All of the research is conducted on farms with farmers volunteering their time and land to the project. Companion planting for a Bug Free Garden. Several plants have been found to help control some kinds of nematodes when grown for several months in soil where they are present, if no good hosts of those nematode species are present. These plants may reduce nematode populations significantly more than fallowing or growing a crop on which the nematodes do not feed.
Control sometimes equals that from chemical treatments.
Excellent article from the State of Florida. Unfortunately, it's a PDF file. Fortunately, you can get Adobe Acrobat here, for free. An interesting list of companion planting pointers. Herb Companion Planting Chart.