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 stem rot becomes very difficult.
Key characteristics of sclerotinia infections 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.
Sclerotinia stem rot rating
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).
Updated assessment protocol for sclerotinia stem rot of canola
Updated disease identification guide for sclerotinia stem rot of canola