Time to Start Watching for White Mold in Soybeans

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

Jaime Willbur, Graduate Research Assistant, University of Wisconsin-Madison

Figure 1. Severe white mold in a soybean field.

Figure 1. Severe white mold in a soybean field.

The warm weather over the last several weeks has pushed the Wisconsin soybean crop quickly toward the reproductive growth stages. By the end of the week, most early-planted soybean fields in the southern portion of Wisconsin will begin flowering (R1 growth stage). This growth stage is a critical time to make a fungicide application decision for white mold (Fig. 1; also called Sclerotinia stem rot). Fungicide decisions should be made for this disease between the R1 and the R3 (pods beginning to form on upper nodes) growth stages. After the R4 growth stage control of white mold using fungicides quickly declines. The decision to apply fungicide during this time should be made based on the weather. As discussed in this FACT SHEET and VIDEO, cool (less than 80F) and wet and/or humid conditions during the R1-R3 growth stages can lead to increased risk of white mold later in the season.

Figure 2 illustrates the white mold cycle. Small hardened black structures called sclerotia survive many years in the soil (Fig. 2A). When conditions are cool and wet during the bloom period small mushroom-like structures called apothecia germinate from the sclerotia (Fig. 2B). The apothecia release spores that land on flower petals and germinate (Fig. 2C) allowing the fungus to infect and colonize the soybean plant. As the fungus continues to colonize the inside of the plant, symptoms will begin to develop around the R5 or R6 soybean growth stages. These include wilting plants and paper bag-brown lesions on stems. Eventually new sclerotia of the fungus begin to develop on the plant (Fig. 2D). These sclerotia become the source for future white mold epidemics. Because the white mold fungus needs the open flowers to infect and colonize soybean, it is important to apply a fungicide during this time to protect the plants from infection IF the weather is conducive for the white mold fungus. It can be difficult to determine what “conducive weather” is and if you need to spray.

In an effort to help define these “conducive” conditions, a model was developed at the University of Wisconsin-Madison in conjunction with Michigan State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold apothecia. This model predicts when apothecia will be present in the field using 30-day averages of maximum temperature, relative humidity, and wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. Based on these predictions, a map is generated (Fig. 3) to indicate areas at no (white), low (blue), medium (yellow), and high (red) risk levels. Fields in yellow or red areas have >40% chance of having apothecia present and may be at risk of white mold developing later in the season. Model predictions must be combined with soybean growth stage and canopy characteristics to aid in timing of fungicide sprays. If the model is predicting medium to high risk in your area, the soybeans are flowering, and the canopy is somewhat closed, then the white mold risk is elevated. If your fields are at-risk, we recommend to consult your local extension personnel or resources for the best in-season management options for your area. To view and download a handy user guide for the model, CLICK HERE.

For Wisconsin soybean growers, regular updates and commentary regarding risk of white mold can be found on this blog. Color coded, state-wide maps will be posted and our recommendations will accompany these posts. So be sure to check back regularly or subscribe to the blog to receive an automatic e-mail update when a new post has been added. You can subscribe via the window immediately to the right of this window. The inaugural post for 2016 can be viewed by clicking here. 

If you have decided to spray soybeans for white mold, what are the best products to use. Over the last several years we have run numerous fungicide efficacy trials in Wisconsin and in conjunction with researchers in other states. Fungicides that have performed well in multi-state studies can be found in the Soybean Fungicide Efficacy Table. In Wisconsin, we have observed that Endura applied at 8 oz at the R1 growth stage performs well. We have also observed that the fungicide Aproach applied at 9 fl oz at R1 and again at R3 also performs comparably to the Endura treatment. Other fungicide options also include Omega and Proline. You can view results of past fungicide evaluations by CLICKING HERE.

For even more detailed information about white mold you can visit the Crop Protection Network page on white mold . You can also find more information about white mold by clicking here and scrolling down to the white mold section.

