Prevention is always the best cure for a pest problem, and root knot nematodes are no exception. So let’s talk about prevention.
First off, plant nematode-resistant varieties. There are a wide variety of different seed suppliers who carry nematode-resistant seed stock, usually notated as an N in the resistance charts.
If you cannot plant nematode-resistant strains, practice good crop rotation. Some species of root knot nematodes are more selective than others. Planting cover crops like marigolds or sudangrass between at-risk crops will also bring down the nematode population.
Remove the roots of old plants when clearing the bed. As root knot nematode juveniles can live in the galls they form on the roots, they will continue to multiply even as the roots are dying out. Removing the remaining root mass can extract those juveniles.
Till the soil 2-3 times in the fall. This breaks up the soil, turning the nematodes up to the surface where they will die off from exposure to the sunlight. This will impact both beneficial and parasitic nematodes, so you may need to re-add beneficial nematodes again in the spring.
Plant overwintering grass cover crops like wheatgrass, ryegrass, or rye. Sudangrass is also good and offers some nematicide properties. Keep these mowed down to a manageable level, and till them under in the spring to add more plant matter to the soil.
Regularly add more organic material to your soil. Adding more composted leaves, grass clippings, and manure to your beds will help naturally control the population of nematodes in the soil, since nematodes prefer living material to decaying plant matter.