Growing Possibilities, A blog by XiteBio | Sclerotinia Stem Rot: How it Kills Your Canola Yields
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Sclerotinia Stem Rot: How it Kills Your Canola Yields

As the cropping season moves along we are starting to see flowering in many crops, and with flowering comes the presence of pests and diseases. Whether it’s on Twitter or in Research publications, the problem of Sclerotinia Stem Rot is on Canola growers’ minds right now. This is one of the most destructive diseases in canola, and has become a yearly concern for canola growers all over Canada, with 2020 being no different. In this week’s edition of Growing Possibilities, we are going to discuss Sclerotinia Stem Rot and what you can do to stop it.

Sclerotinia Stem Rot (sometimes called white mould) is caused by the fungus Sclerotinia sclerotiorum and occurs throughout all Canola growing areas in Canada (1). If a substantial infection is established early in the flowering stage it can lead to significant yield loss for affected plants (up to 80% in the most extreme cases) (2). Sclerotinia overwinters underground and can remain in a viable state for over five years in soil, with some germinating when conditions are right and some staying dormant (3).

Moist soil conditions cause fungus in the soil to germinate into apothecia, which produce airborne spores that start almost all infections in canola. These spore-forming apothecia are short, white fungal stalks resembling golf tees, and are the main visual indicator for Sclerotinia identification. Spores come into contact with dropped petals from canola flowers (their food source) and begin growing before eventually infecting nearby plant stems (4). Stem rot usually occurs later in the season, with indications of infection such as soft, watery lesions on stems and discoloration appearing towards the end of flowering (1).

Factors leading to an increase in disease incidence include:

  • Occurrence of disease in a crop within the last 1-2 years
  • Neighbouring or nearby fields with Sclerotinia presence
  • High crop density/canopy density
  • Excessive moisture and precipitation
  • Moderate temperatures (20-25°C) and high humidity

Diligent field scouting and record keeping are important to spot and manage this disease, and rotation of a non-susceptible crop (such as cereals and grasses) can help break the disease cycle in fungus containing soils (1).

Studies have found the most effective way of managing Sclerotinia to be the application of foliar fungicides (5). Due to the expense of applying fungicides and the yearly variation in incidence of this disease, it is recommended to apply fungicide only when you suspect the presence of Sclerotinia. Pre-spray scouting and recording past incidences of disease should be taken into consideration before deciding to apply (5). Spraying should take place between the 20% and 50% bloom period, with optimal timing being around 30%, as this is when petals will begin to fall off and feed potential fungal apothecia (4). The goal of fungicides is to spray as many petals as possible before they start to fall and feed soil fungus. Fungicide reaching under the canopy can also help mitigate potential infections, but it cannot stop an infection after it has already started, which is why early spraying is a must (1).

Research has shown that certain bacteria species of the genera Bacillus and Pseudomonas have characteristics that prevent and suppress Sclerotinia infections in canola, and could prove useful in the future formulation of a biological control product, reducing the use of fungicides and lowering the environmental impact of stem rot treatments for canola (6).

References:

1) https://www.canolacouncil.org/canola-encyclopedia/diseases/sclerotinia-stem-rot

2) Kong, W., Zhang, C., Cao, F., Liu, F., Luo, S.,Tang, Y., He, Y. (2018) Detection of Sclerotinia Stem Rot on Oilseed Rape (Brassica napus L.) Leaves Using Hyperspectral Imaging. Sensors (Basel): 18(6).

3) Shoute, L.L., Anwar, A., MacKay, S., Abdelrasoul, G.G., Lin, D., Yan, Z., Nguyen, A.H., McDermott, M.T., Shah., M.A., Yang, J., Chen, J., Li, X.S. (2018) Immuno-impedimetric Biosensor for Onsite Monitoring of Ascospores and Forecasting of Sclerotinia Stem Rot of Canola. Sci Rep: 8(12396).

4) https://www.realagriculture.com/2020/07/canola-school-scouting-and-spraying-for-sclerotinia/

5) Kutcher, H.R, F. Dokken-Bouchard, T.K. Turkington, W.G.D. Fernando, S. Boyetchko, L. Buchwaldt, D. Hegedus and I. Parkin. 2011. Managing sclerotinia stem rot in canola.  Canola Research Summit, Winnipeg, MB  April 12-13, 2011

6) Behnam, S., Ahmadzadeh, M., Sharifi Tehrani, A., Hedjaroude, GH A., Farzaneh, M. (2007) Biological Control of Sclerotinia Sclerotiorum (Lib.) De Bary, the Causal Agent of White Mold, by Pseudomonas Species on Canola Petals. Commun Agric Appl Biol Sci: 72(4).

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