To vist other posts on this blog about white mold, click below:

2015 Blog Post

2014 Blog Post

Wisconsin White Mold Risk Map – June 30, 2016

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

Jaime Willbur, Graduate Research Assistant, University of Wisconsin-Madison

Sclero-cast: A Soybean White Mold Prediction Model

**This tool is for guidance only and should be used with other sources of information and professional advice when determining risk of white mold development. We encourage you to read the model how-to guide which can be downloaded by clicking here**

Today is the first public posting of the 2016 experimental white mold risk model map. Postings will be conducted on this blog regularly. We suggest that if you are interested in consulting the risk map for Wisconsin, you subscribe to this blog using the window to the right.

White Mold Risk - June 30, 2016

White Mold Risk – June 30, 2016

 

Risk of apothecial presence and subsequent white mold development is generally low for most of Wisconsin today. Warmer than average conditions and limited rainfall over the past month in much of the state has kept the risk low. A few isolated pockets of moderate or higher risk are located around Shawano Co. and north of Barron Co. Soybeans in this area are likely not at the susceptible growth stage. 

This model was developed at the University of Wisconsin-Madison in conjunction with Michigan State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold apothecia. This model predicts when apothecia will be present in the field using 30-day averages of maximum temperature, relative humidity, and wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. Based on these predictions, this map is generated and indicates areas at no (white), low (blue), medium (yellow), and high (red) risk levels. Fields in yellow or red areas have >40% chance of having apothecia present and may be at risk of white mold developing later in the season. Model predictions must be combined with soybean growth stage and canopy characteristics to aid in timing of fungicide sprays. If the model is predicting medium to high risk in your area, soybeans are flowering, and the canopy is somewhat closed, then the white mold risk is elevated. For further information on how to use and interpret this risk map, CLICK HERE to download a how-to guide.

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.

Time to Consider Your White Mold In-Season Management Plan

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

Damage from white mold in a soybean field under irrigation.

Figure 1. Damage from white mold in a soybean field under irrigation.

Many early planted soybeans in Wisconsin are nearing the R1 (beginning flower) growth stage. This means that soybean growers should consider their in-season white mold management plan and if they are going to implement it. Research has shown that this is the best time to apply fungicides for control of this lethal soybean disease, if conditions are conducive for infection. Before we consider the in-season control options, lets review some basic information from our white mold (Sclerotinia stem rot or SSR) fact sheet. You can also download a PDF version of the fact sheet by CLICKING HERE.

What is Sclerotinia stem rot? Sclerotinia stem rot (SSR), also known as white mold, is a serious and often lethal fungal disease that affects a wide range of agricultural crops in the U.S. including many broadleaf vegetable crops (e.g., carrots, cruciferous plants, peas, potatoes, snap beans) and field crops, especially soybean. SSR is most severe on soybeans in high-yielding environments that have dense, fast-growing canopies (Fig. 1).

Sclerotia of the white mold fungus inside a soybean stem.

Figure 2. Sclerotia of the white mold fungus inside a soybean stem.

What does Sclerotinia stem rot look like? SSR causes sudden wilting of soybean leaves and rapid plant death. Lower stems of affected plants become bleached and under moist conditions (e.g., high humidity, frequent rain), become covered with a cottony white fungal growth. Small, black structures that look like rat or mouse droppings (called sclerotia) form on and inside the stems and pods of affected plants (Fig. 2).

Where does Sclerotinia stem rot come from? Sclerotinia stem rot is caused by the fungus Sclerotinia sclerotiorum which survives as sclerotia in dead plant tissue. Sclerotia can survive for five years or more in soil. A cool, moist environment favors Sclerotinia stem rot development. Under these conditions, sclerotia germinate to produce small, mushroom-like structures (called apothecia) that produce spores. These spores can be spread by wind, insects, or rain splash. In soybeans, most infections occur via open or senescing (i.e., withering) flowers. Occasionally, the fungus will spread from plant-to-plant via direct contact of roots or other plant parts.

How can I save plants with Sclerotinia stem rot? SSR is difficult to control once the disease has occurred. If affected plants are limited to a small area in a field, removal and destruction of plants may help to limit production of sclerotia that can further contaminate and cause long-term problems in the field; however, this strategy usually is not feasible on a large scale. If affected plants are removed, they should be burned. DO NOT compost plants or till them into the soil.

