Time to Consider Your White Mold In-Season Management Plan

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

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

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 hereTo 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

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.