End-of-season assessment of sclerotinia stem rot of canola (PCDMN)

We are fast approaching the time when you can rate
sclerotinia stem rot in your canola. Ideally, plants should be rated within a
week or so before the crop would normally be swathed (if it was being swathed
of course). Once the crop starts to turn, rating becomes very difficult.

Key characteristics of sclerotinia infections in canola include:

Infected tissue has a characteristic bleached white/whitish/tan gray appearance
Affected tissue when it dries is very brittle and shreds and shatters easily when you twist the stems
The pith portion of affected stems is typically not present
In heavily infected stems the pith tissue is destroyed leaving only the outer ring of stem tissue
Sclerotia (black resting bodies) or sclerotial initials (clumps of compacted white hyphae that turn black when mature) are typically produced in or on affected plant tissues
External sclerotia typically develop when you have wet conditions following infection and as the crop progresses towards senescence.

Symptoms not characteristic of sclerotinia:

Infected tissue doesn’t have a bleached white/whitish grey or tan appearance
Affected tissue is not brittle and doesn’t shred and shatter when you twist the stems between your hands
The pith portion of stems is still intact
Sclerotia or sclerotial initials (dense clumps of white hyphae) are not produced in/on suspect symptoms
Yellowing of stems/branches may be due to other factors, such as root disease, nutrient, or weather stress, etc.

For sclerotinia one can look at the incidence of infection, and focus on the main stems and branches. Infections on the main stem and/or branches have the largest impact on yield. However, if incidence is low (<5%), the impact on yield is negligible.

In most canola fields, plants will typically lean in a particular direction. Use this to your advantage and work with the flow of plants.

When assessing stem rot levels, one should look at a representative sample of plants throughout the field. For example, a minimum of 4-6 sites should be looked at in a quarter section field; then at each site between 50-100 plants should be assessed.

Half or full section fields will require more sites. If the field is quite variable then separate assessments in specific areas may be needed, e.g. lodged versus non-lodged.

Assessments can also be made in sprayed versus check areas to determine the usefulness and benefit of in-crop fungicide applications. Unsprayed checks, even small areas, can be quite useful.

An alternative rating scale is one from Kutcher and Wolf
(2006). Although a bit more complicated,
it covers infections on various parts of the plant. Note symptoms in the upper canopy (pods,
etc.) generally have less impact on yield.

Rules of thumb have been developed to assess the potential
loss associated with stem rot infections.
In general the % yield loss is approximately 0.5 times the % of infected
plants (main stem or main branch infections). Thus, if you note 30% infected plants, the potential yield loss is approximately 15%.

The PCDMN has also produced overviews for assessment of stem rot levels and identification of disease symptoms accessible with the following hyperlinks:

• An assessment protocol for sclerotinia stem rot in canola

• A slide deck overview of a guide to scouting and identification of sclerotinia stem rot in canola

• Playing cards describing sclerotinia stem rot of canola symptoms