Assessing the risk of white mold (Sclerotinia stem rot) of soybean in 2014

White mold of soybean (a.k.a. Sclerotinia stem rot), caused by the fungus Sclerotinia sclerotiorum, is a disease that can occur in the northern half of the state in cool, wet years.  The most recent, widespread white mold epidemic in Illinois occurred during the 2009 season, where several fields in the northern half of the state were affected.  In some of the northern-most areas of Illinois, white mold can be considered a more consistent problem.

The white mold fungus overwinters in the soil as, small, black, and dense structures known as sclerotia.  These sclerotia germinate and form mushroom-like structures known as apothecia when soil remains moist for several consecutive days and soil temperatures are at 60 degrees F or below.  These apothecia generally will not form until the soil is shaded from sunlight due to soybean canopy closure.  Spores of the white mold fungus are shot out of the apothecia and land on senescing flower petals, where infections first occur on the soybean plants.  The white mold fungus becomes inactive when temperatures within the soybean canopy are above approximately 82 degrees, so infection and disease development may cease or slow down during periods of hot (above 82 degrees) and dry weather.

So, what does the risk of white mold look like for 2014?  This is not an easy question to answer.  In general, rainfall has been consistent in the northern portion of the state, which would favor white mold, but recent temperatures in the 80s and 90s have not been favorable.  However, the short-term weather forecast shows cooler temperatures (60s and 70s).  If a cool and wet trend continues throughout soybean flowering, then the risk of white mold will be elevated.

In University of Illinois research trials, some fungicide products have shown efficacy against white mold.  Foliar fungicides will not provide complete control of the disease, but may reduce disease.  The results of University of Illinois trials conducted in 2009, 2010, and 2013 are shown in Tables 1-3.  Note that some of the more popular, frequently marketed fungicides are not listed in the tables since many do not have white mold on their label because of no to poor efficacy.  In these trials, the primary targeted growth stage to apply foliar fungicides was at R1 (beginning flower).  In some cases, R1 may occur before canopy closure.  If this is the case, then an application at canopy closure (rather than R1) might be more effective in protecting against white mold.  Also note that some treatments in these research trials were applied twice during the season.

Table 1. Results of soybean foliar fungicide research trials focused on white mold conducted in 2009 at the University of Illinois Northern Agronomy Research Center (DeKalb County).

*All treatments were applied at the R1 growth stage (July 20, 2009).  Treatments followed by “(2x)” were applied again 9 days later.

**Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly differ from one another.

Table 2. Results of soybean foliar fungicide research trials focused on white mold conducted in 2010 at the University of Illinois Northern Agronomy Research Center (DeKalb County).  Funded in part by the Illinois Soybean Association.

*All treatments were applied at the R1 growth stage (July 10, 2010).  Treatments followed by “(2x)” were applied again 7 days later.

**Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly differ from one another.  “NS” indicates that no treatments were significantly different from each other.

Table 3. Results of soybean foliar fungicide research trials focused on white mold conducted in 2013 at the University of Illinois Northern Agronomy Research Center (DeKalb County). Treatments were applied at the R1 growth stage unless indicated otherwise.

*Incognito was applied alone at the R1 growth stage and was followed by Orius applied alone at R3.

**Proline was applied alone at the R1 growth stage, and was followed by Stratego YLD applied alone at R3.

***Least significant difference (alpha level = 0.05).  Treatment values that differ by this number can be considered significantly differ from one another.  “NS” indicates that no treatments were significantly different from each other.

Overall, the highest level of white mold control will be achieved when several management practices are integrated (i.e. choosing the most-resistant varieties, utilizing recommended seeding rates, applying a foliar fungicide, and applying a biocontrol product).  For more information about white mold and management of this disease, go to http://www.soybeanresearchinfo.com/pdf_docs/WhiteMold_NCSRP.pdf, where a 7-page publication on white mold (developed in 2011) can be downloaded.