I Finally Got My Soybeans Planted and Now They Look Sick!

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

Shawn Conley, Soybean and Wheat Extension Specialist, Department of Agronomy, University of Wisconsin-Madison

Phytophthora stem rot of a seedling soybean. Photo Credit: Craig Grau.

The 2019 growing season has been nothing but full of challenges for Wisconsin farmers and farmers throughout the Midwest. Weather and grain markets have not improved, combined with late-planting of all crops, including soybeans. Dryer weather recently has allowed many to catch up a bit on planting, but now the weather is turning wet again. With this wet weather right after planting, we start to get concerned about several seedling and early-season diseases that can show up, and the performance of seed treatments used to protect soybeans against the pathogens that cause these diseases.

What are the Pathogens of Primary Concern?

Soybeans are susceptible to several early diseases. A detailed list of those important in Wisconsin can be found HERE. You will notice in that list that there is an array of fungi and water-molds that can affect soybeans, compromising stands. More recently we have been very interested in tracking the water-molds. These organisms include Pythium and Phytophthora. Pythium can cause diseases such as Pythium seedling blight and root rot while Phytophthora can result in Phytophthora root and stem rot of soybean. When it comes to both of these diseases, several species within each pathogen genus can affect soybeans in Wisconsin. In fact, The Wisconsin Department of Agriculture, Trade and Consumer Protection Pest Survey Program and the Plant Industry Bureau Laboratory has tracked water-molds in soybean fields from since 2008. The latest results of this surveys can be found by CLICKING HERE. You will notice that there are actually two Phytophthora species and more than 5 Pythium species that can affect soybeans in Wisconsin. With the diversity of pathogens in the state and the wet spring we are having, it is no wonder that seedling issues are present in Wisconsin.

Will Seed Treatments Cure Poor Soybean Emergence?

The short answer is no. In the last 10 years we have seen a significant increase in the availability and use of seed treatments in soybeans. These seed treatments can be a simple single-mode-of-action fungicide or combined with multiple fungicides, insecticides, nematicides, and/or plant growth regulators. A detailed list of seed treatment products registered in Wisconsin for soybeans and other grain crops can be found on the What’s On Your Seed fact sheet. While we highly recommend the use of seed treatments to combat seed rots and seedling blights, it is important to realize that they are not perfect and can fail or under-perform for many reasons. Even if you used a seed treatment on your soybean seed in 2019, you may still notice emergence issues. There are many factors that play a role in the success of a seed treatment, including the correct choice of product against the right pathogens, weather, soil type, etc. For more information on the factors that can affect seed treatments check out the fact sheet posted HERE. If you are in a situation where you used a seed treatment and the stand is poor, check out this publication. This can give you some insight on what happened as you work through diagnosing the issue with your agronomist. There is not a one-size-fits-all seed treatment so it is important that if you have had issues with the performance of your seed treatment, you determine if a pathogen is involved and what species it might be. Knowing this information can help guide you in choosing the seed treatment most appropriate for controlling that particular pathogen in the future. If you need help diagnosing a potential seed decay or seedling disease, you can send a sample to the University of Wisconsin-Madison Plant Disease Diagnostic Clinic. Details on how to prepare and send a sample can be found on their website by clicking here.

Does Variety Resistance Help Improve Soybean Stands?

Yes! While resistance to Pythium in soybean isn’t well understood, there are resistant varieties deployed for managing Phytophthora. Both race-specific and field resistance (lower level of resistance to all races) are available in soybean varieties marketed in Wisconsin. There are often one or more race-specific Phytophthora resistance genes in commercial soybean varieties. The genes present in specific soybean varieties are listed each year in the University of Wisconsin Soybean Variety Test Results (UW-Extension publication A3654). Your seed dealer will also have this information.

Monitor the performance of the varieties you choose. When optimum disease conditions develop later in the growing season, scout those areas of the fields to look for stem rot development. If a large number of plants with Phytophthora stem rot are found, choose varieties with a different Rps gene and higher levels of partial resistance for next year. This pathogen does adapt to the Rps genes, but it is a slow process. Careful monitoring of plant performance is all that is needed. A listing of RPS genes and their relative effectiveness in Wisconsin can be found in the table below.

