Meloidogyne incognita, and other Meloidogyne sp.
Infected plants are not vigorous, off color, and progressively decline. The decline becomes rapid as the fruit load increases. Roots are swollen and covered with galls.
The root knot nematode is very difficult to control. Crop rotation with a non-susceptible species is important, and avoiding planting tomatoes after other highly susceptible species (cotton, okra) is a primary control. Many tomato varieties are resistant, so variety selection is also important.