How can I avoid problems with Sclerotinia stem rot in the future? To prevent introduction of the SSR fungus into soybean fields, be sure to plant sclerotia-free soybean seed. Also, harvest fields with SSR last to avoid spreading sclerotia of the SSR fungus from field to field on combines. In fields with a history of SSR, grow soybean cultivars that have been bred for SSR resistance. This is the most economical and successful long-term strategy for SSR control. In addition, consider using no-till production for three to four years as this will reduce the number of viable sclerotia near the soil surface. Rotate soybeans with small grain crops that are not susceptible to SSR (e.g., wheat, barley, oats) to further reduce the number of viable sclerotia in the soil. Increase row spacing and reduce soybean seeding rates to promote a more open canopy that will have better air circulation and thus dry more rapidly. Also, make sure fields are well drained and avoid excessive irrigation especially during flowering. Remember that the SSR fungus prefers wetter conditions; under drier conditions the fungus is less likely to infect. Maintain good broadleaf weed control. Weeds not only decrease air circulation and promote wetter conditions, but can also be hosts for the SSR fungus. Finally, there are fungicides and biological control products available for SSR management. Several biocontrol agents (the most effective being one that contains a fungus called Coniothyrium minitans) have been shown to be effective in controlling SSR.

What are my In-Season Control Options? Fungicides containing an active ingredient that is a succinate dehydrogenase inhibitor (SDHI), such as boscalid, are often effective in SSR control. The active ingredient picoxystrobin (a type of strobilurin fungicide) has also been shown to be effective in SSR control in university research trials. Applications rates are very important for these products. Research in Wisconsin and surrounding states has indicated that Endura (active ingredient is boscalid) should be applied at the 8 oz/acre rate while Aproach (active ingredient is picoxystrobin) should be applied at the 9 fl oz/acre rate. Timing of fungicide applications is critical. Fungicides should be applied during early flowering (R1) to early pod development (R3) growth stages. Research has shown that Endura is best used when applied in a single application at R1. Aproach should be applied twice with the first application at R1, while the second application should be applied at the R3 growth stage with both applications at the rate indicated above. Fungicide applications made at the full pod (R4) growth stage or later will NOT be effective. In addition, applying fungicide treatments after symptoms are visible will not be effective.

In 2014, we conducted a trial using both Endura and Aproach fungicides applied at the R5 growth stage. Details of the trial can be found by CLICKING HERE and scrolling down to page 12. Prior to fungicide application we rated the plots for disease using the SSR disease severity index or DSI. We then rated the plots again 2 weeks later at the R6 growth stage to see if the disease progression slowed. We also harvested plots to determine yield. Initial DSI levels were fairly high, but uniform among all plots (Table 1). DSI ratings at R6 revealed that neither fungicide slowed the disease or reduced the levels of disease compared to the non-treated controls. Yield for all treatments was around 40 bu/acre for all treatments, and was not statistically significant. These results indicate that even the best fungicide products won’t work well on white mold if they are applied at the wrong time. These products have increased yield substantially in research trials where white mold pressure was high, when they were applied at the correct growth stage. In 2013, both programs were among the best in a trial located at the Arlington Agricultural Research Station. Results of that trial can be viewed by clicking here at scrolling down to pages 6 and 7. Timing and preventative application are very important if you choose to use fungicides to control white mold.

Table 1. Sclerotinia stem rot severity and yield after applications of Endura and Aproach fungicides at R5.

Table 1. Sclerotinia stem rot severity and yield after applications of Endura and Aproach fungicides at R5.

Finally, be sure to read and follow all label instructions of the fungicide/biological control product(s) that you select to ensure that you use the materials in the safest and most effective manner possible.

If you would like to learn more about white mold, you can visit the Crop Protection Network white mold page by clicking here. You can also get more information about white mold by clicking here to watch a short video.

