Graduate Thesis Or Dissertation


Decomposition of Brassicaceae Residue in the Willamette Valley Public Deposited

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  • An important inoculum source of Leptosphaeria maculans and L. biglobosa (syn Plenodomus spp.), the pathogens that cause black leg in Brassicaceae crops, is infected plant residues. The general acceptance of conservation tillage in the Willamette Valley, Oregon, USA, has enabled a build-up in the amount of residue on or near the soil surface. The decomposition rate of Brassicaceae species may differ, suggesting that some crops could be a greater source of black leg inoculum than others. Our study found that Brassicaceae crops decompose differently based on the management treatment and whether the residue was on the soil surface or buried. In general, canola (Brassica napus) had lower rates of decomposition compared with turnip (Brassica rapa) and radish (Raphanus sativus) with an average 44, 33, and 27%, respectively, remaining on the soil surface after 24 months. Tilling exposed root and crown tissue and resulted in decreased rates of decomposition for canola and turnip compared to flailing or no-till. Tilling radish increased rates of decomposition compared to flailed and no-tilled residues when on the soil surface. Flailing did not increase rates of decomposition compared to no-tillage. Buried residues had increased rates of decomposition for all crops and treatments compared to soil surface. Buried residue had similar rates of decomposition regardless the treatment or crop. The primary difference in decomposition among treatments and placement were changes in the percentage of cellulose compared with hemi-cellulose and lignin. Little to no lignin degradation was detected over 24 months. Leptosphaeria spp. were quantified on canola and turnip residues 8, 18, and 24 months after harvest using qPCR to determine if tillage treatments impacted the survival of the pathogen. Flailing residue reduced the population of Leptosphaeria spp. compared to no-till and tilled treatments but there was no advantage to an additional flail treatment. Buried residue had increased amounts of the pathogen DNA compared to residue left on the soil surface. The Leptosphaeria spp. DNA detected on residue was determined to be from live pathogens by quantifying pycnidia on infected cotyledons inoculated from with the same residue source used in the qPCR experiment. Although flailing decreased the population, 10% of cotyledons had pycnidia, demonstrating that flailed residue is still infectious. Our results suggest that deep tillage is the best management for Brassicaceae crop residue, followed by minimal tillage for several seasons to allow residue to decompose and not be brought back to the soil surface where infected residue could sporulate.
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  • 2019-04-15 to 2019-11-16



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