Race-specific Phytophthora resistance genes and their effectiveness in Wisconsin

Soybean genes Phytophthora races controlled Effectiveness in Wisconsin
Rps 1a 1, 2, 10, 11, 13-18, 24 limited effectiveness
Rps1b 1, 3-9, 13-15, 17, 18, 21, 22
Rps 1c 1-3, 6-11, 13, 15, 17, 21, 23, 24 effective in 75% of fields
Rps 1k 1-11, 13-15, 17, 18, 22, 24 effective in 99% of fields
Rps 3 1-5, 8, 9, 11, 13, 14, 16, 18, 23, 25
Rps 4 1-4, 10, 12, 16, 18-21, 25
Rps 6 1-4, 10, 12, 14-16, 18-21, 25
Rps 1k, 6 1-11, 12-22, 24, 25

Field resistance, also called field tolerance, is present at different levels in most soybean varieties marketed in Wisconsin. For example, even if a variety has a specific resistance gene that may not be effective, such as Rps 1a, against the races of Phytophthora sojae present in a field, the variety may perform better than other varieties with this gene because it has an adequate level of field resistance to Phytophthora. Field resistance can be overcome by high disease pressure especially in the seedling stage. A final note on resistance – in field where Phytophthora sansomeana is present, Rps genes may have little effect. No data currently exists on soybean variety performance against this fairly new pathogen of soybean in Wisconsin.

How Will Delayed Planting Influence Crop Diseases in 2019?

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

Darcy Telenko, Extension Field Crops Pathologist, Purdue University

Figure 1. The Disease Triangle Concept

We keep getting this question, because as we write this, it is storming yet again in many locations in the Midwest. Rain, rain, and more rain has pushed back timely planting everywhere. Concern is starting to mount about not only yield loss simply from delayed planting, but what increased risk of yield loss due to disease there might be in 2019. As we consider this issue, we will use tar spot of corn and white mold of soybean as just two examples of where this could be an issue.

The Plant Disease Triangle. Remember that the plant disease triangle is the foundation for understanding how plant diseases develop and how to manage them. In order for a plant disease to occur you must have a virulent pathogen, a susceptible host plant, and favorable weather conditions to coincide at the same time. If any one of these three components is missing (or we implement a management strategy that removes or reduces one component) then a plant disease will not occur. When it comes to the host component, it not only matters that the host is generally susceptible but is also at a susceptible growth stage. Consider white mold of soybeans for a minute. All stages of soybean are susceptible to infection by the white mold fungus, but most infections occur through open flowers. Thus, the disease triangle is met when you have (1)white mold fungal spores flying around at the same time that (2)soybean flowers are open (susceptible stage), during, (3) cool and wet weather (favorable environmental condition)completing the triangle (Figure 1). The point here is that if we continue in a cool wet pattern, and delayed planting continues, we may quickly find ourselves with crops at susceptible growth stages when the weather is very conducive to disease.

Figure 2. White Mold in a Soybean Field

Delayed Planting and White Mold of Soybean. In 2017, we had an epic epidemic of white mold on soybean across the upper Midwest (Figure 2). One of the main reasons that the epidemic was so bad is that it was generally cool for a large portion of the season. This resulted in soybeans that moved very slowly from one growth stage to the next. When it came to flowering, soybeans bloomed for an extended period of time. This left them in a susceptible growth stage for about twice as long as normal. These cool conditions also coincided with wet weather that was favorable for the pathogen. In 2018, planting occurred reasonably on-time and we accumulated heat units quickly. Bloom started early in the season and was about half as long as it was in 2017. This meant that soybeans “escaped” infection in large portions of the upper Midwest. Fast-forward to 2019. If this cool rainy cycle persists, and planting is delayed, then soybeans may bloom later and over an extended period of time during wet/humid weather conditions. Keeping an eye on weather before and during the soybean bloom period along with consulting the Sporecaster smartphone app  can help you make the educated decision to spray fungicide or not.

