“Grandmother always planted marigolds around her tomatoes“ Evidently Grandmother had it absolutely correct and here’s why:
Although they can be beneficial against a variety of pests, marigolds are best known for their ability to suppress plant-parasitic nematodes. In India, marigolds have been used for this purpose for hundreds of years (Khan, 1971).
Marigolds can suppress 14 genera of plant-parasitic nematodes, with lesion nematodes (Pratylenchus spp.) and root-knot nematodes (Meloidogyne spp.) the most affected (Suatmadji 1969). Different varieties of marigolds vary in their ability to suppress nematodes. In addition, nematode supression is influenced by crop plants, nematode species, and soil temperature (Ploeg and Maris, 1999; Tables 1-2). Tyler (1938) investigated the effects of 29 varieties of marigolds on nematode populations. Although variation was observed, marigolds had an overall suppressive effect on nematodes.
Marigolds may be resistant to some nematode species but may be very susceptible to others (Table 2). The lesion nematode (Pratylenchus spp.) is a problem is regions like Europe and other countries, but in Florida it is not considered to be a nematode of major concern and probably does not require management. However, French marigold cultivars (T. patula) appear to be most effective against the widest range of nematodes (Lehman, 1979; Belcher and Hussey, 1977; Motsinger et al., 1977; Rickard and DuPree, Jr., 1978; Suatmadji, 1969, Pudasaini et al. 2006, Evenhuis et al. 2004).
Allelopathic Effect
Allelopathy is the ability of an organism to produce chemicals that are toxic to other organisms. Marigold roots release the chemical alpha-terthienyl, one of the most toxic naturally occurring compounds found to date (Gommers and Bakker, 1988). This compound is nematicidal, insecticidal, antiviral, and cytotoxic (Arnason et al., 1989; Marles et al., 1992).The presence of alpha-terthienyl inhibits the hatching of nematode eggs (Siddiqui and Alam, 1988). However if in a field setting, it is unclear if marigolds producing alpha-terthienyl inhibit development because of the alpha-terthienyl itself or because marigolds are a non-host for certain nematodes. Nematodes may not feed or develop on non-host plants even when they do not contain allelopathic compounds. Furthermore, Meloidogyne spp. juveniles were unable to fully develop in the roots of T. erecta (Ploeg and Maris, 1999).
From the IFAS (university of florida extension)