New Sclerotinia Stem Rot Fact Sheet

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A new fact sheet concerning Sclerotinia stem rot (white mold) on soybean has recently been published. The fact sheet describes symptoms of the disease and how to best manage it. To obtain a PDF version of the fact sheet, visit the ‘fact sheet’ section of the UW-Madison Field Crops Pathology website or CLICK HERE TO DOWNLOAD.

Wisconsin Corn and Soybean Disease Update – August 21, 2014

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

Figure 1. IPM Pipe Southern Corn Rust Advisory for August 21, 2014.

Figure 1. IPM Pipe Southern Corn Rust Advisory for August 21, 2014.

I have spent the last several days rating and scouting corn and soybeans in the southern tier of Wisconsin. There are a few active diseases out there to keep track of.

Field Corn

In field corn we have found a few fields with low levels of northern corn leaf blight (NCLB). Levels of NCLB seem to be a bit higher in southwestern Wisconsin. Severity on lower leaves in field corn was in the 10 – 15% range, with no damage apparent on ear leaves. Around the Arlington, WI area, NCLB is very limited with only a few lesions evident every 100 ft. or so.

Eyespot is becoming more evident in field corn.  In fields with corn debris from a previous crop, the severity levels are in the 25-30% range on lower leaves and 10-15% on ear leaves.

Low levels of common rust (less than 5%) can also be found on some field corn hybrids in Southern Wisconsin.

Southern rust has been reported as far north as east-central Nebraska. The southern rust epidemic is being monitored closely in the Midwest. No southern rust has been found or reported in Wisconsin (Fig. 1).

For more information about corn diseases in Wisconsin, see my previous article by clicking here.

Sweet Corn

Several fields with severe epidemics of NCLB on sweet corn have been reported.  These were late-planted fields. Sweet corn is generally more susceptible to NCLB than field corn. Common rust and eyespot can also be found at varying levels on sweet corn in the central and southern portion of Wisconsin.

In research plots at the Arlington Agricultural Research Station, sweet corn planted on June 25th is beginning to tassel. Levels of NCLB are currently low in this field, but common rust is increasing rapidly. Some leaves have 20-25% severity. Any late-planted and/or susceptible varieties of sweet corn should be monitored closely for foliar disease and any decision to spray fungicide should be made by the tasseling/R1 growth stage.

Soybean

The most widespread disease on soybean that we have observed is Septoria brown spot.  Overall levels of Septoria brown spot are low, and can mostly only be found on lower leaves, which is typical for this disease. In many cases a fungicide specifically for this disease is not warranted in Wisconsin, unless there are factors that might lead to increased levels of severity, including continuous soybean rotation, very susceptible varieties, or extremely conducive weather. Most soybean fields are past the R3 growth stage , when a fungicide application might be beneficial for control of foliar diseases. However, this disease should be monitored in fields that were planted late.

Downy mildew has also been observed on soybean in various areas from central to southern Wisconsin. Fungicide application for control of this disease has not proven beneficial in university research trials. Therefore, fungicide application is not recommended for this disease under most circumstances. In soybean fields that are irrigated, the frequency between irrigation events should be lengthened in order to reduce the levels of downy mildew. Warmer, dry weather will also further reduce the level of downy mildew.

Figure 2. Damage from white mold in a soybean field under irrigation.

Figure 2. Damage from white mold in a soybean field under irrigation.

Active white mold has been found in fields in the central and southern portions of Wisconsin. Severity levels vary greatly depending on the fields and level of previous infestation by the white mold fungus. We have observed levels ranging from a few plants in spotty areas of a field to widespread damage with plant mortality across the entire field. The latter case was in a field with a history of white mold and frequent overhead irrigation (Figure 2). Application of fungicide for control of white mold is not recommended after the R3 growth stage. However, fields should be scouted and damage noted to facilitate future planting and management decision in that field. Fields with white mold should be harvested after fields that do not have white mold. The black survival structures (sclerotia; resemble rat droppings) of the white mold fungus can be easily spread on combines from one field to the next. If harvesting white mold infested fields last is not feasible, care should be taken to thoroughly clean combine mechanisms where soybean trash and debris can be trapped, between fields. For more information about white mold and management of the disease, click here.  To watch a short video about white mold you can click here.

