your way to the heavily infested fields. This will help reduce spread of the white mold fungus to fields that aren’t infested. You can also make some decisions on your rotation plan and future soybean variety choices based on these late season observations.

If you would like to learn more about white mold and management of this disease, CLICK HERE to download a fact sheet from the crop protection network. You can also watch a short video about white mold by CLICKING HERE.

Watch for Phomopsis Seed Decay at Harvest

Figure 4. Severe Phomopsis seed decay. Photo Credit: Craig Grau, University of Wisconsin-Madison.

If you remember the 2018 harvest season, you are probably having nightmares right now. Extended rainy periods in October significantly delayed harvest and subjected the standing soybean crop to significant seed decay issues. Watch out for rainy periods during the 2019 harvest. If we end up in these wet patterns again in 2019, we could have a repeat of poor seed quality going into 2020. 

What does Phomopsis seed decay look like?

The fungus that causes Phomopsis seed decay (Fig. 4) can infect soybean plants early in the season and colonize pods and infect seeds near, or at maturity. Infected seed will often be shriveled or undersized and can have a white or chalky appearance. If pods are opened in the field a white cottony “mold” (different than that of white mold) can be observed. Infected seed can pass the Phomopsis seed decay fungus on in seedlings of the next soybean crop. Therefore, it is important to identify Phomopsis seed decay especially in soybean-seed fields.

What conditions are favorable for Phomopsis seed decay?

Warm and wet weather during pod fill and maturity favor the development of Phompsis seed decay. The conditions have been prevalent in areas of Wisconsin in 2019, especially where planting was delayed. Soybean varieties that mature early are also more prone to Phompsis seed decay. Other stresses such as nutrient deficiencies or virus infections can also increase the occurrence of Phompsis seed decay. Infested seed is a likely source of Phompsis seed decay, however, the fungus can survive on soybean debris and certain weeds like velvetleaf.

How should I handle soybeans with Phomopsis seed decay?

Scout fields before harvest to get an idea of how much Phomopsis seed decay you might have in a field. Scout multiple plants in at least 5 locations in a field, opening pods to determine if Phomopsis seed decay is present. In fields where Phomopsis seed decay is observed, harvest should be prioritized as soon as combines can enter the field. Seed infected with the Phomopsis seed decay fungus will continue to rot in the pod until they are harvested.

How should I manage Phomopsis seed decay in the 2020 soybean crop?

Soybean seed producers should try to clean seed to achieve less than 20% damaged seed in a seed lot. Multiple cleaning steps might be needed to achieve this level. While testing germination now is recommended, remember that testing germination again next spring and potentially just prior to delivery will also help you to understand the germination rate and determine if other management strategies need to be employed such as fungicidal seed treatments.

Seed treatments can help improve the germination rate of seed damaged by Diaporthe. However, you will need more than metalaxyl or mefonoxam active ingredients in your seed treatment. Metalaxyl and mefonoxam are good against Phytophthora and Pythium, but not effective against other organisms, like Diaporthe. Seed treatments with Phomopsis on the label have an additional fungicide (either a DMI or SDHI). Table 3-8 of the publication A3646 – Pest Management in Wisconsin Field Crops has a table of some of the seed treatments with Phomopsis on the label. Also available, as stated above, is the seed treatment efficacy table from the Crop Protection Network (CPN). You can download that publication by clicking here.

As farmers begin to look forward to the 2020 growing season we also recommend that you double check the percent germination on every seed lot prior to planting and adjust your seeding rates accordingly. Here are our recommendations for soybean seeding rate based on yield potential and white mold risk: The Soybean Seeding Rate Conundrum.

An Additional Phomopsis Seed Decay Resource

A fact sheet about Pod and Stem blight and Phomopsis seed decay has been developed by a consortium of soybean extension pathologists. You can download that fact sheet by clicking here.

Wisconsin Late-Season Soybean Disease Update

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

The calls have been coming in this past week on a couple of soybean diseases. In the southern third of the state most of the calls have centered on sudden death syndrome or SDS. To the north, most questions pertain to Sclerotinia stem rot or white mold. I’ll discuss SDS in some detail, plus provide a detailed description of brown stem rot (BSR) which also typically shows up this time of year. Finally I’ll provide a brief update on the white mold situation.

