Expression of resistance to Meloidogyne incognita from Gossypium hirsutum L. was studied. Susceptible (M-8), moderately resistant (M-78) and resistant (M-315) cotton germplasms were compared for effects on stage of development of M. incognita. A simple, modified system was used to describe the post-infection development of the nematode in cotton roots with different levels of resistance. This system divided the life cycle of root-knot nematode into seven developmental groups and is based mainly on nematode body shapes; however, the extent of gonad development, presence of esophageal glands and stylet as well as the number of cuticles around the juvenile body were used as references to match with stage of development.

Developmental stage events are described as follows: Stage A is the development stage representing second-stage juvenile (J2) penetration into cotton roots; Stage B is the development stage in which the juvenile becomes sedentary and establishes a parasitic relationship with plant cells. During this stage, the plant cells begin to enlarge and change into giant cells and gall formation; Stage C and D can be considered transitional stages from juvenile to adult. The sexual differentiation of the nematode and significant enlargement of root galls occur at this stage; Stage E, F and G are adult stages and important for determining nematode reproduction. Stage E, adult females before egg laying; Stage F, females with eggs and egg-masses and Stage G, males.

Compared to the susceptible, development was delayed on the resistant cotton line and fewer nematodes completed normal development to egg laying female. Results also indicated that resistance appeared during two important stages of nematode development. About 6-8 days after inoculation (DAI) when nematodes should be in Stage B and again at about 14-18 DAI when nematodes have reached the female development Stage E. We presume that resistant genes are operating at two distinct times. The juvenile at 6-8 DAI may produce a signal, for the plant to form giant cells. This is blocked in M-315 plants with one of the resistant gene. The female at 14-18 DAI may produce another signal, for further development of giant cells which allows the nematode to become established, feed properly, and produce eggs. This signal is blocked in both M-315 and M-78 plants with the second resistant gene. Further experimental designs are necessary to evaluate this hypothesis.