The United States floriculture industry was valued at $6.43 billion in 2021, with Michigan being the third largest producer, producing 10% of all ornamental plants in the United States. A major constraint to the production of bare-rooted ornamental plants grown in the field are plant-parasitic nematodes. In Michigan, plant-parasitic nematodes cause millions of dollars in economic loss each year in the state's $104.7 billion agriculture industry. In the northern United States and Canada, the northern root-knot nematode, Meloidogyne hapla, is the most economically important perennial ornamental pathogen. While this is a known major pathogen of daylily production, one of top commodities in ornamental plant production in Michigan, very little is known about its impact in daylily production fields or how to effectively manage this pest. There are only two main management strategies for M. hapla in ornamental plant fields: hot water dips and preplant fumigation, both of which do not control M. hapla the entire production cycle and are therefore only semi-effective. Therefore, research was conducted to determine alternative management strategies to manage M. hapla in daylily production fields, with the goal to prevent yield loss and exportation rejection, and reduce the economic burden of this pest. Three multi-year field trials at a commercial nursery in Zeeland, MI, and several greenhouse experiments at Michigan State University's Plant Greenhouses, East Lansing, MI, were conducted to test several different management options and combination of management options to find the best new management strategies to control M. hapla in ornamental plant fields. The results of these studies demonstrate that there are more effective solutions for M. hapla management in ornamental plant field production compared to current practices and highlight three new management options: Indemnify as a soil drench, Majestene 304, and TerraClean 5.0 have been shown to provide the best M. hapla management in daylily fields, with a reduction in M. hapla population levels by 39.5%, 34.7%, and 28.8%, respectively, compared to the control. Indemnify also reduced the number of galled roots by 80% compared to the control plants, which is considerable and can lead to less fields being quarantined and fewer shipment rejections, significantly increasing the profits of the ornamental plant industry. The Indemnify treatment was additionally shown to have a significant positive effect on plant growth, producing plants with some of the largest overall plant biomass, such as plant heights, shoot weights, crown widths, and, most importantly, yield. Plants where Indemnify was applied as a soil drench always had higher yields (on average 41.3% higher) compared to the control plants and higher yields (on average 40% higher) compared to Telone II fumigation. These experiments also show that the annual application of treatments throughout the production cycle is crucial and provides significantly better M. hapla management compared to current practices, which only focuses on managing nematodes at the beginning of the production cycle. Most importantly, these trials show that there was no impact on plant growth, health, and yield from annual treatment applications. Even though M. hapla is well established in these monoculture, long-term ornamental plant fields, a trial determining possible soil suppression showed that natural suppression may not be occurring in ornamental plant fields in Michigan, but more experiments are needed. Two greenhouse trials tested the damage potential and host status of Hemerocallis spp. to M. hapla and Paratylenchus spp., and determined the threshold level of M. hapla. These greenhouse experiments show that daylily is an excellent host to M. hapla, with a threshold level as low as 13 M. hapla/100 cm3 soil. The data also suggests that even though M. hapla affects plant growth, daylily plants may actually be tolerant to M. hapla; over the length of the daylily growth cycle, the plants became more tolerant of its feeding and grew to similar sizes of the nematode-free plants. Lastly, daylily was shown to not be a host to Paratylenchus spp., and therefore, these nematodes do not need to be included in management decisions. Through the application of the new alternative and more sustainable management strategies described in this dissertation, M. hapla can be effectively and efficiently managed in ornamental plant fields leading to a significant advancement in the floriculture industry in Michigan, the northern United States, and Canada.
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