Sudden death syndrome (SDS)

Symptoms of sudden death syndrome on soybeans

The first noticeable symptoms of SDS are chlorotic (i.e., yellow) blotches that form between the veins of soybean leaflets. These blotches expand into large, irregular, chlorotic patches (also between the veins), and this chlorotic tissue later dies and turns brown. Soon thereafter entire leaflets will die and shrivel. In severe cases, leaflets will drop off leaving the petioles attached. Taproots and below-ground portions of the stems of plants suffering from SDS, when split open, will exhibit a slightly tan to light brown discoloration of the vascular (i.e., water- conducting) tissue. The pith will remain white or cream-colored. In plants with advanced foliar symptoms of SDS, small, light blue patches will form on taproots and stems below the soil line. These patches are spore masses of the fungus that causes the disease.

Foliar symptoms of SDS can be confused with those of brown stem rot. However, in the case of brown stem rot (BSR), the pith of affected soybean plants will be brown. In addition, roots and lower stems of plants suffering from BSR will not have light blue spore masses.

Once symptoms of SDS are evident, yield losses are inevitable. Yield losses can range from slight to 100%, depending on the soybean variety being grown, the plant growth stage at the time of infection and whether or not SCN is present in a field. If SDS occurs after reproductive stages R5 or R6, impact on yield is usually less compared to the development of SDS at flowering that can lead to substantial yield losses. When SCN is present, the combined damage from both diseases can be substantially more than the sum of the damage expected from the individual diseases.

SDS is caused by the soilborne fungus, Fusarium virguliforme (synonym: F. solani f. sp. glycines). F. virguliforme can overwinter freely in the soil, in crop residue, and in the cysts of SCN. The fungus infects soybean roots (by some reports as early as one week after crop emergence), and is generally restricted to roots as well as stems near the soil line. F. virguliforme does not invade leaves, flowers, pods or seeds, but does produce toxins in the roots that move to the leaves, causing SDS’s characteristic foliar symptoms.

SDS cannot be controlled once plants have been infected. Foliar fungicides have NO effect on the disease.Recently a new seed treatment has been identified that has efficacy against SDS. The active ingredient fluopyram can be found in the seed treatment iLeVo and is rated “very good” in multi-state trials. Other methods of control include using SDS-resistant varieties whenever possible in fields with a history of the disease; however, keep in mind that SDS-resistant varieties with maturity groups suitable for Wisconsin and other northern regions (groups I and II) can be limited. If SDS and SCN are both problems in the same field, planting an SCN-resistant soybean variety may also be beneficial in managing SDS. Do not delay planting soybeans to avoid symptoms of SDS.  In Wisconsin, it has been demonstrated that the benefits to yield when planting early outweigh the benefits of reduced SDS symptoms if planting is delayed. Improve soil drainage by using tillage practices that reduce compaction problems. Rotation, while useful in managing other soybean diseases, does not appear to significantly reduce the severity of SDS. Even after several years of continuous production of corn, F. virguliforme populations typically are not reduced substantially. Research from Iowa State University has shown that corn (especially corn kernels) can harbor the SDS pathogen.

For more information CLICK HERE to download a full color fact sheet on SDS. A short video on SDS can also be viewed by CLICKING HERE.

Brown stem rot (BSR)

Symptoms of BSR in soybean stems compared with a healthy soybean stem in the center.

Symptoms of BSR are usually not evident until late in the growing season and may be confused with signs of crop maturity or the effect of dry soils. The most characteristic symptom of BSR is the brown discoloration of the pith especially at and between nodes near the soil line. This symptom is best scouted for at full pod stage. Foliar symptoms, although not always present, typically occur after air temperatures have been at to below normal during growth stages R3-R4, and often first appear at stage R5, peaking at stage R7. Foliar symptoms include interveinal chlorosis and necrosis (i.e., yellowing and browning of tissue between leaf veins), followed by leaf wilting and curling. Yield loss as a result of BSR is generally greatest when foliar symptoms develop. The severity of BSR symptoms increases when soil moisture is near field capacity (i.e., when conditions are optimal for crop development).