Other diseases such as brown stem rot, sudden death syndrome, and stem canker have been found at extremely low levels in soybean fields in Wisconsin this season. This situation should be monitored closely as soybeans approach the R6 and R7 growth stages. These two diseases may become more apparent at that time. Again, good record keeping of where these diseases are found can facilitate future management decisions for those fields.

Managing White Mold in Soybean

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Wilting and plant death as a result of Sclerotinia stem rot. Photo Credit: Craig Grau.

Wilting and plant death as a result of Sclerotinia stem rot. Photo Credit: Craig Grau.

Damon Smith, Extension Field Crops Pathologist, University of Wisconsin

Kiersten Wise, Extension Specialist for Field Crop Diseases, Purdue University

Martin Chilvers, Extension Field Crops Pathologist, Michigan State University

Carl Bradley, Extension Plant Pathologist, University of Illinois

Daren Mueller, Extension Plant Pathologist, Iowa State University

Farmers in the Great Lakes area of the U.S. may be concerned about white mold (also called Sclerotinia stem rot) in soybean this year. The disease, caused by the fungus Sclerotinia sclerotiorum, is not common every year in in the Great Lakes region, but farmers that have battled the disease in the past will want to assess the risk of white mold development as soybeans approach flowering (growth stage R1 – plants have at least one open flower at any node).

White mold development is favored by cool, cloudy, wet, humid weather at flowering. The disease is more problematic in soybeans in high-yield environments where high plant populations, narrow row spacing, and an early-closing canopy are commonly used. No single management strategy is 100% effective at eliminating white mold, and in-season options for at-risk fields are limited. For more information on white mold, the disease cycle, and additional management options click here and scroll down to “White Mold.”

There are fungicides available for in-season management of white mold, however not all commonly used fungicides are labeled for use against white mold in soybean. For information on which fungicides are labeled for disease control and recommendations on fungicide efficacy, please click here. Fungicide recommendations are developed by the NCERA-137 national soybean disease committee, and recommendations are based on replicated research data collected from University trials.

In Wisconsin in 2013 numerous products were evaluated for white mold control in soybean. Results of this trial can be viewed by clicking here and scrolling down to pages 6 and 7. Consistent with results of the NCERA-137 research, our Wisconsin research identified several products having a rating of ‘good’ for white mold management, including Aproach, Endura, and Proline. If using fungicides for white mold management, keep in mind that efficacy may be based on the ability of the fungicide to penetrate into the canopy, and the timing of the fungicide application. Fungicides will be most effective at reducing the impact of white mold when applied at, or close to, growth stage R1. Wisconsin research data indicates that fungicides applied up to growth stage R3 (early pod – pods are 3/16-inch long at one of the four uppermost nodes) may be effective, but later applications will likely not be effective at reducing disease. Once symptoms of white mold are evident, fungicides will have no effect on reducing the disease. Fungicide applications for white mold management may be most useful on fields where varieties rated as susceptible to white mold are planted in a field with a history of the disease.

If a soybean field is diagnosed with high levels of white mold, this field should be harvested last. This will help reduce the movement of the survival structures of the white mold fungus by harvesting equipment, to fields that are not infested. Also, be sure to clean all harvesting equipment thoroughly at the end of the season to avoid inadvertent infestation of fields. Rotations of 2-3 years between soybean crops can help reduce the level of the fungus causing white mold in fields. Using corn or small grains crops such as wheat, barley, or oats in rotation with soybean is recommended.

There are several resources available to help farmers and agribusiness personnel manage white mold. Extension plant pathologists across the North Central Region have developed a useful resource in collaboration with the North Central Soybean Research Program to describe the disease and optimal management strategies. This resource, along with downloadable fact sheets, can be found here.

This group also developed a podcast series to facilitate learning about white mold on-the-go.  This series can be accessed by clicking here.

There is also a University of Wisconsin Cooperative Extension video that shows symptoms of white mold and discusses management options for the disease.  The video can be found on YouTube by clicking here.