Disease Surveillance in Soybeans, Peas, and Dry Beans and On-Farm Fungicide Evaluation Results
Laura Schmidt, Production Specialist – West, MPSG
SURVEILLANCE OF CROPS across the province helps us track diseases of concern over time and keeps us abreast of any emerging disease issues to inform research priorities and advise on areas that may need further investigation.
A representative sample of soybean, field pea and dry bean fields are surveyed each year for root, foliar
and stem diseases across Manitoba. These surveys are a collaborative effort between Agriculture and Agri-Food Canada (AAFC), Manitoba Agriculture and Manitoba Pulse & Soybean Growers.
The On-Farm Network (OFN) has also tested foliar fungicides in soybeans, peas and dry beans over the past several years to evaluate the frequency of yield response to these products under a range of environments.
2022 Disease Survey Results
In 2022, 54 soybean fields in Manitoba were surveyed for foliar and stem diseases at R4 (full pod) to R5 (beginning seed). Roots were collected and submitted to AAFC for root disease analysis. Soybeans were visually assessed for infection by bacterial blight, Septoria brown spot, downy mildew, frogeye leaf spot, northern stem canker, white mould, pod/stem blight and anthracnose.
Bacterial blight and Septoria brown spot were the most common foliar diseases in soybeans, infecting 91% and 81% of fields surveyed, respectively (Figure 1). Severity levels of these two diseases were below 1 (scale 0–5), indicating that only trace symptoms of disease were found. Root rots were found in every field surveyed and had an average severity of 3.8 (range: 3.4– 4.2). Phytophthora root rot (PRR) was confirmed in 11% of fields surveyed.
Northern stem canker was the most common stem disease in 2022 infecting 15% of surveyed fields. White mould was not found during the survey but was more common to find in soybean fields in 2022 than in previous years.
Phytophthora Root Rot Pathotype Identification
In 2022, agronomists piloted a commercial soil test from AYOS technologies to identify PRR pathotypes in 11 farmer’s fields with suspected PRR presence (Table 1). Of those 11 fields where PRR was confirmed, 100% of fields had PRR pathotypes that overcame soybean Rps genes 1a and 1c. Rps genes 1k and 3a were defeated at 55% and 64% of fields tested, respectively, and Rps 6 was defeated at 27% of PRR-infected fields. Soil tests to identify PRR pathotypes present in fields planned for soybeans will be an important tool to inform variety selection so farmers can choose effective Rps genes that are resistant to the PRR pathotypes present in a given field.
On-Farm Evaluation of Soybean Fungicides
Since 2014, 66 replicated and randomized field-scale trials have evaluated a single application of foliar fungicide in soybeans vs. none through the On-Farm Network. The main disease target for fungicide application in soybeans is white mould. Over the history of these trials, a single application of fungicide has resulted in a significant soybean yield response 17% of the time (Figure 2). Only six of those significant trials (9%) have resulted in a positive return on investment where the yield increase was great enough to cover the cost of the fungicide.
To determine if a soybean crop needs a fungicide application, evaluate the likelihood of white mould development. Risk is greatest if warm, humid conditions persist around flowering (15-25°C and 1-2 inches of rain within 1-2 weeks of flowering), if the crop canopy is thick
and dense (plant populations above 180,000 plants/ac on narrow rows), on fields where there are several susceptible broadleaf crops in rotation (canola, dry beans and sunflowers) and if the previous crop had heavy white mould pressure leading to more sclerotia carryover.
2022 Disease Survey Results
In 2022, 49 pea fields in Manitoba were surveyed for root diseases at R1 (early flower) and 48 fields were surveyed for foliar and stem diseases at R4 (full pod). Soil samples were collected for Aphanomyces root rot detection.
Figure 3 shows root rot was found in all pea crops with an average severity of 4.2 (range: 1.4–7.4) on a scale of 0–9. Fusarium root rot was the most common root disease and detected in every field. Aphanomyces root rot was also detected in 98% of fields in 2022. (Read more about this disease on page 44).
Mycosphaerella blight was the most common foliar disease, found in 100% of fields. Severity of Mycosphaerella blight was 2.2, on average, (range: 1.0–5.1) on a scale of 0–9. Bacterial blight was present in 83% of fields and downy mildew was found in 31% of fields. White mould was found at trace levels in only 4% of pea fields.