Figure 3. Tar Spot Signs and Symptoms on Corn Leaves

Delayed Planting and Tar Spot of Corn. In 2018 Tar spot of corn (Figure 3) created quite a stir. The epidemic was widespread and caused some significant yield losses in areas that it occurred. The tar spot fungus is residue-borne. There is also decent evidence that it can survive over-winter on corn residue (Figure 4). Our laboratories have been investigating tar spot fungal survival on corn residue collected after snow-melt in Wisconsin and Indiana. Regardless of whether there was fall tillage performed or not, survival of tar spot fungal spores (ascospores) on the residue collected ranged between 15 and 40%, with an average around 20%. These are VERY preliminary findings (and the numbers might change once we finish counting and analyzing data), but the point is that there is viable tar spot fungal inoculum present in Midwest corn fields. Therefore, one component of the triangle is met! As for the other two components, corn is being planted later than normal and conditions are cool and wet. Again, if this cycle of cool and wet holds, conditions will be favorable for the fungus. Delayed planting of corn will also push corn into conducive growth stages for the fungus to infect and cause heavy yield losses (although, we have seen infection at all growth stages as long as there was green tissue available). One of the reasons that the 2018 tar spot epidemic was so significant, was that many areas of the upper Midwest had cool and excessively wet conditions around the V6 growth stage and again near or after the VT growth stages. When foliar diseases of corn start at early growth stages (V6 or V8) the risk for yield loss can be much higher than if they start after R2 or brown silk. Keep an eye on the weather between the V6 and R2 growth stages and consult with your local extension personnel to decide if a fungicide might be warranted for corn to prevent tar spot, or other foliar diseases.

Figure 4. Signs of the Tar Spot Fungus on Corn Residue

Scouting and Watching Weather Reports Might Pay in 2019. Once corn and soybeans are planted, take the time to scout and pay attention to the weather. While thorough scouting can take time, it may be worth it in 2019. Catching a plant disease early can be the difference in being successful in managing it or not. Pay attention to the weather leading up to, and during, the critical crop growth stages. This can also help you make an educated decision about in-season application of fungicides. If it is cool and humid/rainy, and the crop is at a susceptible growth stage, then a fungicide application might be warranted. If it is hot and dry and the crop moves quickly through susceptible growth stages, then a fungicide might not be warranted. Study the disease triangle and use it to your advantage. The 2019 field season could be a year that this knowledge might be handy!

For in-season updates follow us on Twitter and Consult our websites at the links below:

Dr. Damon Smith

@badgercropdoc

https://badgercropdoc.com/

 

Dr. Darcy Telenko

@DTelenko

https://extension.purdue.edu/fieldcroppathology/

 

For More information about tar spot, white mold, and fungicide efficacy consult the following resources:

  1. Tar spot Fact sheet
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. White Mold Fact Sheet
  5. Short White Mold Video
  6. White Mold Webinar
  7. Corn Fungicide Efficacy Table
  8. Soybean Fungicide Efficacy Table

Poor Soybean Seed Quality and Preparing for the 2019 Field Season

Damon Smith, Associate Professor and Extension Specialist, Department of Plant Pathology, University of Wisconsin-Madison

Shawn Conley, Professor and Extension Specialist, Department of Agronomy, University of Wisconsin-Madison

Figure 1. Severe Phomopsis seed decay. Photo Credit: Craig Grau, University of Wisconsin-Madison.

As we finish off 2018 and look ahead to the 2019 planting season, soybean farmers need to be prepared for some potential seed quality issues. The 2018 crop was plagued by several problems, but one of the most substantial was a large amount of white, chalky, or black, and damaged seed (Figure 1). This damaged seed is impacting germination rates of soybean seed slated for the 2019 crop.

What caused this issue?

