By Sam Markell
Extension Plant Pathologist, Broad-leaf Crops
Are Conditions Favorable for White Mold?
White mold is a concern in most broadleaf crops once they enter bloom. Canola is blooming in some of the state and soybeans and dry edible beans are just entering bloom or will be shortly. Assessing your risk for white mold is very important in managing the disease.
How does white mold occur?
Sclerotinia survives in the soil as sclerotia; hard, black structures. When there is ample soil moisture, at least 1 to 2 inches of water a week or two before bloom, the sclerotia will germinate, produce apothecia (little mushrooms), and release ascospores.
Once spores are released, they need to land on a nutritional source to begin the infection process; usually the flower petals. This is the reason that most broadleaf crops are considered susceptible once they begin to flower. Once the flower petals become colonized, the pathogen easily penetrates the plant and produces the characteristic light tan / white lesion (it looks like dry bone), takes on a shredded appearance, and black sclerotia are produced.
What are favorable conditions for white mold?
- Soils need to be moist before bloom. Generally, 1-2 inches of rain falling in a 1-2 week period before plants enter bloom is the minimum needed for sclerotia to germinate, produce apothecia, and release ascospores.
- Moderate temperatures and wetness during bloom. High temperatures above 85 degrees F inhibit disease. In years where we hit the 90’s F during bloom, we rarely have white mold. Sclerotinia infection and development best when daytime highs are cooler; 60’s- 70’s F. The long range forecast is for much of the state is forecast to be warm – good news.
- Similarly, the canopy needs to be wet. Rain, fog, and heavy dews during bloom are all favorable for disease. Paying attention to the long term rain forecast is important if deciding to make a fungicide application.
- Canopy density and canopy closure make a big difference on the environment in the field. Once canopy closure occurs, the crop is likely to have a more favorable environment for infection and disease development.
- Although not environmental, crop rotation and white mold history make a difference. A field with a history or white mold and short rotations among broadleaf crops is more likely to have white mold problems than a field with no white mold history and/or long crop rotations.
- Crop makes a difference. Not all broadleaf crops are equally susceptible to white mold. Sunflowers and dry edible beans consistently seem to be very susceptible, and little resistance is available. Similarly, canola can be hit hard when the environment is favorable. Soybeans can be infected, but they typically do not experience the yield loss the other crops do. Additionally, some varieties of soybean are much less susceptible to white mold than others.
How do you manage white mold with fungicides?
Fungicides can help manage the disease and on some crops they can be very effective. Dry bean applications can be very beneficial in favorable environments; canola application can be as well. Fungicide applications to soybeans are much more variable however and favorable economic returns are less common. Part of the reason is soybeans are naturally less susceptible than dry beans or canola. Sunflower is very susceptible to white mold, but fungicides are not recommended because they are generally not effective at reducing disease.
If you choose to make a fungicide application, timing is very important. Applications made relatively early in the bloom stages (20-50% flower in canola, R1-R2 in dry beans) are preferred because it helps manage infections that can occur right after the plant enters bloom. The early infections tend to do the most damage because they have the greatest time to develop through the season. Later applications may also prevent infections, but those later infections do less damage.
What resources are available?
Excellent data on white mold applications on soybeans and dry beans and other pathosystems can be found through the Carrington Research Extension Center website. Carrington has one of the best (perhaps the best) white mold research programs in the world. I would strongly recommend visiting this site.
Colorado and Nebraska have developed a fungicide decision checklist for dry beans.
The NDSU canola sclerotinia risk map uses environmental conditions favorable for sclerotinia, so it can be helpful for all broadleaf crops.