Foliar symptoms of BSR can be confused with those of sudden death syndrome (see description below). However, in the case of sudden death syndrome (SDS), the pith of affected soybean plants will remain white or cream-colored. In addition, roots and lower stems of plants suffering from SDS (but not those suffering from BSR) often have light blue patches indicative of spore masses of the fungus that causes SDS.

BSR is caused by the soilborne fungus Cadophora gregata. There are two distinct types (or genotypes) of the fungus, denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregata Type A also is associated with higher yield loss. P. gregata survives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregata survives longer when soybean residue is left on the soil surface (e.g., in no till settings) where the rate of residue decay is slow. P. gregata infects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients.

Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, C. gregata and soybean cyst nematode (Heterodera glycines) frequently occur in fields together, and there is evidence that BSR is more severe in the presence of this nematode.

BSR cannot be controlled once plants have been infected. Foliar fungicides and fungicide seed treatments have NO effect on the disease. Use crop rotations of two to three years away from soybean with a non-host crop (e.g., small grains, corn, or vegetable crops), as well as tillage methods that incorporate plant residue into the soil. Both of these techniques will help reduce pathogen populations by promoting decomposition of soybean residue. Also, make sure that soil fertility and pH are optimized for soybean production to avoid overly low phosphorus and potassium levels, as well as overly low soil pH. Finally, grow soybean varieties with resistance to BSR. Complete resistance to BSR is not available in commercial varieties. However several sources of partial resistance that provide moderate to excellent BSR control are available. Also, some, but not all, varieties of soybean cyst nematode (SCN) resistant soybeans also are resistant to BSR. Most soybean varieties with SCN resistance derived from PI 88788 express resistance to BSR. However, the same is not true of varieties with SCN resistance derived from Peking. Therefore growers should consult seed company representatives about BSR resistance when selecting a variety with SCN resistance derived from this source. You can download a full color fact sheet on BSR by clicking here. You can also CLICK HERE to view a short video on BSR.

White Mold

White, fluffy growth of the white mold fungus on a soybean stem

Symptoms of white mold are becoming pretty apparent in Wisconsin. White fluffy growth (mycelium) is readily evident. Incidence in the northern half of the state is higher. We have visited fields as far north as Wausau and Pulaski, Wisconsin and have observed incidence ranging from 0% to 30% of plants infected. Reports from areas in the northwest indicate white mold present, but not as high of incidence levels. As we move to the southern portion of Wisconsin, white mold can be found, but at reasonably low levels. Most of the soybean crop is at the R5 growth stage, with some earlier maturing fields approaching R6. Questions have arisen about spraying fungicide now to reduce the damage caused by white mold and preserve yield. The short answer is NO. The reason is that the primary means of infection by the white mold fungus is through soybean flowers. These infections happened weeks ago. Therefore, the optimal time to spray would be when flowers were out. A low level of plant-to-plant transmission can occur late in the season in soybeans. However, this rate is low enough, that spraying to prevent it does not produce favorable results.

How much soybean yield might I lose from white mold?

Research has demonstrated that for every 10% increase in the number of plants that are infected with white mold at the R7 growth stage, you can expect between 2 to 5 bushels of yield loss. Thus, the fields I mentioned earlier will likely range from little detectable yield loss (3% incidence) to as high as 10 bushels lost (20% incidence).

What should I do if I see white mold in my soybean field now?

Get out and survey your fields for white mold. It is a good idea to determine how much white mold you have in your fields, so you can make some educated harvest decisions. One way to move white mold from one field to the next is via combines. You could clean your combine between each field, but this can be time consuming. So by determining which fields have no white mold and which fields have the most white mold, you can develop a logical harvest order by beginning your harvest on fields with no white mold and working your way to the heavily infested fields. This will help reduce spread of the white mold fungus to fields that aren’t infested. You can also make some decisions on your rotation plan and future soybean variety choices based on these late season observations.

If you would like to learn more about white mold and management of this disease, CLICK HERE to download a fact sheet from the crop protection network. You can also watch a short video about white mold by CLICKING HERE.

