Rimmer: Effect of herbicide and disease resistance on survival and inoculum production of Leptosphaeria maculans, the causal agent of blackleg
Date: March 2002
Term: 3 years
Status: Completed
Researcher(s): Dr. S. Roger Rimmer, Agriculture and Agri-Food Canada and Dr. R. Kutcher, AAFC Melfort
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: Canola Council of Canada, AAFC Matching Investment Initiative
Project Summary
Leptosphaeria maculans, the cause of blackleg, causes significant yield loss in canola crops. In this study, researchers compared pathogen survival and inoculum production of infested residues in both blackleg resistant cultivars and blackleg susceptible herbicide tolerant cultivars. The results showed that ascospore release occurred from residues from both susceptible and resistant canola cultivars, and is not highly associated with the disease ratings observed at harvest.
Leptosphaeria maculans, the cause of blackleg, causes significant yield loss in canola crops. The pathogen survives on infested residues from previous years’ crops. There is evidence that infection of resistant cultivars occurs similarly to infection in susceptible cultivars, but symptom expression in resistant cultivars is inhibited or delayed. This suggests, that even on resistant cultivars, the fungus will colonize the crop residues, reproduce and provide inoculum for subsequent canola crops.
Previous studies have indicated that certain herbicides and fungicides are inhibitory to L. maculans and might reduce the survival of the pathogen on infected residues. Two of these herbicides are glufosinate‐ammonium (Liberty) and glyphosate (Roundup), which are now widely used in western Canada to control weeds in herbicide tolerant canola (HT canola) cultivars.
The objective of this study was to determine the effects of two herbicides, Liberty and Roundup, and three fungicides, Tilt, Sportak and Quadris on pathogen survival and inoculum production of infested residues in both blackleg resistant cultivars and blackleg susceptible herbicide tolerant cultivars. The study also tried to determine the optimal application of pesticides for control of blackleg on residues, and to assess the risk of development of new and more pathogenic strains of the fungus.
Research results indicate that the use of RoundUp or Liberty herbicides on HT canola cultivars probably does not result in reduced numbers of spores released from stem residues when applied at recommended rates for weed control and under normal agronomic situations. Although these chemicals have shown inhibitory effects on the pathogen under laboratory conditions, this does not seem to occur under field conditions. Even fungicide applications to infected residues were not effective in subsequently reducing inoculum production.
The study also found that the timing of ascospore release from infected canola stem residues varied significantly by location, but supports the currently held view that short crop rotations are conducive to high blackleg disease levels. The results showed that ascospore release occurred from residues from both susceptible and resistant canola cultivars, and is not highly associated with the disease ratings observed at harvest. Understanding the effects of these pesticides in canola cultivars with blackleg resistance or herbicide tolerance will provide researchers with a rationale for improved options in disease management.