Most of this damage is a result of infection and colonization by a group of fungal species called Diaporthe. This group is implicated in diseases such as stem canker, pod and stem blight (Figure 2), and Phomopsis seed decay (Figure 3). Excessive rains at the end of August and throughout September and October resulted in a large amount of pod infection by Diaporthe. These infections combined with delayed harvest allowed for extensive seed colonization by these fungi. This resulted in Phomopsis seed decay which has led to visually damaged seed and the germination issues we are now seeing. To learn more about this group of fungi and the diseases they cause, visit the Crop Protection Network (CPN) website on pod and stem blight and Phompsis seed decay by clicking here. You can also download a PDF version of the CPN fact sheet on the same subject by clicking here.

How Do I manage this Problem at Planting in 2019?

Figure 2. Pod and stem blight of soybean

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). Page 157 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 is the seed treatment efficacy table from the Crop Protection Network (CPN). You can download that publication by clicking here.

Figure 3. Damaged soybean seed as a result of Diaporthe infection.

We also recommend that as a farmer, 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.

If I’m a Seed Producer, What Should I Do to Prevent this Problem Next Year?

Foliar fungicide applications during the growing season could reduce the damage from Diaporthe. Some work has demonstrated that fungicide applications between the R3 to the R5 growth stages might be useful in reducing damage. This might help improve seed quality, but not necessarily improve yield. For a list of fungicide products with efficacy ratings for soybean, take a look at this additional publication from the CPN by clicking here.

2018 Wisconsin Field Crops Pathology Fungicide Tests Summary Now Available

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

Each year the Wisconsin Field Crops Pathology Program conducts a wide array of fungicide tests on alfalfa, corn, soybeans, and wheat. These tests help inform researchers, practitioners, and farmers about the efficacy of certain fungicide products on specific diseases. The 2018 Wisconsin Field Crops Fungicide Test Summary is now available. These tests are by no means an exhaustive evaluation of all products available, but can be used to understand the general performance of a particular fungicide in a particular environment. Keep in mind that the best data to make an informed decision, come from multiple years and environments. To find fungicide performance data from Wisconsin in other years, visit the Wisconsin Fungicide Test Summaries page. You can also consult publication A3646 – Pest Management in Wisconsin Field Crops to find information on products labeled for specific crops and efficacy ratings for particular products. Additional efficacy ratings for some fungicide products for corn foliar fungicides, soybean foliar and seed-applied fungicides, and wheat foliar fungicides can be found on the Crop Protection Network website.

Mention of specific products in these publications are for your convenience and do not represent an endorsement or criticism. Remember that this is by no means a complete test of all products available.  You are responsible for using pesticides according to the manufacturers current label. Some products listed in the reports referenced above may not actually have an approved Wisconsin pesticide label. Be sure to check with your local extension office or agricultural chemical supplier to be sure the product you would like to use has an approved label.  Follow all label instructions when using any pesticide. Remember the label is the law!

Sporebuster, a New White Mold Fungicide Value Calculator

Damon L. Smith, Ph.D., Associate Professor and Extension Specialist, Department of Plant Pathology, University of Wisconsin-Madison

When a fungicide application is needed to control white mold in soybeans, Sporebuster can help determine a profitable program. You enter your expected soybean price, expected yield, and treatment cost. Sporebuster instantly compares ten different treatment plans at once to determine average net gain and breakeven probability of each. You can mark, save and share by email, the best plans for your farming operation.

The purpose of Sporebuster is to assist soybean farmers in making a fungicide program decision that is profitable for their operation. Sporebuster is meant to complement Sporecaster, which is a tool that can be used to make the decision whether a fungicide application is even needed. Once Sporecaster recommends a fungicide application, Sporebuster can be used to determine a profitable program. To learn more about Sporebuster, how to use it, and to download it, click here.

Badger Crop Doc with Ashley Davenport for an AgPro Podcast

Dr. Damon Smith, or @badgercropdoc, is an extension specialist and associate professor at the University of Wisconsin, Madison. We’re talking about different agronomic issues farmers have faced this year, including a newer one: tar spot. You can listen to the Podcast here.

