Sclerotinia sclerotiorum (Lib.) de Bary is a filamentous fungus that causes above-ground disease symptoms on many field and horticultural crops. This pathogen is so omnivorous that there are over 60 disease names on more than 300 plant species. In many cases, Sclerotinia diseases are difficult to manage and the effects are highly destructive. As a result, this pathogen is responsible for millions of dollars in crop losses annually.
One of the most damaging diseases caused by S. sclerotiorum is white mould on dry beans (Figure 1).
This disease cycle begins in June or July when dense, melanized resting structures in the soil (called ‘sclerotia’) germinate to form small mushroom-like structures called an apothecia. The apothecia forcibly discharges ascospores which become airborne, and drift short distances on the wind, hoping to find a suitable host tissues to colonize. Senescing bean flower petals are the most commonly exploited initial food source in bean fields, and once growing on a decaying petal, the fungus can begin attacking green tissues like leaves, stems and pods.
The fungus must kill the host tissues in order to absorb nutrition from the plant, and it can move quickly through the crop. This is why infections can appear suddenly and be highly destructive. As the dead tissues are consumed, the fungus produces more sclerotia which eventually drop to the soil and remain inactive until suitable conditions coax them out of dormancy.
Two of the main pillars of crop disease management are crop rotation and disease resistance and many diseases can be successfully managed with these two tools. Unfortunately, these two are not sufficient for management of white mould because of the pathogen’s extremely broad host range, ability of sclerotia to survive dormant in soils for two to five years, and the lack of resistant germplasm available to plant breeders. As a result, fungicide applications are a critically important in managing white mould.
The fungicide vinclozolin was the go-to product for managing white mould on dry beans for many years, until 2005, when it’s use on beans was revoked by Health Canada’s Pest Management Regulatory Agency. Over the next 10 years after the loss of vinclozolin, many new fungicides were registered for control or suppression of white mould in Canada. The products varied in active ingredients, and in price. Naturally, bean growers wondered if they varied in efficacy, so we evaluated seven registered fungicides in side-by-side comparisons in replicated, small-plot trials for three years at Brooks, AB.
The results of the three years are shown in Figure 2.
Our studies confirmed much of what we already know:
- Weather (primarily rainfall) is the primary driver of white mould incidence and severity.
- In 2016 there was plenty of rainfall and therefore high levels of disease leading to low yields. In 2016 there were significant yield responses to fungicide applications.
- In 2015, a dry year, there was very low disease pressure and very high yields. There was very little response to fungicide application and no statistically significant differences.
- In 2014, the season was dry early, but turned wet late. As a result, high levels of mould developed near the end of the season, but yields were only affected by small amounts. Additionally, there was no significant yield responses to fungicide applications.
- Treatments with the lowest disease did not always have the highest yield, indicating that factors other than white mould were contributing to yield.
- Lance was a top performer at reducing disease in all three years, regardless of the environment. All the other fungicides varied in performance depending on the environmental conditions. There were no statistically significant differences between fungicides in 2014 or 2015.
These results indicated that there were some years that fungicide applications were not needed because there was little to no disease pressure. Similarly, there were some years where disease came late enough that yield loss was small, and a fungicide application during the bloom period did not protect against any yield loss. Finally, there were years where disease potential could cut into yield potential, and a fungicide application was well worth the effort. It is during these years (like 2016) where a fungicide application can protect as much as 800 kg/ha in yield.
Finally, it is important to keep in mind that the data presented here is only three site-years of data, only one of which had significant disease pressure that threatened yield. As a result, major decisions regarding fungicide selection should be made cautiously. All of the fungicides showed the ability to reduce disease and/or improve yield, and in two of the three years there were no significant differences between them.
Therefore, it is recommended to first focus on the decision to spray or not, and the timing of the application, since they will be much more important than which fungicide is chosen.
When considering these results, and other information that is known about white mould, some recommendations are:
- Always employ foundational cultural practices to minimize risk, such as good crop rotation (three-year break between highly susceptible crops like beans, sunflowers and canola), and choosing cultivars with the best tolerance or ability to avoid disease.
- Some years may not require a fungicide, but always be ready to apply one. It’s easier to call off an application last minute than it is to initiate one.
- Monitor white mould risk as the crop is coming into flowering (humidity and rainfall) to help determine the current risk.
- Check weather forecasts regularly to know if a rain event is forecast.
- When it appears that the environment could drive disease potential to the point that it may threaten yield potential, one or more fungicide applications can protect against yield loss to white mould.
- If you really want to know if your fungicide made you money, leave an untreated check strip that allows you to calculate how much yield/money you gained (or lost) with your fungicide application.
- Keep detailed records for each field. Over time, these records may help you predict risk for individual fields.
MPSG’s On-Farm Network Highlights
Foliar fungicides have been evaluated for pinto and navy beans at nine sites in the On-Farm Network since 2016. A single application of foliar fungicide was applied at R2 (early pin bean) and compared with an untreated control.
On average, a single application of foliar fungicide on dry beans significantly increased yield by 55 lbs/ac when compared with the untreated check. With low rainfall in recent years, white mould disease pressure has not been severe in these trials.
Monitor weather conditions as flowering approaches to determine your white mould risk. To aid in spray decisions, refer to MPSG’s Fungicide Decision Worksheet for Managing White Mould in Dry Beans.