Assessing the risk of white mold (Sclerotinia stem rot) of soybean in 2013

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 white mold epidemic in Illinois occurred during the 2009 season, where several fields in the northern half of the state were affected.

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 and dry weather.  The 2013 season started out similar to the 2009 season, with frequent rainfall and cool temperatures, but warmer and drier conditions have been observed more recently.

So, what does the risk of white mold look like for 2013?  This is not an easy question to answer since we have had fairly favorable weather for white mold initially with some stretches of non-favorable weather in-between.  In fields that were not planted late and have been in full-canopy for the last couple of weeks, it is likely that apothecia have developed and that some infections may have occurred already.  However, the progression of infection and disease development likely came to a stop with the hot and dry conditions observed last week.  For fields planted later that just recently closed their canopy (or have not yet closed their canopy), apothecia likely have not emerged because of the hot dry weather experienced last week.  The weather from this point forward will likely dictate how bad white mold will be in 2013, although, I believe that it is safe to say that it will be less severe and less widespread than it was in 2009.

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 and 2010 are shown in Tables 1 and 2.  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).  Because of the late-planted soybean fields this year, 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 (R1 and again 7-9 days later).

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 different 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 different 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.planthealth.info/whitemold_basics.htm, where a recently-developed 7-page publication on white mold can be downloaded and several podcasts on white mold can be accessed (all funded by the North Central Soybean Research Program).