2018 Pest Management Update Meeting Series Announced

The schedule for the Wisconsin Pest Management Update meeting series has been set. Presentations will include agronomic pest management information for Wisconsin field and forage crops. Speakers include Mark Renz and Rodrigo Werle, weed scientists, Damon Smith, plant pathologist, and Bryan Jensen, entomologist.

The format will be the same as in 2017. Meetings will either be in the morning or afternoon On November 12-16, 2018. Simply choose a day/location to attend with each meeting running 3 hours. Note that several locations and contacts have changed since 2017 (marked with * in the meeting flier). Please read the informational flier carefully and make sure you contact the appropriate person at your desired location.

2018 Pest Management Update Highlights:

  • Integrated Pest Management Updates in corn, soybeans, alfalfa, and small grains: Update on new products and/or use of existing products as well as brief highlights of the 2018 pest situations in each crop.
  • Waterhemp management
  • Dicamba off-target research
  • Pollinator Training
  • Soybean cyst nematode training and management

Please make your reservation with the host contact at least one week prior to the scheduled meeting date.

Three hours of Certified Crop Advisor CEU credits in pest management are requested for each session.

To download a PDF of the flier, CLICK HERE.

Wisconsin Late-Season Soybean Disease Update

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.

Wisconsin White Mold Risk Maps – July 29, 2018

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

**These maps are for guidance only and should be used with other sources of information and professional advice when determining risk of white mold development. For field-specific predictions, we encourage you to use the Sporecaster smartphone application. These predictions will be most accurate for your specific location. Information about Sporecaster and how to download can be found by clicking here. Sporecaster takes into account crop phenology, in addition to weather parameters, to make field-specific recommendations. The maps below are based on weather only and you must determine if your crop is currently phenologically at risk for infection. For more information on white mold and how to manage it, see my previous post.**


Map Legends: No color = model is inactive and risk of apothecia in the field is not likely; Gray = apothecia might be present, but likelihood of apothecial presence is extremely low; Blue = low risk of apothecia; Yellow = medium risk of apothecia in the field; Red = high risk of apothecia in the field. Model predictions must be combined with soybean growth stage and canopy characteristics to aid in timing of fungicide sprays.


 

Figure 1. White Mold Risk Map for Non-irrigated Fields – July 29, 2018

Cooler, dry weather has changed the white mold risk map over the last several weeks for non-irrigated (dryland or rain-fed) soybean fields (Figure 1). Cool weather has generated some areas of high risk in the southwest, western, and north-central regions of the state. However, the dry conditions have dissipated much of the moderate and low risk areas that showed several weeks ago. Risk in general is much more spotty than it has been. The soybean crop is also moving through growth stages much more rapidly than in 2017. Thus, the risk window for infection by the white mold fungus will end soon. Remember that once the crop has finished flowering risk of new infections is low to non-existent. In addition, late applications (R4 and later growth stages) of fungicide will not be needed for white mold control.

 

Figure 2. White Mold Risk Map for irrigated fields planted on 15″ row-spacing – July 29, 2018

Risk remains high across much of the state for irrigated soybeans planted to 15″ rows (Figure 2). Risk is starting to dissipate in areas in the east-central region, due to dryer conditions. However, a fungicide application should be considered if irrigating and soybeans are flowering and planted to a 15″ row-spacing. Note that if you have irrigation in your field but are not actually irrigating, you should be using the non-irrigated model above to make an accurate prediction of white mold risk.

 

Figure 3. White Mold Risk Map for irrigated fields planted on 30″ row-spacing – July 29, 2018

Risk of white mold in irrigated fields planted to 30″ row-spacing has decreased a bit in some areas, since the last post (Figure 3). However, Much of the state is at risk if irrigating on 30″ spaced soybeans. In these areas, a fungicide application should be considered if soybeans are being irrigated, canopy is nearly closed, and flowers are present. Note that if you have irrigation in your field but are not actually irrigating, you should be using the non-irrigated model above to make an accurate prediction of white mold risk.


