Managing White Mould
The main disease controlled by fungicide in dry beans is white mould (Sclerotinia sclerotiorum) (Figure 1). Fungicides can also offer control or suppression of anthracnose, powdery mildew and rust, depending on the product. However, these diseases are less frequent and less severe in Manitoba-grown dry beans. White mould has a wide host range, infecting other broadleaf crops such as canola, soybeans and sunflowers.
Foliar fungicides for suppression or control of white mould are preventative, meaning they must be applied before disease symptoms are visible. The R2/early pin bean stage is the best time to apply fungicide for effective white mould control. This stage coincides with flowering (100% of plants with an open blossom) and early pod development (first pin-pods are 0.5 to 1″ long).
If warm, humid conditions persist throughout flowering and pod development, consider a second fungicide application 7 to 14 days later.
The Fungicide Decision Worksheet for Managing White Mould in Dry Beans (also available in the Bean App) was created to help determine the risk of white mould disease development, based on environmental and agronomic factors.
If you are on the fence about applying fungicide and find yourself in the “moderate risk” category according to the decision worksheet, consider leaving one more more check strips to determine the effectiveness of fungicide in a given year.
Another option would be to conduct an on-farm trial by following the protocol on our On-Farm Network page.
White Mould (Sclerotinia) Life Cycle
- The life cycle begins with fungal sclerotia bodies that overwinter in the soil, from which apothecia (small, mushroom-like structures) form. Apothecia release ascospores in to the crop canopy, infecting blossoms and other dead or senescent plant tissues.
- Warm (15 to 25°C), wet soil about ten days before flowering favours the development of apothecia. Prolonged plant surface wetness for 40+ hours is favourable for infection.
- Disease development begins lower in the crop canopy. Leaves, branches, stems and pods can all show symptoms. A thick plant canopy will contribute to disease development, as it helps maintain ideal moisture and temperature conditions for white mould.
- Crop rotation including non-host crops, upright plant architecture, resistant varieties, tillage and wider row spacing can help reduce white mould disease development.
Dry Bean Disease Survey
In 2020, 39 dry bean fields were surveyed for foliar and stem diseases during mid-August when plants were starting to mature. Most fields were in southern Manitoba and 10% of surveyed fields were outside of the traditional bean-growing regions. Fields were visually assessed for common bacterial blight, halo blight, white mould, anthracnose and rust.
Common bacterial blight was the most prevalent foliar disease in Manitoba (Table 2). It infects plants following any kind of tissue abrasion or damage from strong storms. Foliar products are available, but their effectiveness has been variable so far and they often require multiple applications throughout the season. Note that fungicides will not control this bacterial disease.
White mould prevalence and severity in dry bean fields has been low in the last three dry years (2018-2020). Whether fungicide applications will be warranted in 2021 will depend on the yield potential of the crop and the conditions surrounding flowering.
Fungicide application timing for dry beans is in July at the start of flowering to protect plants from the potential spread of sclerotinia ascospores through flower petal drop. Intuitively, the amount of dry bean crops infected by white mould follows the trend of July precipitation year to year (Table 3). High rainfall in 2015 and 2016 led to greater disease levels, and it is likely that sclerotia bodies produced from those crops led to higher disease loads in 2017. Looking at the dry years that followed, disease levels were brought down.
For 2021, use rainfall amounts leading up to and during dry bean flowering to anticipate the development of white mould in your fields. Consider field history of host crops and disease pressure. If white mould levels were high in previous crops, the carryover of sclerotia bodies in the soil will increase your risk of disease development. Also scout for signs of disease development ahead of fungicide application and use the Fungicide Decision Worksheet for Managing White Mould in Dry Beans.
On-Farm Network Evaluation of Foliar Fungicide
Since 2016, 14 replicated and randomized trials have been conducted to evaluate foliar fungicide application in dry beans through the On-Farm Network. White mould and anthracnose are the disease targets of foliar fungicide in dry beans, but white mould is the main concern. Anthracnose was not found during the 2020 survey and has not been a recent issue for dry beans due to variety resistance.
Over the duration of these on-farm trials, we have not seen any statistically significant yield responses to foliar fungicide in dry beans. Why? Trial sites were dry and did not have any white mould pressure. If the disease pressure is not there, fungicide will not provide much of a service. It should also be noted that dry beans in these trials were grown on wide rows (30 inches). These trials will continue in 2021 to capture more locations and environments.
Additional Resources
Are Foliar Fungicide Applications Necessary in Dry Beans to Control White Mould in a Dry Year? – Pulse Beat article by Greg Bartley, MPSG On-Farm Network Specialist
White Mould Fungicide Efficacy Research in Dry Beans – Pulse Beat article by Michael Harding, Alberta Agriculture and Dr. Syama Chatterton, Agriculture and Agri-Food Canada
Fungicide Efficacy Testing Results from NDSU’s Carrington CREC
Improving the Management of White Mold in Dry Beans – 2018 Bean Day presentation by Dr. Michael Wunsch, North Dakota State University
Optimizing fungicide application timing – 2019 presentation by Dr. Michael Wunsch, North Dakota State University