Cassandra Tkachuk, Research Specialist, MPSG – Spring 2022 Pulse Beat
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. Below are the results from the 2021 surveys.
The On-Farm Network (OFN) has also tested foliar fungicides in soybeans, dry beans and field peas over the past several years to evaluate the performance of these products under a range of environments. A summary of the long-term fungicide results can be found within each section below.
Soybean Disease Survey
IN 2021, 60 soybean fields in Manitoba were surveyed for root diseases, including Fusarium, Pythium and Rhizoctonia root rots, and disease isolates from 40 fields were tested. A total of 67 fields were surveyed for Phytophthora root rot and suspect samples were collected. Surveying took place from mid-July to mid-August and Phytophthora assessment extended into early September. At least 10 plants were uprooted at each of three random sites within a field and taken back to the AAFC–Brandon lab for root disease identification.
Fusarium root rot was the only root disease confirmed in 2021. It was present in all 40 fields at an average severity of 4.7, on a scale of 0–9 (Table 1). Despite the prevalence and relatively high severity of Fusarium (yield loss incurred at a rating of 4), root rot pressure was exceptionally low in 2021 due to dry conditions. Root rot-infected plants were also typically isolated to low spots within a field, meaning the incidence of Fusarium was low. Take note of root diseases in your fields moving forward, as they persist in soil and crop residue, and can be significant yield robbers under wet conditions.
A total of 58 soybean fields were surveyed in 2021 for foliar and stem diseases at the R5–R6 stages. 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.
Septoria brown spot and bacterial blight remain the most common foliar diseases of soybeans, found in 81% and 67% of fields across Manitoba, respectively (Table 2). However, the severity of these diseases stayed low in 2021, meaning there was minimal impact on yield. Downy mildew was present in as many as 25% of fields in the southwest region, but it was not detected in all regions and its prevalence and severity were low overall.
Frogeye leaf spot was found in one field within the central region and one in the southwest, and white mould was found in only one field in the eastern/Interlake region (data not shown).
ON-FARM EVALUATION OF SOYBEAN FUNGICIDE
Since 2014, 66 replicated and randomized field-scale trials have been conducted through the OFN to evaluate foliar fungicide in soybeans. The main goal for fungicide application in soybeans is to control white mould. However, white mould (Sclerotinia) is not the yield robber in soybeans that can be in other crops, and the continuous flowering period of indeterminate soybeans can make control difficult.
According to the aggregated on-farm results, there were no soybean yield responses to fungicide 83% of the time (55/66 trials), statistically significant yield increases 17% of the time (11/66 trials) and economical yield increases only 9% of the time (6/11 significant yield trials), where the yield bump was great enough to cover the cost of fungicide.
The best strategy for making an economical fungicide decision is to determine the likelihood of disease development each season and in each field, since profitability depends on the presence of white mould. Disease is likely to develop if conditions are wet and cool (<21°C) leading up to flowering, if your field has a history of white mould pressure and if there are signs of disease development in the current crop (e.g., apothecia on the soil surface ahead of flowering). ■
Dry Bean Disease Survey
IN 2021, 40 dry bean fields were surveyed for root, foliar and stem diseases in Manitoba. Root diseases included Fusarium, Pythium and Rhizoctonia root rots, foliar diseases included common bacterial blight, halo blight and rust, and stem diseases included white mould and anthracnose. Most surveyed fields were in the traditional bean-growing regions of southern Manitoba and 10% of fields were outside of these regions.
Fields were surveyed for root disease during mid- to late-July at the early flowering to beginning seed stages. At least 10 plants were sampled at each of three random sites within each field. Fusarium root rot was the only root disease detected in 2021, found in all 40 fields at an average severity of 4.5, on a 0–9 scale (Table 3). A severity rating of 4 is the level at which yield loss is incurred.
Fields were visually assessed for foliar and stem diseases in mid-August when plants were starting to mature. Common bacterial blight (CBB) was the most prevalent disease, found in 98% of fields at an average severity of 1.3, on a scale of 0–5 (Table 3). Foliar products are available for CBB, but their efficacy is variable and often require multiple applications. Halo blight was found in 5% of fields and white mould in 3% — both at very low severity levels. Rust and anthracnose were not detected.
ON-FARM EVALUATION OF DRY BEAN FUNGICIDE
Since 2016, 16 replicated and randomized trials have been conducted to evaluate foliar fungicide application in dry beans through the OFN. White mould and anthracnose are the disease targets of foliar fungicide in dry beans, but white mould is typically the main concern. Anthracnose was not found during the 2021 survey and has not been a recent issue for dry beans due to variety resistance.
Over the duration of these on-farm trials (2016–2021), we have not seen any statistically significant dry bean yield responses to foliar fungicide due to dry conditions across sites and minimal white mould pressure. It is important to note that most bean fields tested in these trials were grown in wide rows (30 inches). Dry bean fungicide trials are planned to continue in 2022 to capture more locations and environments.
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. The amount of July precipitation tends to coincide with the amount of crops infected by white mould. White mould pressure was more severe in the wetter year of 2016, compared to subsequent, drier years with less disease pressure (Table 4).
Assessing rainfall amounts leading up to and during dry bean flowering is one way to anticipate white mould development in your fields. Also consider field history of host crops and disease presence. If past white mould levels were high, 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. ■
Field Pea Results
IN 2021, 46 pea fields were surveyed for root diseases and 41 fields were surveyed for foliar diseases during mid- to late-July at the R3 to R4 (flat to full pod) stages. Foliar diseases included Mycosphaerella blight, bacterial blight, downy mildew, white mould, powdery mildew, anthracnose, rust and Septoria leaf blotch.
A detailed summary of 2021 results can be found in Table 5 and a comparison of disease results from 2016 to 2021 in Table 6. Fusarium root rot was the most common root disease, found in all surveyed fields at an average severity of 3.1, on a scale of 0–9. Rhizoctonia was not detected in any fields and Aphanomyces data is unavailable for 2021 due to ongoing pandemic constraints. MPSG is committed to gaining a better understanding of Aphanomyces in the province moving forward.
Mycosphaerella blight was the most prevalent disease and has been found at some level of severity in every surveyed pea field since 2016 (Table 5). Downy mildew was found in 12% of fields and bacterial blight in 7%. However, disease pressure and severity were low overall due to the widespread dry conditions.
ON-FARM EVALUATION OF PEA FUNGICIDE
Since 2017, the OFN has conducted 31 randomized and replicated, field-scale trials across Manitoba to evaluate foliar fungicide in peas. To date, there have been 20 single vs. no fungicide trials, one double vs. none, two double vs. single vs. none and eight double vs. single. Mycosphaerella blight and white mould are the targets of foliar fungicide in peas, but Mycosphaerella is usually the main concern.
According to aggregated data comparing peas with and without fungicide (including single vs. none, double vs. none and double vs. single vs. none), there have been no yield responses to fungicide 74% of the time (17/23 trials), statistically significant yield increases 26% of the time (6/23 trials) and economic yield increases 9% of the time (2/6 significant yield trials), where the yield bump at least covered the cost of the product. Sites experienced mostly dry conditions over the years, which contributed to the lack of yield responses.
When conditions are conducive to disease development, that is when one might consider a second fungicide application. The aggregated results from double vs. single trials tell us that double application resulted in no yield increase 62% of the time (5/8 trials), a statistically significant yield increase 38% of the time (3/8 trials), and in all three of these cases, the yield bump was economical.
Fungicide decisions should be customized for each field, each year using the Fungicide Decision Worksheet for Managing Mycosphaerella Blight in Field Peas. ■