Sclerotinia fungicide improves yield, but check ROI

Key result: Carefully consider the risk scenario and potential return on investment before spraying fungicide for sclerotinia stem rot or selecting current varieties with the sclerotinia-tolerant trait.

Project title, Principal investigator: “Effects of genetic sclerotinia tolerance and foliar fungicide applications on the incidence and severity of sclerotinia stem rot infection in argentine canola,” Chris Holzapfel, Indian Head Agricultural Research Farm

Funding: SaskCanola

Sclerotinia stem rot is a major canola disease that causes significant yield loss in Western Canada each year. However, the impact of the disease is still hard to predict, as incidence and severity in individual fields varies dramatically depending on environmental conditions. This can make the grower’s decision to spray sclerotinia fungicide difficult. Although hybrids with increased sclerotinia tolerance are now available, sclerotinia infection is not eliminated in them and conditions where foliar fungicide applications are still needed can occur.

This study evaluates the relative effectiveness of genetic tolerance and foliar fungicides to reduce sclerotinia stem rot infection in canola. It also investigates if, and under what conditions, foliar fungicide applications may still be required when growing a hybrid with genetic tolerance to sclerotinia. The three-year field study (2013-15) was carried out at three locations in Saskatchewan (Indian Head, Melfort and Outlook) and two in Manitoba (Melita and Brandon). Two of the locations had access to irrigation and all of the locations were considered to have at least a moderate risk for sclerotinia in canola. Each location compared susceptible hybrid 45H29 RR and tolerant hybrid 45S54 RR, and utilized four foliar fungicide treatments: untreated check, fungicide applied at 20 per cent bloom, fungicide applied at 50 per cent bloom, and fungicide applied twice (at both 20 and 50 per cent bloom).

This study was carried out in small plot trials (which can be criticized for having an edge effect, increased air flow and being on well-drained, uniform ground) rather than field scale sites (which can have low-lying wet areas with higher incidence and severity), but the plot sizes were as large as possible and at least one location had disease incidence and severity consistent with nearby field scale trials.

Despite selecting locations considered to have at least a moderate risk for sclerotinia, the actual disease incidence in 2013-15 was generally low, which may have impacted the range in treatment effects. Under the conditions encountered, disease levels in tolerant 45S54 were frequently lower than those in susceptible 45H29. Under this low disease pressure, however, there were no measurable benefits to applying fungicides with a tolerant hybrid.

A significant yield increase was seen with fungicide use when averaged across locations and hybrids. However, the economic returns (under the conditions encountered) were marginal at best, depending on grain prices and fungicide costs.

Although not statistically significant, there was some evidence that the yield response was slightly larger and more consistent with susceptible hybrids than tolerant ones. Tolerant hybrids were shown to be effective for reducing disease and less likely to benefit from fungicide (see Table 1). However, susceptible hybrids may frequently yield higher, at least under low disease pressure as encountered in these trials.

Dual applications provided no benefit over single applications (neither visual symptoms nor actual seed yields), regardless of location or application timing. Applying fungicide between 20 and 50 per cent bloom, before too many petals drop, was important (for fungicide efficacy).

Improvements in management decisions for sclerotinia stem rot will contribute to long-term economic sustainability of canola in Western Canada. The greatest challenge for managing sclerotinia in canola continues to be accurately predicting whether yield responses to costly fungicide applications are likely. Genetic tolerance is an exciting advancement that has potential to reduce dependence on fungicides and provide adequate protection under low to moderate disease pressure. However, to be widely adopted and utilized to its full potential, sclerotinia tolerance should be incorporated into broader range of hybrids and, given the sporadic and unpredictable nature of this disease, yields must remain competitive with susceptible hybrids.