Volvo, I feel for you. Discovering nematodes in the garden is not fun. I've been doing battle with them for at least five years. They only appeared when I had a load of top soil delivered to add to 12 newly built raised beds when I retired. Then they spread to other beds probably because I was using shovels, etc in an infested bed and then taking that implement right to an uninfected bed and using it in there. Nematodes will slowly but surely also spread via rain runoff.
My battle with them is documented here along with links for further reading:
http://www.tomatoville.com/showthrea...matodes&page=7
Lots of reading done, lots of experimenting. One thing for sure is that if you have 'todes, you have them. There is no getting rid of them. You can only beat them into temporary submission. More organic material in the soil and letting infected beds go fallow a season help set them back. I have 16 raised beds 4'x18' so that's not too hard but it can be tough if you have only a few beds or no beds as in open areas that are row cropped.
Nematodes are inactive in cool soils (don't harm plants) and become active again when soil temps reach 65 degrees. Because of that, fall/winter things don't seem to be bothered. I have a hunch, not proven, that spring stuff planted as early as it can be in an infected area will develop roots old enough by the time the 'todes wake up to keep damage to a minimum. Perhaps roots toughen up as they age to a point where it's harder for nematodes to penetrate them? That varies year to year. Too many other variables to account for too.
I've tried growing and tilling in mustard but it hasn't worked very well. It has to be intensively planted, chopped up well and watered in. Pain in the butt. However, I happened to notice within the past year or so that infected beds that had fall/winter brassicas grown in them had a depopulation of 'todes to the point where I could successfully grow a susceptible crop in that same bed come spring. My usual broccoli, cabbage, kale, collards etc (fall and spring plantings) are what's been planted. Since none of that was tilled in I can only guess something is coming off the plant roots to knock them back.
Planting things that nematodes don't mess with helps starve them for a season. It's akin to letting the soil go fallow. They don't mess with my alliums or corn, some of which (popcorn, very early sweet corn) gets planted in beds where nematodes have been the season before. Flowers don't seem to be bothered so far, at least not zinnias, marigolds, coneflowers, daisies, etc.
Sanitation has become a routine. I rinse off implements that have just been used in *any* bed and spray with a 10% bleach solution before using them in another bed.
I think that this year the soil building, sanitation, fallowing, temporarily reducing populations by planting stuff they don't eat is having a positive effect. I would suggest trying to plant brassicas wherever you see nematode damage starting to try to reduce spread. Map your garden and be sure to note where you find nematodes and concentrate efforts there. Sanitize.
Good luck and please update with your efforts to battle them and any results!