Should I Spray for White Mold, and if so, When?

​Foliar fungicide applications can be an effective and profitable white mold management tactic provided the following conditions are met:

• Fields have a history of white mold 
• Current and future weather conditions favor white mold development (air temps below 85 degrees Fahrenheit and rainy or overcast)
• Fungicide of choice has a proven performance on white mold 
• Fungicides are applied at the optimum time for white mold suppression

 
As the white mold fungus infects through flowers or fallen flower petals, fungicides for soybean white mold management should be applied at or between the R1 (beginning flowering) and R3 (one pod 3/16” long on the upper four nodes having a fully expanded leaf on the main stem) growth stages. However, we have noted that in some years later or earlier applications within this window favor disease management. The variation in responses to fungicide timing from year to year is driven by the timing of apothecia development. 

Is it possible to scout for apothecia? Not really. Apothecia are small in size (up to ¼ inch) and you don’t need many of them to generate spores to infect plants. Apothecia are also often confused with mushrooms of other fungi such as the birds nest fungus, see figures.
 
To address this variable fungicide timing response and the difficulty in scouting for apothecia, “Sporecaster” was developed. Sporecaster is designed to predict the probability of apothecial presence. However, fields still need to be scouted to determine if the soybean crop meets thresholds such as canopy closure and reproductive stages. 
To use the “Sporecaster” app download it onto your phone from the Apple Store or Google Play. The app allows users to locate and setup multiple fields and run the apothecial risk prediction model using weather data from a third-party provider (Dark Sky API). 

Once Sporecaster is opened you can create multiple fields to determine their apothecial risk. The app will prompt the user for information, such as field name, row spacing, if the field is irrigated and the field location. Then the risk of apothecial presence can be calculated. The model will only run if it is told that flowers are present and if canopy closure meets threshold (for 30” row spacing only). A forecast risk expressed in percentage units is then shown, with red being above the 40% action threshold for a fungicide application. It is possible to rerun the model as desired and even go back to previous years to examine previous risk.
 
What about spraying for other soybean foliar diseases? In general, we don’t see a great deal of other foliar soybean diseases in Michigan. However, southern Michigan may warrant a fungicide application for frogeye leaf spot. Under heavy frogeye leaf spot conditions, we have seen a 20 bu/A protection over an untreated when sprayed at R3. However, this was a field that we had purposefully infected with frogeye and were irrigating to promote disease. Occasionally we will also see Septoria brown spot moving up the canopy. A fungicide application tends to be warranted when Septoria infected leaves are found halfway up the canopy. Most often when not treating for a specific disease, fungicide applications result in a breakeven proposition, however there is variability in response from field to field. If you make the decision to spray, check strips should be included to learn and verify the decision. 

Figure: White mold apothecia and sclerotia on soil surface (Jill Check)

Figure: Very common "bird's nest" fungus, a harmless fungus often confused with white mold apothecia.

Figure: Screen capture from Sporecaster, a phone App for predicting the risk for white mold apothecia (mushrooms) to develop, showing low (blue) medium (yellow) to high (red) risk depending on the location and other field-specific information.