I get this on a couple of my plants every year and tried a new approach this season. My experiment was to dig up the soil in the affected area and replace it with a 18-gal plastic tote box filled with new soil. I drilled holes in the bottom of the tub for drainage. I did this for three plants had no evidence of the root knot when I pulled them up two weeks ago. The plant's roots had gone through the the drainage holes but no root knot was seen on the roots below the box.

My theory was that the nematodes exist in the top foot of soil. My tote boxes were two feet deep. As a control, I had two plants in the affected soil without any protection. They both developed the root knot.