UVM Theses and Dissertations
Format:
Print
Author:
Nishanthan, Tharshani
Dept./Program:
Plant and Soil Science
Year:
2013
Degree:
PhD
Abstract:
Natural suppressive soils hold potential for managing soybean cyst nematode, Heterodera glycines (SCN); however, ecological mechanisms leading to the suppressive state are poorly understood. The overall objective was to use a broad ecological approach to characterize the composition and function of nematode communities and microbial functions in suppressive soils. The general working hypothesis was that certain production practices alter soil community composition and ecological function that create long-term suppression of SCN populations and/or manifestation of disease. A three-year experiment was conducted in two fields naturally suppressive to SCN in Waseca County, Minnesota, where soybean was cultivated continuously for more than 15 years under a no-tillage system. The experiment was designed as a split plot and replicated three times. Main plots were cultivation (no-till, conventional till) and subplots were five crop sequence-biocide combinations. Treatments were chosen to identify management practices that disrupt natural suppression of SCN. Nematodes were enumerated and identified to genus. Activity of fourteen extracellular enzymes was quantified to assess function of the microbial community. Soil physicochemical properties were measured as co-variables of the biology and as dependent variables.
Cultivation, application of biocides, and rotation to corn all increased abundance of SCN and the impact increased progressively within the three years of the experiment. Cultivation decreased abundance of plant-parasitic and fungivorous nematodes and increased abundance of bacterivorous nematodes. Among plant-parasitic nematodes, the proportion that was Helicotylenchus was correlated negatively with Heterodera glycines. Perhaps, Heterodera glycines and Helicotylenchus compete for space and/or nutrients in the rhizosphere. When soybean was rotated to corn, the relative abundance of fungivorous nematodes (especially Aphelenchoides) increased: Values of [sigma]MI25 and trophic, genera diversity indices correlated positively with SCN suppressive soils suggesting that complex soil communities with later stages of ecological succession foster natural disease suppression. Naturally suppressive soils contained greater activity of chitinase and aminopeptidase, as well as a greater volume ofsmall inhabitable pores volume than conducive soils.
Mannase and arabinase activity relates to crop rotation, reflecting relative proportions of carbohydrate monomers in the cell walls of com and soybean, respectively. Our results suggest that disease suppression is to be more closely aligned with fungi than bacteria. Natural suppression of SCN appears to be associated with the microbial community fostered by a combination of no-till and soybean monoeulture. We conclude that agronomic practices used to manage SCN have detrimental non-target effects on other plant-parasitic and free-living nematodes and the functional composition of decomposer microbial communities.
Cultivation, application of biocides, and rotation to corn all increased abundance of SCN and the impact increased progressively within the three years of the experiment. Cultivation decreased abundance of plant-parasitic and fungivorous nematodes and increased abundance of bacterivorous nematodes. Among plant-parasitic nematodes, the proportion that was Helicotylenchus was correlated negatively with Heterodera glycines. Perhaps, Heterodera glycines and Helicotylenchus compete for space and/or nutrients in the rhizosphere. When soybean was rotated to corn, the relative abundance of fungivorous nematodes (especially Aphelenchoides) increased: Values of [sigma]MI25 and trophic, genera diversity indices correlated positively with SCN suppressive soils suggesting that complex soil communities with later stages of ecological succession foster natural disease suppression. Naturally suppressive soils contained greater activity of chitinase and aminopeptidase, as well as a greater volume ofsmall inhabitable pores volume than conducive soils.
Mannase and arabinase activity relates to crop rotation, reflecting relative proportions of carbohydrate monomers in the cell walls of com and soybean, respectively. Our results suggest that disease suppression is to be more closely aligned with fungi than bacteria. Natural suppression of SCN appears to be associated with the microbial community fostered by a combination of no-till and soybean monoeulture. We conclude that agronomic practices used to manage SCN have detrimental non-target effects on other plant-parasitic and free-living nematodes and the functional composition of decomposer microbial communities.