Greenhouse Studies on the Effect of Marigolds (Tagetes spp.) on Four Root-knot Nematode Species (Meliodogyne spp.)

The effects of pre-planted marigold on tomato root-galling and multiplication of Meloidogyne incognita, M. javanica, M. arenaria, and M. hapla were studied. Marigold cultivars of Tagetes patula, T. erecta, T. signata, and a Tagetes hybrid all reduced galling and numbers of second-stage juveniles (J2) in subsequent tomato compared to the tomato-tomato control. All four Meloidogyne spp. reproduced on T. signata ‘Tangerine Gem.’ Several cultivars of T. patula and T. erecta suppressed galling and reproduction of Meloidogyne on tomato to levels lower than or comparable to a fallow control. Phytotoxic effects of marigold on tomato were not observed. It is concluded that several of the tested marigold cultivars are now ready for full scale field evaluation against Meloidogyne spp. Root-knot nematodes (Meloidogyne spp.) are the most economically important plant-parasitic nematodes in tropical and subtropical agriculture (Sasser, 1979). In California, control of these nematodes is dependent primarily on nematicides and resistant crop cultivars. However, as the use of nematicides is being severely restricted and resistant cultivars are available for only a limited number of crops (Roberts, 1990), development of alternative control strategies is urgently required. Marigold (Tagetes spp.) has long been known to possess nematicidal activity. Initial reports on suppression of root-knot nematodes by marigold (Steiner, 1941; Tyler, 1938) were later confirmed for M. arenaria, M. hapla, M. incognita, and M. javanica (Daulton and Curtis, 1963; Hackney and Dickerson, 1975; McSorley and Frederick, 1994; Motsinger et al., 1977; Rickard and Dupree, 1978; Suatmadji, 1969). However, results from these studies were often equivocal. Thus, T. erecta did not suppress M. hapla in one study (Suatmadji, 1969), but efficiently controlled it in two other studies (Bünte and Müller, 1996; Rickard and Dupree, 1978). In studies with T. patula, M. arenaria was suppressed (Motsinger et al., 1977; Suatmadji, 1969) or not suppressed (McSorley and Frederick, 1994; Rickard and Dupree, 1978). The reasons for these conflicting results are not known, but it is likely that intraspecific differences in the plants and in the nematodes play an important role (Motsinger et al., 1977; Suatmadji, 1969). Results with M. incognita and M. javanica were more consistent. Strong suppression of these nematodes, particularly by T. patula, was observed in several studies (Daulton and Curtis, 1963; Hackney and Dickerson, 1975; McSorley and Frederick, 1994; Siddiqi and Alam, 1988; Suatmadji, 1969). Suppression of lesion nematodes (Pratylenchus spp.) by marigolds also has been reported (Hackney and Dickerson, 1975; Hutchinson, 1962; McKenry, 1988; Suatmadji, 1969; Visser and Vythilingam, 1959) and appears less variable than suppression of root-knot nematodes. Despite numerous reports on suppression of lesionand root-knot nematodes, few studies have included the effects of marigolds on nematode infestation or yields of subsequent crops. Oostenbrink et al. (1957) reported yield increases in roses and apple on lesion nematode-infested soils after marigolds comparable to nematicide treatments. Later, Oostenbrink (1960) suggested that marigolds increased yields of lesion nematode-susceptible crops on sandy and peaty soils by 10% to 40%. Significant yield increases in lesion or rootknot nematode-susceptible crops after marigold were also reported by Bünte and Müller (1996), Miller and Ahrens (1969), Siddiqi and Alam (1988), and Suatmadji (1969). Seinhorst and Klinkenberg (1963) obtained significant yield increases after marigold, but suggested that these results might not be attributed solely to nematode control as yields of onions and sugarbeets were 1.4x and 1.2x higher, respectively, after marigolds in fields without plant-parasitic nematodes. In contrast, McKenry (1988, 1991), achieved significant reductions in P. vulnus populations, but did not observe expected yield increases in subsequent plum plantings. McKenry (1988) attributed the lack of yield increase to phytotoxicity of marigold that nullified the beneficial effects of nematode control, and concluded that marigold is unlikely to be of use for controlling nematodes in perennial crops. Given the conflicting results on the suppression of Meloidogyne species by marigold and effect of marigold on nematode infestation and yield responses of subsequent crops, this study was initiated to identify marigold cultivars that would effectively suppress Meloidogyne populations, decrease infestation levels in a subsequent susceptible crop, and be non-phytotoxic.