Prairie soil moisture levels and need to scout for apothecia of the sclerotinia stem rot pathogen, July 16-23, 2023
Based on satellite data from July 10-16, 2023, large areas of the Prairies are still dealing with dry conditions and would only have <5 to 15% of surface soil moisture (VMC) (Figure 1). Corresponding, satellite images for July 10-16, 2023, 2023 show below long term average soil moisture levels for large areas of the Prairies, including regions where sclerotinia stem rot can occur (Figure 2). Previously, some areas including the Edmonton region to eastern Alberta and a small area of northwestern Saskatchewan were seeing somewhat higher soil moisture levels, but during the period of July 10-16 moisture levels appear to be decreasing (Figures 1 and 2). Note since July 9, regional weather and storm systems may have resulted in localized rainfall events that can contribute to and maintain soil moisture, which may promote sclerotial germination and apothecia production. Regions with 20 to over 95 mm accumulated rainfall in the past 14 days as of July 23, 2023 include broad areas of central and northern Alberta and pockets in the Peace Region, Saskatchewan, and Manitoba (Figure 3).
As of July 16-23, the need to scout fields for apothecia of the stem rot fungus and/or to assess in-field risk are generally low, although the risk may be somewhat higher where rainfall has occurred and crops have not been compromised by dry conditions in June and July (Figure 3). Most canola is well into flowering, while early seeded and/or drought stressed fields are likely finishing flowering. In Prairie regions, past and current rainfall will likely trigger the start and/or continuation of the sclerotial germination process, which can take up to three weeks (Figure 3). Further rainfall may increase the risk of sclerotial germination and apothecia production. Some areas in Alberta, Saskatchewan and Manitoba may still have soil moisture conditions somewhat conducive for sclerotial germination, especially were localized rainfall has occurred (Figure 3). In these areas, in-crop scouting for apothecia and the use of available stem rot risk assessment tools may be needed over the next 1-2 weeks as part of fungicide use decisions (https://www.canolacouncil.org/canola-encyclopedia/diseases/sclerotinia-stem-rot/#forecasting-and-risk-assessment, https://tinyurl.com/bp6dchvx).
Previously, the PCDMN tweeted some tips for practical assessment of the risk of sclerotinia stem rot, including the wet pants test, and these will complement the other tools that are available. See: https://twitter.com/pcdmn/status/1143578383537889281 and https://drive.google.com/file/d/1cC1gNcLq-L5Dk2tUXG-vmEmW_6oJ-kAd/view?usp=sharing.
Background information on the soil moisture maps can be found at the following link: New for 2023: Prairie Soil Moisture Maps.
Given dry conditions, especially prior to mid-June, sclerotial germination would be delayed, although recent rains in some regions have likely triggered the germination process. Normally it takes about 3 weeks of wet soil conditions and moderate temperatures for germination and production of apothecia. This period may be as short as 2 weeks if some moisture was received earlier in June, which may pre-condition sclerotia. Delayed germination and apothecia development may mean that sclerotinia spore loads in the air are limited as the crop or portions of the crop come into bloom in 2023. This may mean that an early bloom fungicide application may not represent the best target. However, in crops with reasonable yield potential and where risk tools indicate increasing risk as flowering continues, later fungicide application at full bloom may be considered. Note in areas were rainfall occurred and soil moisture levels are conducive, later seeded canola may be at risk during the early bloom; to mid-bloom period. Tools such as the Spornado spore trap (https://2020seedlabs.ca/services/spornado/) and petal testing services (https://www.seedtesting.com/disease-testing-sclerotinia/https://quantumgenetix.com/product/sclerotinia-white-mold/) can provide guidance related to the level of sclerotinia inoculum and stem rot risk as the crop starts and continues to flower. Note these need to be combined with other risk tools to provide a complete picture of stem rot risk in relation to host, pathogen and environment factors, i.e. the disease triangle. If fungicides are being considered, accounting for the most common crop growth stage in a field should be considered to help guide the timing(s) used. Fortunately, most fungicide products have a window from 20-50% bloom, which represents a range of growth stages. Note available risk assessment tools will provide information related to stem rot risk and whether a fungicide is needed from 20-50% bloom.
Figure 1. Percent saturated surface soil moisture from Soil Moisture and Ocean Salinity (SMOS) data, July 10-16, 2023. Courtesy of Agriculture and Agri-Food Canada’s Agroclimate Geomatics and Earth Observation Division.
Figure 2. Difference from long term average, percent saturated surface soil moisture from the Soil Moisture and Ocean Salinity (SMOS) data, July 10-16, 2023. Courtesy of Agriculture and Agri-Food Canada’s Agroclimate Geomatics and Earth Observation Division.
Figure 3. Fourteen day cumulative rainfall (mm) as of July 23, 2023 observed across the Canadian prairies. Prepared by Agriculture and Agri-Food Canada’s Science and Technology Branch. Data provided through partnership with Environment Canada, Natural Resources Canada, Provincial and private agencies.
Disclaimer
Information related to soil moisture is OFFERED TO THE PUBLIC FOR INFORMATIONAL PURPOSES ONLY. Agriculture and Agri-Food Canada, and their employees assume no liability from the use of this information. Soil moisture conditions will vary depending on soil texture, and localized rainfall and temperature conditions. Sclerotinia stem rot risk will vary depending on host, pathogen and weather factors, and farmers and consultants are encouraged to use currently available risk assessment tools (https://www.canolacouncil.org/canola-encyclopedia/diseases/sclerotinia-stem-rot/#forecasting-and-risk-assessment). Mention of a particular product or service does not represent an endorsement by PCDMN or the organizations involved with the PCDMN.