Epidemiology of Sclerotinia sclerotiorum, causal agent of Sclerotinia Stem Rot, on SE US Brassica carinata
Type of DegreeMaster's Thesis
Entomology and Plant Pathology
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Brassica carinata is a non-food oil seed crop currently being introduced to the Southeast US as a winter crop. Sclerotinia sclerotiorum, causal agent of Sclerotinia Stem Rot (SSR) in oilseed brassicas, is the disease of most concern to SE US winter carinata, having the potential to reduce yield and ultimately farm-gate income. A disease management plan is currently in development which will include the use of a disease forecasting system. Implementation of a disease forecasting system necessitates determining the optimal environmental conditions for various life-stages of SE US isolates of this pathogen for use in validating or modifying such a system. This is because temperature requirements and optimums for disease onset and subsequent development may vary depending on the geographical origin of S. sclerotiorum isolates. We investigated the optimal conditions necessary for three life-stages; conditioning of sclerotia, germination of apothecia, and ascospore infection of dehiscent carinata petals. Multiple isolates of S. sclerotiorum were tested, collected from winter carinata or canola grown in the SE US. In the conditioning test sclerotia of 4 isolates (HA, FH, TI, and BT) were subjected to 4 temperatures (15/3ᵒC, 18/6ᵒC, 21/9ᵒC day/night, and 12ᵒC constant), 3 moisture frequencies (moisture event every 1, 3, or 6 days), and 3 conditioning durations (4, 6, and 8 weeks). We found high variability amongst isolates in their response to temperature during conditioning and that a moisture event every day resulted in significantly less germination of sclerotia for most isolates. In the apothecia germination test we subjected uniformly conditioned sclerotia of 4 isolates (BR, BT, QC, and TI) to 3 light levels and 2 temperatures (18/6°C and 26/11°C) where we found a decrease in cumulative apothecia as light level increased and no significant difference between temperatures but significant differences between isolates with respect to apothecia accumulation after a 4 week germination period. In the petal infection test ascospores of two isolates (FH and TI) inoculated onto carinata petals were subjected to 4 temperatures (15/3°C, 18/6°C, 21/9°C, and 26/11°C) and 4 humidity levels (about 97, 94, 84, and 74% RH). We found that infection increased as temperature and as RH increased, with no infection occurring below 84% RH at any temperature and no significant difference between isolates in terms of proportion of infection to non-infection of petals. A management plan is currently being developed for controlling SSR in Southeastern US winter carinata which will include fungicide recommendations. An in vitro fungicide assay was done in order to supplement fungicide field trials in developing management recommendations for growers. Six fungicides were tested: chlorothalonil, boscalid [Endura®], pyraclostrobin [Headline®], fluxapyroxad + pyraclostrobin [Priaxor®], flutriafol [Topguard®], and metconazole + pyraclostrobin [Twinline®]. These fungicides were tested at varying rates, based on maximum recommended label rate, at 0.02, 0.05, 0.1, 0.5, and 1.0x, on three isolates (BR, FH, and TI) of S. sclerotiorum from the Southeast US. Significant differences among isolates’ growth, relative to their sensitivity to fungicides (P < 0.001) were noted, though these differences in growth were only seen with less than 0.5x maximum recommended label rate, with growth on concentrations above 0.5x being similar. Only flutriafol, a demethylation inhibitor (DMI), and boscalid, a succinate dehydrogenase inhibitor (SDHI), were unable to prevent growth of any isolate regardless of fungicide concentration. This suggests that growers should not use DMI’s or SDHI’s to control SSR on winter carinata unless in combination with another fungicide chemistry.