Survival of blackleg pathogen inoculum in canola stubble under simulated flooding conditions
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Blackleg of canola (Brassica napus), caused by two Leptosphaeria spp, is a significant constraint to canola production worldwide except in china where only the less virulent L. biglobosa has been reported. In China, the disease is caused by a less pathogenic species, L. biglobosa, and there is a concern that importing canola from Canada may introduce the virulent L. maculans, impacting the crop there negatively. In China, canola (or rapeseed) production is centered in several eastern and central provinces where winter rapeseed is often followed by paddy rice that normally is flooded for weeks during late spring and summer. L. maculans or L. biglobosa in diseased canola stubbles serves as the key inoculum source to cause blackleg, and it has been questioned if the flooding practice may help suppress the inoculum. A study was initiated to determine the effect of flooding temperature (12 to 40°C) and duration (2 to 12 weeks) on survival of blackleg pathogen in canola stubbles. Experiments were set up on a Thermogradient Plate that is capable of simultaneously creating 96 independent temperature settings. Diseased stubbles with > scale-3 level of basal stem-canker symptoms used for the experiments were collected from a Westar canola plot in Melfort after 2011 harvest. Flooded stubbles were sampled every two weeks, surface sterilized, and incubated on V8-juice medium amended with antibiotics for 10 days to observe pycnidia cultures of L. maculans or L. biglobosa as the evidence of pathogen survival. Two trials were set up in RCBD with four replications, and pathogen incidence data (based on 25 stubble pieces per replicated) were subject to ANOVA. Significant reduction (P= 0.01) of pathogen incidence was observed at 2-week flooding treatment relative to control (non-flooded) and there was no pathogen recovery after 4weeks of flooding till 12 weeks of experiment. Lower flooding temperatures of 12oC and 16oC appeared to be slightly less effective than higher temperatures (20-40 oC) in reducing pathogen survival. Stubble tissues degraded sharply after 2weeks (contrast, P= 0.05) in response to the flooding temperature and the dry weight was reduced more substantially (40%) at higher temperatures. Virulence of any survived pathogen propagule after flooding is still intact and survival at any temperature or duration of flooding does not differentiate between L. maculans or L. biglobosa. High proportion of survived blackleg pathogen (pycnidia) from flooding were L. maculans (67%) and the rest L. biglobosa (33.0%) under Westar cotyledon test.
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