These models were developed at the University of Wisconsin-Madison in conjunction with Michigan State University and Iowa State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold fungus apothecia. Weather information and maps are provided by the Soybean Integrated Pest Information Platform for Extension and Education (iPIPE), which is managed by ZedX, Inc. These models predict when apothecia will be present in the field using combinations of 30-day averages of maximum temperature, relative humidity, and maximum wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. Based on these predictions, a map is generated under three scenarios (non-irrigated soybeans, soybeans planted on 15″ row-spacing and irrigated, or soybeans planted on 30″ row-spacing and irrigated). The maps are colored to show the likelihood of apothecial presence within a region.  If the model is predicting high risk (red) in your area for your planting scenario, the soybeans are flowering, and the canopy is somewhat closed, then the white mold risk is high. If your fields are at-risk, we recommend to consult your local extension personnel or other research-based resources for the best in-season management options for your area.

Wisconsin White Mold Risk Maps – July 15, 2018

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

**These maps are for guidance only and should be used with other sources of information and professional advice when determining risk of white mold development. For field-specific predictions, we encourage you to use the Sporecaster smartphone application. These predictions will be most accurate for your specific location. Information about Sporecaster and how to download can be found by clicking here. Sporecaster takes into account crop phenology, in addition to weather parameters, to make field-specific recommendations. The maps below are based on weather only and you must determine if your crop is currently phenologically at risk for infection. For more information on white mold and how to manage it, see my previous post.**


Map Legends: No color = model is inactive and risk of apothecia in the field is not likely; Gray = apothecia might be present, but likelihood of apothecial presence is extremely low; Blue = low risk of apothecia; Yellow = medium risk of apothecia in the field; Red = high risk of apothecia in the field. Model predictions must be combined with soybean growth stage and canopy characteristics to aid in timing of fungicide sprays.


Figure 1. White Mold Risk Map for Non-irrigated Fields – July 15, 2018

Hot, dry weather continues to push risk for white mold down in non-irrigated (dryland or rain-fed) soybean fields (Figure 1). While risk is high in some locations of the state, there is much more yellow on the map compared to last week, indicating medium risk. Continued areas of high risk are present along Lake Michigan and in the central and southwest portions of Wisconsin. In these areas, a fungicide application should be considered if the soybean canopy is nearly closed and flowers are present.

Figure 2. White Mold Risk Map for irrigated fields planted on 15″ row-spacing – July 15, 2018

Risk remains high across the state for irrigated soybeans planted to 15″ rows (Figure 2). A fungicide application should be considered if irrigating and soybeans are flowering and planted to a 15″ row-spacing. Note that if you have irrigation in your field but are not actually irrigating, you should be using the non-irrigated model above to make an accurate prediction of white mold risk.

Figure 3. White Mold Risk Map for irrigated fields planted on 30″ row-spacing – July 15, 2018

Risk of white mold in irrigated fields planted to 30″ row-spacing has increased significantly since last week (Figure 3). Much of the state is at risk if irrigating on 30″ spaced soybeans. In these areas, a fungicide application should be considered if soybeans are being irrigated, canopy is nearly closed, and flowers are present.


These models were developed at the University of Wisconsin-Madison in conjunction with Michigan State University and Iowa State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold fungus apothecia. Weather information and maps are provided by the Soybean Integrated Pest Information Platform for Extension and Education (iPIPE), which is managed by ZedX, Inc. This model predicts when apothecia will be present in the field using combinations of 30-day averages of maximum temperature, relative humidity, and maximum wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. Based on these predictions, a map is generated under three scenarios (non-irrigated soybeans, soybeans planted on 15″ row-spacing and irrigated, or soybeans planted on 30″ row-spacing and irrigated). The maps are colored to show the likelihood of apothecial presence within a region.  If the model is predicting high risk (red) in your area for your planting scenario, the soybeans are flowering, and the canopy is somewhat closed, then the white mold risk is high. If your fields are at-risk, we recommend to consult your local extension personnel or other research-based resources for the best in-season management options for your area