Jonathan Reich and Dr. Syama Chatterton, Lethbridge Research and Development Centre, Agriculture and Agri-Food Canada
PREDICTING WHITE MOULD OUTBREAKS IN DRY BEAN FIELDS
Disease forecasting models have been hugely successful for some crops and diseases, with benefits including increased yields and reduced fungicide use. However, some plant diseases have been fiendishly difficult to predict and manage using disease forecasting methods up to this point. White mould, the most economically important disease in dry bean production in Canada, is one of these difficult to predict diseases and is currently one of the areas of research of a CAP Pulse Cluster project at the Lethbridge Research and Development Centre aimed at optimizing disease management of dry beans.
White mould is caused by the fungus Sclerotinia sclerotiorum, which infects many other crops on the prairies, most notably canola. S. sclerotiorum has a relatively simple life cycle (Figure 1). It spends the vast majority of its life dormant in the soil as small (< 1 cm diameter), hard, black structures called sclerotia. Following a cold period like winter and in the presence of abundant soil moisture, these sclerotia germinate to produce small (< 1 cm in diameter), mushroom-like structures called apothecia. Once mature, a single apothecium releases hundreds of thousands of infectious spores into the air over its two-week life span. The spores are transported on the wind and, once they land on a susceptible host plant, can infect and cause white mould. As the disease progresses in the host plant, S. sclerotiorum develops sclerotia which drop to the soil and remain there until conditions are conducive to germination once again.
Despite its well-understood biology — at least in lab conditions — the spread and growth of S. sclerotiorum under field conditions has been much more difficult to understand. In some years or locations, it causes devastating disease, while in other years, it is hardly noticeable. This seemingly sporadic appearance of disease is one reason why white mould epidemics have been so difficult to predict. Many factors are known or hypothesized to contribute to disease development under field conditions (e.g., irrigation, soil type, crop rotations, bean variety, fungicide applications), but these factors do not always explain white mould levels in bean fields.
We hypothesized that the missing piece of this puzzle is the levels of airborne spores present in a field throughout the growing season. Simply put, fields with no spores will have no disease and fields with lots of spores will have lots of disease.
So, how do you sample airborne spores? It turns out there are lots of ways to collect air samples, but we are using cyclone samplers manufactured in the U.K. (Figure 2) — the birthplace of research into airborne microbes. The cyclone samplers are essentially vacuums that suck in air and deposit all particles, including spores of S. sclerotiorum, into a small vial. The vial is taken back to the lab for DNA extraction and quantification of S. sclerotiorum DNA. Collaborators in Manitoba and Ontario are replicating this procedure in each of those provinces to make this a pan-Canadian research project.
What have we found so far? First, as expected, white mould disease surveys in southern Alberta show that the disease is widespread but varies significantly in its intensity between fields. Many fields had no symptoms of white mould, but highly infected fields had almost every plant infected. Second, because the fungus only releases spores at one point in its lifecycle, we expected to see a single spike in the number of spores over the course of the growing season but found the opposite: S. sclerotiorum spores are commonly present throughout the growing season (Figure 3). This could be because microclimates cause the fungus to grow at different rates or that the spores are coming from other fields with different environments.
Some evidence comes from the trends in the three provinces where mean daily ascospore numbers were highest in southern Alberta (irrigated production) compared to Manitoba and Ontario (Figure 4). Unexpectedly, there is no clear relationship between the number of ascospores present in a field and the final disease level in that field.
This last finding has prompted us to continue expanding the search for contributing factors to disease development. In addition to the surveys and air monitoring, we are performing more in-depth interviews with growers to tease out management practices that could be contributing to differences in disease levels between fields. For instance, it is no secret that certain market classes of beans
are less susceptible to white mould, findings which were supported in the 2020 growing season. The 2021 field season will be the most comprehensive yet, with researchers hoping to find trends that will greatly enhance white mould prediction in dry beans.
EFFECT OF SEED TREATMENT AND SEED SOURCE ON BACTERIAL DISEASES OF DRY BEANS
The other activity in our Pulse Cluster project is evaluating whether some new experimental seed treatments and seed source will reduce bacterial diseases of dry beans. With the loss of streptomycin for agriculture usage, there are no seed treatments available for seed-borne bacterial pathogens.
Field trials were conducted in 2019 under irrigation in Vauxhall, AB; Harrow, ON; and Morden, MB. Seven cultivars were used in this trial — AAC Explorer, AC Black Diamond, AAC Black Diamond 2, L16PS461 and AC Island were sourced from Alberta and Idaho, and Envoy and Portage were sourced from Manitoba and Idaho. While Portage and Envoy were only planted in Morden and Harrow, AC Island was only seeded in Vauxhall. All seed batches were treated with three experimental seed treatment products or not treated control. Plots were assessed for four bacterial diseases: halo blight, common bacterial blight, bacterial brown spot and bacterial wilt (Figure 5) over the growing season.
Bacterial disease progression in the small-plot field trials was low, likely due to the hot and dry growing conditions in Vauxhall and Harrow in 2019. There were higher disease levels in Morden, but the severity and incidence were still below economic threshold levels. At Morden, halo blight occurred early in the season (mid-June) in 5–10% of plants, but at very low severity. Common bacterial blight appeared in late July to early August in 15–20% of plants, but disease severity was also low (less than 5% of leaf surface with lesions, on average). Due to low disease incidence and severity, there were no differences in bacterial disease levels or yields observed due to seed source or seed treatment (Figure 6). The trials will be repeated in 2021 and 2022 with hopefully higher bacterial disease pressure.