MARIGOLDS AS COVER CROPS

K. E. Dover, R. McSorley, K. -H. Wang

Department of Entomology & Nematology, University of Florida

Last updated October, 2003

Contents:

Introduction

Nematode Suppression

Tables

Mode of Action

Allelopathic Effect

Enhancement of Nematode Antagonistic Microorganisms

Planting

Considerations

References

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⁾	--	--
Radopholus similis	--	Resistant ⁽¹⁰⁾	--
Rotylenchulus reniformis	Resistant ^(9,11)	Resistant ⁽⁸⁾	--
Tylenchorhynchus claytoni	Resistant ⁽¹³⁾	--	--

8. Alam et al., 1978.

9. Caswell et al., 1991.

10. Charles, 1995.

11. Ko and Schmitt, 1993.

12. Merwin and Stiles, 1989.

13. Miller and Ahren, 1969.

14. Oostenbrink, 1960.

15. Reddy et al., 1986.

16. Reynolds et al., 2000.

17. Rhoades, 1980.

18. de Waele et al., 1990.

19. Winfield, 1985.

III. Mode of Action

A. Allelopathic Effect

Research has shown that marigolds have nematicidal properties, and can help to manage root-knot nematodes when planted as a cover crop. The nematicidal compound released by marigold roots, a-terthienyl, is one of the most toxic naturally-occurring compounds found to date (Gommers and Bakker, 1988). The active ingredient of an effective synthetic nematicide, metam sodium (Vapam), is related to this chemical. The compounds is nematicidal, insecticidal, antiviral, and cytotoxic (Arnason et al., 1989; Marles et al., 1992).

Pratylenchus spp. (lesion nematodes) and Meloidogyne spp. (root-knot nematodes) can be managed by planting marigolds, and certain cultivars of the French marigold (T. patula) seem to give the most effective control of these damaging pests (Lehman, 1979; Belcher and Hussey, 1977; Motsinger et al., 1977; Rickard and DuPree, Jr., 1978; Suatmadji, 1969).

The presence of α-terthienyl inhibits the hatching of nematode eggs (Siddiqui and Alam, 1988). In addition, penetration of and development of Rotylenchulus reniformis was reduced within T. patula, a poor host (Caswell et al., 1991), and Meloidogyne spp. juveniles were unable to fully develop in roots of trap crop T. erecta (Ploeg and Maris, 1999).

B. Enhancement of Nematode Antagonistic Microorganisms

There are a number of microorganisms in the soil that may keep nematodes in check. Since planting marigolds may help suppress nematodes but not other soil microorganisms (Topp et al., 1998), populations of antagonists such as nematode-trapping and other nematophagous fungi are likely to increase. However, in a greenhouse and field study of pineapple, field soil planted with T. erecta ‘Cracker Jack’ for three months, then incorporated into the soil did not increase nematode-trapping fungal propagules (colony forming units, or cfu) per gram of soil better than a bare soil treatment (Wang et al., 2002). Yet in another pineapple field experiment where T. erecta was planted for an extended period of time (21 months through multiple plantings of T. erecta), nematode-trapping fungal population densities eventually built up to a peak of (16 cfu/g soil) at 19 months after planting. However, its population density crashed to 3 cfu/g soil when the crop was incorporated into the soil (Wang et al., 2003). This is an indication that marigold is an inefficient cover crop to be used to enhance the nematode-trapping fungal populations.

IV. Planting Tips

Marigold is summer crop in most of the United States, but can be grown year-round in parts of Florida. When used in crop rotations for nematode management in a nematode-susceptible vegetable crop, marigold needs to be planted at least two months before the desired vegetable crop. Marigolds are only effective if they are planted in the same site in which the vegetable crop will be planted (see “Considerations” section below). Of course, nematode populations will rebound over time when susceptible crops are grown (McSorley et al., 1999). The North Carolina Department of Agriculture and Consumer Services (NCDA&CS) suggests taking soil samples to determine which, if any, fertilizers or lime amendments are needed, since nutrient deficiencies or toxicities may exacerbate nematode problems (NCDA&CS, 2000).

Planting should be dense to ensure the best nematode control. Spacing less than 15 cm is recommended. One suggested pattern is to plant both marigold rows and plants within rows 7 inches apart. This flower grid allows for mechanical weed control (Vann et al., 2003; Gay, undated). Keeping the plot weed-free is very important because nematodes can reproduce on weed roots, thereby nullifying the effects of marigolds. Prior to planting the next crop, marigolds can be disked into the soil in the fashion of a green manure. Marigolds can be rotated to (the same) problem areas at least every other year until nematode problems are ameliorated (Doubrava and Blake, 1999).

V. Considerations

Not all marigold varieties control all types of nematodes. A single variety can also vary in its effect on different root-knot species. For example, T. erecta ‘Cracker Jack’ may show good control of the southern root-knot nematode (Meloidogyne incognita), but may also attract other nematodes such as the reniform (Rotylenchulus spp.), stubby-root (Trichodorus spp.) and spiral (Rotylenchus spp.) nematodes. Therefore, growers should determine which marigold variety to use based on their target nematode pests in their field. Furthermore, populations of the same root-knot nematode species can vary in their aggressiveness in different fields or locations (Carpenter and Lewis, 1991), so it is important to verify the effect of marigolds on local nematode populations before attempting management on a large scale.

