I. Introduction

Marigold (Tagetes spp.) is a commonly-used ornamental plant, but its attractive blooms aren’t the only benefit of planting this crop. Marigolds have been seeded between beds of solanaceous plants in India for hundreds of years for nematode and insect pest management (Khan, 1971). The levels of nematode control by 29 different varieties of marigolds were evaluated back in 1938 and, although considerable variation was present, an overall suppressive effect was observed (Tyler, 1938).

When tested against 40 strains of bacteria and fungi, essential oil of Tagetes was found to have a 100% inhibitory effect against Gram-positive bacteria and fungi, and a 95% inhibitory effect against fungi (Hethelyi et al., 1986). Thiophenes, one of several compound classes found in marigold, show significant antiviral capabilities (Soule, 1993). In fact, 7 of the 10 most effective antiviral thiophenes are found in Tagetes (Atkinson et al., 1964; Hudson, 1990). A chemical released by marigold roots called α-terthienyl has drawn much attention for its nematicidal characteristics.

II. Nematode Suppression

Suatmadji (1969) reported that marigold prevented the population increase of 14 genera of plant-parasitic nematodes. Of these genera, lesion nematodes (Pratylenchus spp.) and root-knot nematodes (Meloidogyne spp.) are most affected. Nematode suppressive effects of marigolds differed among marigold species, cultivars, targeted nematode species, and even soil temperature (Ploeg and Maris, 1999).

African marigolds (Tagetes erecta) and French marigolds (Tagetes patula) are commonly-used species of this plant. Each boasts nearly 30 varieties, and nematicidal effects can vary as much as the flower characteristics themselves. Varietal differences include bloom size, shape and color, as well as plant size and leaf shape.

A summary of the susceptibility of the various marigold species or varieties to different plant-parasitic nematodes is listed in Tables 1, 2, and 3.

Table 1. Susceptibility of marigold varieties to root-knot nematodes.

Marigold Variety	Reduction in Root Galling Caused By:
	M. incognita	M. arenaria	M. hapla	M. javanica
*African Marigold (T. erecta)*
Unknown variety ⁽⁷⁾	--	--	--	Resistant
‘Toreador’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Diamond Jubilee’ ⁽⁶⁾	Resistant	Intermediate	Resistant	Resistant
‘Alaska’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Crackerjack’ ⁽⁵⁾	Resistant	Resistant	Resistant	Resistant
‘Flor de Muerto’ ⁽⁵⁾	Resistant	Resistant	Resistant	Resistant
*Triploid Hybrid Marigold (T. erecta* x T. patula)**
‘Red Nugget’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Polynema’ ^{(4, 5)}	Resistant	Intermediate	Resistant	Intermediate
*French Marigold (T. patula)*
‘Bolero’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Dwarf Primrose’ ⁽³⁾	Resistant	Resistant	--	Resistant
‘Goldie’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Petite’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Petite Harmony’ ⁽⁶⁾	Resistant	Intermediate	Resistant	Resistant
‘Single Gold’ ^{(4, 5)}	Resistant	Resistant	Susceptible	Resistant
‘Tangerine’ ⁽⁶⁾	Resistant	Resistant	Resistant	Resistant
‘Bonita Mixed’ ⁽⁵⁾	Resistant	Resistant	Resistant	Resistant
‘Gypsy Sunshine’ ⁽⁵⁾	Resistant	Resistant	Resistant	Resistant
‘Scarlet Sophie’ ⁽⁵⁾	Resistant	Resistant	Resistant	Resistant
*SignetMarigold (T. signata pumila)*
‘Golden Gem’ ⁽⁶⁾	Susceptible	Susceptible	Resistant	Intermediate
‘Tangerine Gem’ ⁽⁵⁾	Susceptible	Susceptible	Intermediate	Susceptible
*Mexican Marigold (T. minuta)* ^{(1, 2)}
	Resistant	Susceptible	Susceptible	Resistant

1. Belcher and Hussey, 1977.

2. Lehman, 1979.

3. McSorley and Frederick, 1994.

4. Ploeg, 2002.

5. Ploeg, 1999.

6. Rickard and DuPree, Jr., 1978.

7. Sipes and Arakaki, 1997.

Table 2. Marigold varieties that offer some resistance to Meloidogyne incognita.

African marigold (T. erecta)
	‘Toreador’ ⁽⁶⁾
	‘Diamond Jubilee’ ⁽⁶⁾
	‘Alaska’ ⁽⁶⁾
	‘Flor de Muerto’ ⁽⁵⁾
	‘Crackerjack’ ⁽⁵⁾
Triploid hybrid marigold (T. erecta x T. patula)
	‘Red Nugget’ ⁽⁶⁾
	‘Polynema’ ⁽⁵⁾
French marigold (T. patula)
	‘Bolero’ ⁽⁶⁾
	‘Bonita Mixed’ ⁽⁵⁾
	‘Goldie’ ⁽⁶⁾
	‘Gypsy Sunshine’ ⁽⁵⁾
	‘Petite’ ⁽⁶⁾
	‘Petite Harmony’ ⁽⁶⁾
	‘Petite Gold’
	‘Scarlet Sophie’ ⁽⁵⁾
	‘Single Gold’ ⁽⁵⁾
	‘Tangerine’ ^{(5, 6)}
Mexican marigold (T. minuta)^{(1, 2)}

Note: Check references for marigold varieties used, since results may depend on variety

1. Belcher and Hussey, 1977.

2. Lehman, 1979.

5. Ploeg, 1999.

6. Rickard and DuPree, Jr., 1978.

Table 3. Susceptibility of three marigold species to various plant-parasitic nematodes

Nematode	Marigold variety / species
Nematode	T. patula	T. erecta	T. minuta
Belonolaimus longicaudatus	Susceptible ⁽¹⁷⁾	--	--
Ditylenchus destructor	--	Resistant ⁽¹⁸⁾	--
Dolichodorus heterocephalus	Susceptible ⁽¹⁷⁾	--	--
Helicotylenchus multicinctus	--	Resistant ⁽¹⁰⁾	--
Hoplolaimus galeatus	Intermediate⁽¹⁷⁾	--	--
Hoplolaimus indicus	--	Resistant ⁽¹⁰⁾	--
Paratrichodorus christiei	Susceptible ⁽¹⁷⁾	--	--
Pratylenchus brachyurus	--	--	Resistant ⁽¹⁵⁾
P. nanus	Resistant ⁽¹⁹⁾	--	--
P. penetrans	Resistant ^{(12, 13, 14, 16)}	Resistant ⁽¹⁶⁾	--
P. pratensis	Resistant ⁽¹⁴⁾