Mid-Season Soybean Issues in Wisconsin

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

The 2015 season has again been an interesting adventure. We managed to plant soybeans early or on-time in much of the state and then the rains started and have kept things fairly wet. Combined with cooler temperatures, this moisture has brought about the risk for several disease on soybean in Wisconsin. Of course the most notable disease is WHITE MOLD, but I already have written about this disease and discussed management strategies. If you would like to revisit this topic, CLICK HERE. You can also watch a short video on the subject HERE. Other diseases to watch out for during the mid-season include Septoria brown spot, brown stem rot (BSR), sudden death syndrome (SDS), stem canker and pod and stem blight, and soybean vein necrosis disease (SVND). I will consider each one of these diseases below.

Leaf spots caused by the fungus Septoria glycines. Photo credit: Brian Hudelson, UW Plant Disease Clinic

Leaf spots caused by the fungus Septoria glycines. Photo credit: Brian Hudelson, UW Plant Disease Clinic

Septoria brown spot

Septoria brown spot is a common disease occurring on soybean each year. The spores of this fungus are typically rain splashed from old soybean debris, to the growing plants.  Septoria brown spot is usually not considered a yield limiting disease, but in certain cases, it has been attributed to significant yield loss.  This is usually the case where a susceptible variety is grown in a location conducive to the disease and rain is frequent and heavy.  In a situation like this, fungicides might be required during the reproductive phase of growth to preserve yield. However, most of the time, Septoria brown spot is observed early in the season and again late in the season during periods of heavy rainfall and does not affect yield.

Symptoms include dark brown spots on both upper and lower leaf surfaces.  Adjacent lesions frequently merge to form irregularly shaped blotches.  Leaves become rusty brown. Symptoms of Septoria leaf spot can also develop on stems and pods of plants approaching maturity.  Stem and pod lesions have indefinite margins, are dark in appearance and range in size from flecks to lesions several inches in length, but they are not distinct enough to be diagnostic.  Seed are infected but symptoms are not conspicuous.

The onset of Septoria leaf spot symptoms is influenced by the relative maturity of the soybean variety, and symptoms appear earlier in the season on an early-maturing variety. Complete resistance has not been identified in soybean varieties or lines, but varieties do differ in partial or rate-reducing resistance which can be used effectively. Crop rotation is an effective preventative strategy. Septoria leaf spot is more severe in continuously cropped soybean fields. The host range includes most species of Glycine, other legume species, and common weeds such as velvetleaf. For fields with very high levels of Septoria leaf spot, plow under soybean straw to promote rapid decay. Application of fungicides to soybean foliage from bloom to pod fill has effectively reduced the severity of Septoria leaf spot in research trials over the last several seasons. However, yield has not been effected by these applications. Therefore, the odds of a positive return on investment when using a foliar fungicide on soybean to control Septoria brown spot are low. You can check out the results of foliar fungicide trials on soybean in Wisconsin by CLICKING HERE. You can also view the soybean fungicide efficacy table HERE.

brown stem rot

Browning of the internal stem (left) is diagnostic for BSR. The middle stem may be developing symptoms. Compare to healthy, white pith in the stem on the right.

Brown Stem Rot (BSR) 

Symptoms of BSR are usually not evident until late in the growing season and may be confused with signs of crop maturity or the effect of dry soils. The most characteristic symptom of BSR is the brown discoloration of the pith especially at and between nodes near the soil line. This symptom is best scouted for at full pod stage. Foliar symptoms, although not always present, typically occur after air temperatures have been at to below normal during growth stages R3-R4, and often first appear at stage R5, peaking at stage R7. Foliar symptoms include interveinal chlorosis and necrosis (i.e., yellowing and browning of tissue between leaf veins), followed by leaf wilting and curling. Yield loss as a result of BSR is generally greatest when foliar symptoms develop. The severity of BSR symptoms increases when soil moisture is near field capacity (i.e., when conditions are optimal for crop development).

Foliar symptoms of BSR can be confused with those of sudden death syndrome (see description below). However, in the case of sudden death syndrome (SDS), the pith of affected soybean plants will remain white or cream-colored. In addition, roots and lower stems of plants suffering from SDS (but not those suffering from BSR) often have light blue patches indicative of spore masses of the fungus that causes SDS.

BSR is caused by the soilborne fungus Phialophora gregata. There are two distinct types (or genotypes) of the fungus, denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregata Type A also is associated with higher yield loss. P. gregata survives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregata survives longer when soybean residue is left on the soil surface (e.g., in no till settings) where the rate of residue decay is slow. P. gregata infects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients.

Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, P. gregata and soybean cyst nematode (Heterodera glycines) frequently occur in fields together, and there is evidence that BSR is more severe in the presence of this nematode.

BSR cannot be controlled once plants have been infected. Foliar fungicides and fungicide seed treatments have NO effect on the disease. Use crop rotations of two to three years away from soybean with a non-host crop (e.g., small grains, corn, or vegetable crops), as well as tillage methods that incorporate plant residue into the soil. Both of these techniques will help reduce pathogen populations by promoting decomposition of soybean residue. Also, make sure that soil fertility and pH are optimized for soybean production to avoid overly low phosphorus and potassium levels, as well as overly low soil pH. Finally, grow soybean varieties with resistance to BSR. Complete resistance to BSR is not available in commercial varieties. However several sources of partial resistance that provide moderate to excellent BSR control are available. Also, some, but not all, varieties of soybean cyst nematode (SCN) resistant soybeans also are resistant to BSR. Most soybean varieties with SCN resistance derived from PI 88788 express resistance to BSR. However, the same is not true of varieties with SCN resistance derived from Peking. Therefore growers should consult seed company representatives about BSR resistance when selecting a variety with SCN resistance derived from this source. You can download a full color fact sheet on BSR by clicking here. You can also CLICK HERE to view a short video on BSR.

Soybean Leaf with Symptoms of SDS

Soybean Leaf with Symptoms of SDS

Sudden Death Syndrome (SDS)

The first noticeable symptoms of SDS are chlorotic (i.e., yellow) blotches that form between the veins of soybean leaflets. These blotches expand into large, irregular, chlorotic patches (also between the veins), and this chlorotic tissue later dies and turns brown. Soon thereafter entire leaflets will die and shrivel. In severe cases, leaflets will drop off leaving the petioles attached. Taproots and below-ground portions of the stems of plants suffering from SDS, when split open, will exhibit a slightly tan to light brown discoloration of the vascular (i.e., water- conducting) tissue. The pith will remain white or cream-colored. In plants with advanced foliar symptoms of SDS, small, light blue patches will form on taproots and stems below the soil line. These patches are spore masses of the fungus that causes the disease.

Foliar symptoms of SDS can be confused with those of brown stem rot (see description above). However, in the case of brown stem rot (BSR), the pith of affected soybean plants will be brown. In addition, roots and lower stems of plants suffering from BSR will not have light blue spore masses.

Once symptoms of SDS are evident, yield losses are inevitable. Yield losses can range from slight to 100%, depending on the soybean variety being grown, the plant growth stage at the time of infection and whether or not SCN is present in a field. If SDS occurs after reproductive stages R5 or R6, impact on yield is usually minimal. If SDS occurs at flowering however, yield losses can be substantial. When SCN is present, the combined damage from both diseases can be substantially more than the sum of the damage expected from the individual diseases.

SDS is caused by the soilborne fungus, Fusarium virguliforme (synonym: F. solani f. sp. glycines). F. virguliforme can overwinter freely in the soil, in crop residue, and in the cysts of SCN. The fungus infects soybean roots (by some reports as early as one week after crop emergence), and is generally restricted to roots as well as stems near the soil line. F. virguliforme does not invade leaves, flowers, pods or seeds, but does produce toxins in the roots that move to the leaves, causing SDS’s characteristic foliar symptoms.

SDS cannot be controlled once plants have been infected. Foliar fungicides have NO effect on the disease. Recently a new seed treatment has been identified that has efficacy against SDS. A multi-state university-based research team has demonstrated that ILevo seed treatment can reduce the effect of SDS and increase yields in fields where SDS is a problem. Other methods of control include using SDS-resistant varieties whenever possible in fields with a history of the disease; however, keep in mind that SDS-resistant varieties with maturity groups suitable for Wisconsin and other northern regions (groups I and II) are somewhat scarce at this time. If SDS and SCN are both problems in the same field, planting an SCN-resistant soybean variety may also be beneficial in managing SDS. Avoid planting too early. Wisconsin growers typically prefer to plant soybeans before May 10 to extend the length of the growing season and maximize yields. However, planting when soils are cool and wet makes plants more vulnerable to infection by F. virguliforme. Improve soil drainage by using tillage practices that reduce compaction problems. Rotation, while useful in managing other soybean diseases, does not appear to significantly reduce the severity of SDS. Even after several years of continuous production of corn, F. virguliforme populations typically are not reduced substantially. Research from Iowa State University has shown that corn (especially corn kernels) can harbor the SDS pathogen.

