Plant cover crops such as cereal rye to add organic matter and disrupt the life cycle of root knot nematodes. The nematodes can't feed effectively on the rye, and it may be toxic to them. Plant them this spring. I've been told these annuals need to stay put for at least three months to be effective. Till the soil several times this summer to expose and dry it, depriving the nematodes of needed moisture.
Solarization is another technique that can be used in summer to literally cook the pests.
Till the garden area, and water until there's moisture at least eight inches down. Cover the area with clear plastic and secure the edges by burying them. As the heat builds, the nematode population declines. This process also will take out weeds. Remove the clear plastic in a month.