Systemic Acquired Resistance and Susceptibility to Root-Knot Nematodes in Tomato

Changes in host suitability of tomato (Lycopersicon esculentum 'Celebrity') to host-incompatible Meloidogyne incognita and host-compatible M. hapla were determined after concomitant and sequential inoculations of split-root assays. Initially, infective second-stage juveniles (J2) of M. hapla or M. incognita were applied to one-half of split-root systems, and 0, 5, 10, 15, or 20 days later, the other half was challenge-inoculated with the same or other species. Each challenge-inoculation had a corresponding control in which the same nematode species was applied to only one-half of a split-root system. Host suitability, based on nematode eggs (Pf) per unit of initial inoculum density (Pi) of 2,000 J2, was determined 60 days after challenge-infection. Prior inoculation with M. incognita significantly suppressed reproduction of challenge M. hapla applied 5 days after or later. Reproduction ratios (Pf/Pi) of challenge M. hapla were 20, 13, 6, 5, and 4, whereas corresponding controls were 21, 18, 17, 15, and 12. Concomitant inoculations with both species did not alter host suitability to either species nor did sequential inoculations with M. incognita as both prior and challenge species. Prior inoculation with M. hapla significantly enhanced reproduction of challenge M. incognita about four times relative to controls. These results indicate that prior infection of plants with incompatible or compatible nematode species induced systemic resistance or susceptibility, respectively, to later nematode infections.