In addition, the nematode population before planting marigolds is very important in determining the success of the following crop. Marigolds may suppress nematodes in the soil, but the pests are not eradicated. Therefore, although marigolds may have an effect on the plant-parasitic nematodes, they may not affect a (high) nematode population enough to protect the successive crop (Lehman, 1979).

Research has shown that the nematicidal compound (α-tertheinyl) released by marigold roots is only active at the site in which the flowers are grown. In other words, there is no benefit to amending a planting site with marigolds that were grown at a different site. These compounds are chemically altered by near-UV light and they become inactive when taken out of the soil. Toxic properties are only present in exudates from living roots, and not in extracts of homogenized roots or leaves (Wang, 2001; Ploeg, 2000).

Biomass generated by a marigold species also needs to be taken into consideration when selecting a suitable cover crop. Tagetes erecta produced more biomass than T. patula, and thus provided a longer nematode suppressive period (Supratoyo, 1993).

Some researchers have tested intercropping marigold with a cash crop to manage plant-parasitic nematodes. However, a marigold-cucurbit intercropping system was less productive than cucurbit monoculture, and no effect on plant parasitic nematodes was observed (Powers et al., 1993).

VII. References

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Anonymous. 2000. Root-knot nematodes: Biocontrol with French marigolds. Nema Note, North Carolina Department of Agriculture and Consumer Services, Agronomic Division, Raleigh, NC. http://www.ncagr.com/agronomi/nnote1.htm.

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Atkinson, R. E., R. F. Curtis, and G. T. Phillips. 1964. Bi-thienyl derivatives from Tagetes minuta L. Tetrahedron Letters 43: 3159-3162.

Bearce, B. C. Marigolds (Tagetes sp.). Undated. Center for Agriculture, Natural Resources, & Community Development, West Virginia University, Extension Service, Morgantown, WV. http://www.wvu.edu/~agexten/hortcult/flowers/marigold.htm.

Belcher, J. V. and R. S. Hussey. 1977. Influence of Tagetes patula and Arachis hypogaea on Meloidogyne incognita. Plant Disease Reporter 61: 525-528.

Carpenter, A. S., and S. A. Lewis. 1991. Aggressiveness and reproduction of four Meloidogyne arenaria populations on soybean. Journal of Nematology 23: 232-238.

Caswell, E. P., J. deFrank, W. J. Apt, and C.-S. Tang. 1991. Influence of nonhost plants on population decline of Rotylenchulus reniformis. Journal of Nematology 23: 91-98.

Charles, J. S. K. 1995. Effect of intercropping antagonistic crops against nematodes in banana. Annals of Plant Protection Sciences 3: 185-187.

Doubrava, N. and J. H. Blake. 1999. Root-knot nematodes in the vegetable garden. Publication number HGIC 2216. Home and Garden Information Center, Clemson University, Cooperative Extension Service, Clemson, SC. http://hgic.clemson.edu/factsheets/HGIC2216.htm.

Gay, J. D. Undated. Nematode control in the home vegetable garden. Leaflet 209. College of Agriculture and Environmental Sciences, University of Georgia, Cooperative Extension Service, Athens, GA. http://www.ces.uga.edu/pubcd/l209-w.html.

Gommers, F. J. and J. Bakker. 1988. Physiological diseases induced by plant responses or products. Pp. 3-22 in: Diseases of nematodes. G. O. Poinar, Jr. and H. -B. Jansson, eds., Vol. I. CRC Press, Inc., Boca Raton, FL.

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Hudson, J. B. 1990. Antiviral compounds from plants. CRC Press, Inc., Boca Raton, FL.

Khan, A. M., S. K. Saxena, and Z. A. Siddiqi. 1971. Efficacy of Tagetes erecta in reducing root infesting nematodes of tomato and okra. Indian Phytopathology 24: 166-169.

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Miller, P. M., and J. F. Ahrens. 1969. Influence of growing marigolds, weeds, two cover crops and fumigation on subsequent population of parasitic nematodes and plant growth. Plant Disease Reporter 53: 642-646.

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Ploeg, A. T. 2000. Effects of amending soil with Tagetes patula cv. Single Gold on Meloidogyne incognita infestation on tomato. Nematology 2: 489-493.

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Ploeg, A. T. and P. C. Maris. 1999. Effect of temperature on suppression of Meloidogyne incognita by Tagetes cultivars. Journal of Nematology 31(4S): 709-714.

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Wang, K.-H., B. S. Sipes, and D. P. Schmitt. 2002. Management of Rotylenchulus reniformis in pineapple, Ananas comosus, by intercycle cover crops. Journal of Nematology 34: 106-114.

Wang, K.-H., B. S. Sipes, and D. P. Schmitt. 2003. Intercropping cover crops with pineapple for the management of Rotylenchulus reniformis. Journal of Nematology 35: 39-47.

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