Root-knot Nematodes: Biocontrol with French Marigold

Root-knot nematodes (Meloidogyne spp.) can be effectively managed in home gardens without pesticides. One alternative to using nematicides is to intercrop and rotate vegetables with marigolds. Most cultivars of African marigold (Tagetes erecta) and French marigold (Tagetes patula) are effective in reducing the most common root-knot populations—Meloidogyne incognita and M. javanica (Table 1).

The roots of these attractive flowering plants contain chemicals that kill nematodes. As a method of biocontrol, growing marigolds is not only pleasing to the eye but economical and environmentally sound as well. One drawback, however, is that the benefits are not realized until the following year. Also, the treatment frequently needs to be repeated with marigolds and vegetables grown in alternate years.

Be careful when you buy marigolds to help control nematodes. Not all marigolds sold as bedding plants are effective against nematodes. Tagetes signata cv. Tangerine Gem, for example, will not control root knot.

To use marigolds to manage root-knot nematodes, follow these steps.

• Collect a soil sample from the root zone of a living, but apparently unhealthy, plant. Include roots in the sample. Submit this sample to the Nematode Assay Section of the NCDA&CS Agronomic Division to verify that root knot is the problem. It is also a good idea to send a soil sample to the Soil Testing Section for lime and fertilizer recommendations.
• At the end of the growing season, remove as many roots as possible from the soil by pulling, plowing or tilling. Doing so will reduce the number of safe places where nematodes can survive during the winter.
• In winter, till the soil several times to expose nematodes to the sun and weather.
• Apply lime according to soil test recommendations.
• In spring, plant half of the garden with marigolds and half with root-knot-resistant vegetable cultivars. Plantings in blocks or strips are easy to manage (Figure 1). Strips may comprise one or several rows of vegetables. Contact your county Cooperative Extension agent for Plant Pathology Note 6, which lists nematode-resistant vegetable varieties. Table 2 indicates the amount of marigold seeds or plants needed based on area.
  • Use a marigold variety known to be Tagetes erecta or T. patula.
  • Space marigold plants, or thin seedlings, so they are 7 inches apart (Figure 2).
• Fertilize according to soil test recommendations. Nutrient imbalances can make nematode problems worse.
• Early in the growing season, cultivate between marigolds regularly to keep weeds under control.
• Before the first frost, remove as many seed-bearing flower heads as possible. Then, you will have seed for next year's marigold patches, and fewer volunteer marigolds will sprout among your vegetables.
• In the fall before crop residue destruction, take another soil sample for nematode assay from the marigold area, if you want to monitor effectiveness.
• Till the remaining marigolds into the soil.
• The following spring, repeat the process with this exception: plant marigolds where you planted vegetables the previous year and vice versa.

Figure 1. Strip-crop arrangement.

Figure 2. Detail of marigold spacing on 7-inch centers.

Table 2. Marigold seed or plant requirements per treatment area.

Total Area in Marigolds	Seed Required	Plants Required
100 sq. ft.	6 packets (~350 seeds)	300
1,000 sq. ft.	0.5 ounce	3,000
1 acre	17.5 ounces	130,680

Additional information on nematode problems can be obtained from your county Cooperative Extension office. Assistance in interpreting the nematode control recommendations can also be obtained from the NCDA&CS regional agronomist for your area. For more information about the Agronomic Division and its services, see our website at www.ncagr.com/agronomi/ or call 919-733-2655.