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EFFECT OF COVER CROPS AND SOIL CHARACTERISTICS ON THE OCCURRENCE OF METARHIZIUM ROBERTSII IN AN ORGANIC CROPPING SYSTEM
Restricted (Penn State Only)
Randhawa, Puneet Kaur
Master of Science
Date of Defense:
May 04, 2017
Mary Ellen Barbercheck, Thesis Advisor
Maria Del Mar Jimenez Gasco, Committee Member
Dawn S Luthe, Committee Member
Nina Ellen Jenkins, Committee Member
organic feed grain system
Organic crop production does not allow synthetic chemical inputs, but relies instead on cultural practices and biological processes to control insect pests. In addition to insect natural enemies, insect pathogenic fungi in the genus Metarhizium can impact insect populations either through application of a formulated product or as a naturally-occurring or conserved biological control agent. The development of effective biological control, especially through conservation of endemic strains, can contribute to environmental quality and reduced health risks in agricultural systems. Plants influence belowground soil organisms through the release of diverse carbon rich root exudates and Metarhizium has been shown to survive in the rhizosphere of plants. In addition, soil is a major habitat of Metarhizium and soil characteristics such as organic matter, pH and elements such as phosphorus, potassium, calcium etc. are known to affect the habitable niches of Metarhizium. Therefore, I focused my research on the impact of cover crops and soil characteristics on Metarhizium with the long-term goal of improving conservation biocontrol. This thesis consists of 3 chapters, focused on understanding the effect of a commonly used production practice – cover cropping – and soil characteristics on the detection of Metarhizium robertsii in an organic feed grain system. This thesis reports research conducted from 2013 through 2015 as a component of a larger study to assess the benefits and tradeoffs associated with increasing the diversity of cover crops in an organically-managed feed grain production system. In Chapter 1, I reviewed the scientific literature aimed at understanding the role of Metarhizium as an insect pathogen, its role in pest management in organic systems and the effects of agricultural practices on entomopathogenic fungi. Further, I reviewed literature on cover crops, effects of plant diversity and plant species on the soil microbial community and the impact of soil characteristics on the occurrence of entomopathogenic fungi, and specifically, Metarhizium, in soil. In Chapter 2, I investigate the effect of twelve cover crop treatments that vary in species diversity in an organically-managed corn-soybean-wheat rotation on the occurrence of the entomopathogenic fungus, M. robertsii. I consider the effects of the nominal treatments as well as the expressed diversity of a subset of the cover crop mixtures. Cover crops influence entomopathogenic fungi through the release of root exudates which are a source of nutrients for M. robertsii. Because the effects of cover crops may manifest over time, I examine the effects of cover crop biomass from the current year as well as previous fall on detection of M. robertsii. Accordingly, in this chapter, I hypothesized that: 1) cover crops with increasing diversity will result in greater detection and diversity of Metarhizium as compared to cover crop monocultures; 2) cover crop biomass in the fall will be correlated with Metarhizium detection in spring; 3) cash crops will significantly affect Metarhizium detection; and 4) expressed diversity of cover crops will significantly affect Metarhizium detection. Against my expectations, the results revealed no effect of cover crop diversity on detection of Metarhizium. Only one species, M. robertsii, was detected from the experimental site. In addition, the frequency of Metarhizium detection did not vary between monocultures and mixtures comprised of diverse species of cover crops, i.e. polycultures. I suggest that the unexpected results are due to low initial species diversity from the previous long-term effects of conventional management at the research site. Further, I found that plant species (i.e., corn vs soybean) significantly affect Metarhizium detection. I suggest that this difference in detection between corn and soybean is due to plant identity effects. Furthermore, my results reveal the existence of legacy effects of the expressed cover crop species biomass in the previous fall on Metarhizium detection in spring. Taken together, my research suggests that the effect of cover crop biomass on M. robertsii detection is not immediate but appears with a time lag. In Chapter 3, I examine the effects of 13 soil characteristics at the experimental site on M. robertsii to determine if particular soil characteristics may be associated with the conservation of this beneficial fungus. Specifically, I hypothesized that labile carbon, gravimetric moisture and electrical conductivity will be negatively related, whereas calcium, phosphorus, potassium, magnesium will be positively related, and elements such as copper, zinc and sulfur and pH will be positively or negatively associated to the detection of Metarhizium. I observed that gravimetric soil moisture, pH and sulfur were positively related to the frequency of detection of Metarhizium. The range of sulfur at our experimental site was within the normal agronomic range for crop production in Pennsylvania soils. The remaining soil characteristics were not instrumental in M. robertsii detection in soil. Thus, my research identifies soil characteristics which could be manipulated by the farmers to conserve M. robertsii in the soil at site.
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