Role of Salicylic Acid and Phenols in the Resistance of Tagetes species Against Meloidogyne incognita

To understand the basis of mechanism of host resistance in marigold spp. against root-knot nematode, Meloidogyne incognita, marigold, Tagetes erecta cv. Hazara and YD, and T. patula cv. Jafri were grown in pots inoculated with 500, 2500 and 5000 juveniles of M. incognita/kg soil, and the biochemical (chlorophyll, total phenol and salicylic acid), histopathological (root penetration), and morphological responses of the host plant (length, fresh and dry weight of shoot and root) and soil population of the nematode were determined. The marigold cultivars were found resistant to root-knot nematode and did not supported gall formation or egg mass production. Nematode population drastically decreased over time, and only 3–11% larvae could penetrate the roots, the penetrated juveniles remained confined to cortex and did not attain maturity. Plant growth variables and leaf pigment were not influenced, but total phenol and salicylic acid contents of leaves of marigold cultivars growing in the infested soil showed drastic increase (P≤0.001) and varied linearly with the inoculum level. The concentration of the biochemicals was relatively greater in the host cultivar which supported lesser root penetration. This resistance was mainly biochemical rather than morphological. The small proportion of nematode population that penetrated into roots was not able to grow normally due to synthesis of greater amount of phenols and salicylic acid. Marigold cultivars not only antagonized the penetrated juveniles, but also suppressed the nematode larvae in soil and caused mortality to them apparently through root exudates.