For more information CLICK HERE to download a full color fact sheet on SDS. A short video on SDS can also be viewed by CLICKING HERE.

Patches of soybean plants killed by stem canker.

Patches of soybean plants killed by stem canker.

Stem Canker

There are actually two different types of stem canker caused by related, but different fungi. The fungus Diaporthe caulivora causes northern stem canker, while southern stem canker is caused by Diaporthe meridionalis. These two pathogens are part of the larger Diaporthe-Phomopsis complex, which consists of Phomopsis seed decay, pod and stem blight (see link to fact sheet below), and stem canker. In Wisconsin, northern stem canker is the most common stem canker disease, however, southern stem canker has been found.

Cool, wet conditions in the spring and early summer favor infection by the northern stem canker fungus. The symptoms of the disease become apparent later in the season. Considering the cool and rainy weather that has been prevalent over much of the state this season, it isn’t surprising that northern stem canker is prevalent.

Initially symptoms of northern stem canker appear as small reddish-brown lesions near nodes. As lesions expand, they can become more brown or gray, but the red border will remain. Eventually lesions of northern stem canker will get large enough to girdle the stems and may be confused with Phytophthora root and stem rot. The best way to tell these two diseases apart is to look for the location of the lesion.  Generally with northern stem canker, lesions begin at nodes away from the soil line on the main stem and move upward. Phytophthora stem lesions will progress upward from the soil line. Northern stem canker can also occur in patches and damage plants in wide swaths Northern stem canker can also be confused with white mold when diagnosing above the canopy. Because the lesions can girdle stems, leaf flagging and death can resemble that of white mold damage. Therefore, careful scouting and inspection of the lower canopy and stems in necessary to tell the difference between white mold and northern stem canker.

Spores of the stem canker pathogen originate mostly from soybean debris from the previous crop. Therefore, severity of northern stem canker can be higher in fields with minimal tillage. Burying debris can help reduce the severity of the disease. Stem canker can also be more prevalent in fields with high fertility and high organic matter.  Stem canker-resistant varieties are also available. Choose varieties with the highest resistance rating possible within the appropriate maturity group for your area. Soybeans rotated with alfalfa may also have a higher incidence of the disease, because alfalfa is an alternate host of Diaporthe. Fungicide application is not recommended for this disease.

You can download a new full color stem canker fact sheet by clicking here and for pod and stem blight by clicking here.

Symptoms of soybean vein necrosis disease (SVND)

Symptoms of soybean vein necrosis disease (SVND)

Soybean Vein Necrosis Disease (SVND)

Soybean plants with SVND exhibit vein clearing (i.e., lightening of vein color) and chlorosis (i.e., yellowing), as well as mosaic patterns (i.e., blotchy light and dark areas) on affected leaves. Initially, symptoms develop around the veins of leaves and eventually expand outward. As the disease progresses, vein and leaf browning and necrosis (i.e., death) occur.

SVND is caused by soybean vein necrosis virus (SVNV). SVNV is in the viral genus Tospovirus. This group of viruses includes common vegetable viruses [e.g., Tomato spotted wilt virus (TSWV) and Iris yellow spot virus (IYSV)] and ornamental viruses [e.g., Impatiens necrotic spot virus (INSV)] that can cause severe damage and substantial loss of yield and crop quality. Tospoviruses tend to have wide host ranges and are transmitted by several species of thrips. SVNV is also thought to be thrips-transmitted, but this has yet to be confirmed. SVNV may have been introduced to Wisconsin via thrips moving north on wind currents from the southern United States.

Currently very little is known about SVND. Thus there are no specific management practices recommended for SVND at this time. No specific control recommendations are in place. Researchers at universities across the country are attempting to determine what impact SVNV will have. Additional research is needed to determine how SVNV affects soybeans, how it is transmitted, how it overwinters, and what can be done to slow its spread.

To learn more about SVND you can download a fact sheet assembled by a multi-state working group of soybean pathologists and also view a short video.