I had never heard of nematodes before, I guess that they are not as much of a problem in New England? Down here in Houston they are a problem.
When you bring a plant home, carefully check the roots before planting. Look for knots and swollen areas on the roots. Once you add root knot nematodes to your garden, they are there forever. So it is best not to bring them home. ( University of Ontario has an article with several good pictures ). Be especially sure to check your tomato plants since that is the most likely way you’ll pick them up down here.
Nematodes are microscopic worms some are good, some are bad. The bad ones have a needle like part they use to puncture plant roots. They then inject an enzyme into the root which softens root, then they suck that back out of your plant roots.
You can reduce the number of nematodes in your soil by adding grated orange peel about 1%-5% to your soil. Nematodes must be in damp or wet soil. They die when they dry out, but the eggs live on to reappear when soil gets wet.
Marigolds prevent and destroy nematodes