Small-Plot Fungicide Comparison Trials
Beginning in 2022, MPSG has initiated product evaluation and comparison trials to generate simple, straightforward results for farmers. At the Parkland Crop Diversification Foundation at Roblin and at AAFC-Portage la Prairie sites, small-plot pea fungicide trials compared Delaro 325 SC, Miravis Neo 300 SE, Dyax, Acapela and an experimental BASF fungicide. These trials were conducted in partnership with Assiniboine Community College (ACC) and yield results are expected soon.
At these two sites, samples of infected peas were taken to test if the Mycosphaerella blight pathogen populations were resistant to Group
11 (strobilurin) fungicides. At Portage, Group 11 resistance was not detected in the pathogen. However, at Roblin, 20% of the Mycosphaerella blight samples were resistant to Group 11 fungicides. This confirms that the resistance mutation to Group 11s is present in some Mycosphaerella disease populations in Manitoba. Most pea fungicides contain multiple active ingredients and are still effective against this disease but using a Group 11 fungicide alone is discouraged.
On-Farm Evaluation of Field Pea Fungicides
Since 2017, 44 On-Farm Network trials have investigated pea yield response to foliar fungicide applications. Of those trials, 25 have compared a single application vs. none and 16 have compared a single vs. double application.
Among the 25 trials comparing a single application of fungicide at early flower vs. untreated strips, there have been seven statistically significant yield responses. A single fungicide application increased pea yields 28% of the time over no application (Figure 4). Yield increases ranged from 1.4-12.5 bu/ac (average: 4.6 bu/ac). Assuming a product cost of $21.25 and a pea sell price of $10/bu, five of the seven significant trials were economical, providing a return on investment of $1.75- 104.08/ac (average: $36.05/ac).
During the dry years of 2019, 2020 and 2021, it was more common to ask if
a fungicide application was necessary at all due to the dry growing conditions. In those years, risk of disease development was low, leading to fewer instances where foliar fungicides paid.
Among the 16 trials comparing two fungicide applications to a single pass, there have been seven statistically significant yield responses (Figure 5). Two fungicide applications increased pea yield 44% of the time, improving yield by 5.1 bu/ac, on average (range: 2.7–7.1 bu/ac). Considering the same economic assumptions as above, all seven of those yield responses were economical, providing an average return on investment of $29.70/ac (range: $5.75–50.15/ac).
To determine if a fungicide application is likely to be beneficial in-season, consult MPSG’s Fungicide Decision Worksheet for Managing Mycosphaerella Blight in Field Peas.
2022 Disease Survey Results
In 2022, 27 dry bean fields in Manitoba were surveyed during mid-July to early August for root diseases and during late August for foliar and stem diseases (Figure 6).
Fusarium root rot was detected in 100% of fields surveyed with an average severity of 4.0 (range: 3.4–5.2) on a scale of 0–9. Common bacterial blight was the most common foliar disease, found in every dry bean crop surveyed with an average severity of 1.1 (range: 0.3–2.7)
on a scale of 0-5. Bacterial brown spot, a disease that was added to the survey this year, was the next most common foliar disease observed in 89% of fields with an average severity of 1.6 (range: 0.7–3.0) on a scale of 0-5. White mould was found in 57% of dry bean fields and the average percentage of plant tissue infected (stems and pods) was 6.5%.
On-Farm Evaluation of Dry Bean Fungicides
Since 2016, a single application of foliar fungicide applied at R2 (early pin bean) has been compared to untreated dry beans at 16 On-Farm Network trials. White mould and anthracnose are the main disease targets of foliar fungicides in dry beans.
Until 2022, trial sites were dry and did not have white mould disease pressure present, resulting in no statistical difference between treated and untreated strips. In 2022, there was a positive yield response of 175 lbs/ac to fungicide application, however, no white mould nor anthracnose were detected in the trial at R4 when disease ratings were taken (Figure 7).
To determine if a fungicide application is likely to be beneficial in your dry bean fields, use the Fungicide Decision Worksheet for Managing White Mould in Dry Beans to asses disease development risk. A research project led by Dr. Baljeet Singh at ACC has been underway to develop a weather-based fungicide application decision support tool for dry bean farmers in Manitoba.