Holly Derksen, Manitoba Agriculture and Dr. Bryan Cassone, Brandon University
The annual soybean disease survey provides important information on disease issues faced by growers across Manitoba. Sixty-seven soybean fields distributed across the province were surveyed in 2017. Areas with higher soybean production were targeted, but almost all areas growing soybeans were represented in the survey.
Both foliar and root diseases were included and the following article outlines the foliar diseases detected across the province and the implications of these results. For more information on the root diseases surveyed, readers are encouraged to refer to the 2018 spring edition of Pulse Beat for the article 2017 Survey of Soybean Root Rot in Manitoba.
Figure 1. Septoria brown spot (right) and bacterial blight (left) are two common foliar diseases present in Manitoba soybean fields, though rarely found at levels that would result in yield loss.
Disease assessments were made in the field focusing on common diseases like septoria brown spot, bacterial blight and downy mildew as well as less common diseases like white mould, pod/stem blight, anthracnose and frogeye leaf spot. Most foliar diseases in soybean are able to infect throughout the growing season, so fields were surveyed early in July (at the onset of flowering) and later in August (during pod fill).
Two survey timings allowed for an accurate depiction of diseases showing up early and late in the growing season. Some foliar stem diseases, like white mould, pod/stem blight and anthracnose only show up later in the season in most years. It is important to note that the disease ratings presented here were only based on visual assessment and were not individually verified through laboratory analysis.
Not surprisingly, bacterial blight and septoria brown spot were found in the majority of fields surveyed (Table 1). These two foliar diseases are very common, but are rarely present at levels which cause economic concern in the form of yield loss.
Both of these diseases, along with downy mildew, were assessed for both incidence (the proportion of surveyed plants showing infection) and severity (the level of disease based on proportion of the canopy infected).
At the later survey timing, bacterial blight, septoria brown spot and downy mildew had average severities of 1.9, 1.8 and 1.8, respectively. Severity levels this low are not expected to cause yield loss (disease severity levels are explained in Table 2).
It is important to note that, while the provincial average severities remained low, there was variation between fields. For example, one field surveyed was visually assessed to have bacterial blight present on all plants surveyed with an average severity of 3.9 and septoria brown spot present on more than 90% of plants with an average severity of 3.3. In cases like this, verification of the causal agent of the symptoms is important if management practices are to be employed. Practices such as crop rotation and tillage can be used to manage both bacterial blight and septoria brown spot, but the use of fungicides (while rarely economical in Manitoba) would only be effective against septoria brown spot.
Less common diseases detected as part of this survey include white mould, phomopsis pod/stem blight, anthracnose, and frogeye leaf spot (Table 3). Most of these diseases are considered of minor importance to soybean production in Manitoba due to their relatively lower prevalence in comparison with the diseases listed in Table 1.
However, white mould is an important soybean disease issue in other soybean-producing areas of North America and has the potential to be a yield-limiting issue in Manitoba. In fields with thick canopies, high moisture, and/or high humidity, this disease can cause economic losses. In most years, fungicide use is not necessary for management of this disease in soybeans (unlike other crops like canola or dry beans), but individual fields should be assessed during flowering to determine the microclimatic conditions, and therefore, risk of infection.
In addition to making disease assessments, samples were collected for molecular analysis as part of a project through Brandon University. Using a novel diagnostic approach previously described in the 2017 spring edition of Pulse Beat for the article An Innovative Technology for Early Detection of Foliar Diseases in Manitoba, disease-causing microorganisms (pathogens) are identified in symptomatic leaves.
Not only does this validate the visual diagnoses, it also detects pathogens that may be new to the region and not yet on the radar of agronomists. This approach largely confirmed the seasonal and regional distributions of the disease survey (Table 4).
In addition, pathogens causing bacteria pustule, leaf spot and leaf blight were detected, along with several viruses. This included Alfalfa mosaic virus (AMV), which had not been previously reported to infect soybean in the province. AMV can cause plant stunting, leaf distortion, mottling and reduced pod numbers.
While most of these diseases are not considered economically important, high incidence of bacteria pustule has been shown to cause significant yield losses on susceptible varieties in other soybean growing regions of North America.
The results of these surveys paint an important picture of soybean production in Manitoba. Agronomists can use this information when assessing prevalent diseases and their impact on the crop. This survey also provides important information for researchers, breeders and funding agencies.
As soybean production continues to expand in Manitoba and western Canada, the continuation of pest surveys such as this will cast light on issues that may accompany expanding acres and tight rotations. This survey is in its infancy, but as more years of data are collected, trends and regional differences will begin to emerge and guide agronomy research and extension in the future.
The Manitoba soybean disease survey is a coordinated effort by Manitoba Agriculture, Manitoba Pulse & Soybean Growers, Agriculture and Agri-Food Canada, Brandon University and the University of Manitoba.