Natural Crop Protection in the Tropics: Nematodes

Nematodes are commonly called threadworms, eelworms, or roundworms. There are four major types of nematodes – bacterial-feeding, fungal-feeding, root-feeding and predatory. There are ecto- and endoparasitic root-feeding nematodes. It is estimated that as many as three billion nematodes may live in one acre of soil, with most of those in the top three inches.

Damage pattern

It is the root-feeding nematodes which are damaging to plant growth. As few as one endo-parasitic nematode per plant may be enough to result in decreased productivity or death, while plants may tolerate several hundred ecto-parasitic nematodes per root system without reduction in production. If a crop suffers nematode attacks, symptoms are not very specific but include stunting, yellowing of the leaves, poor growth and abnormal roots. Thus, nematodes can lead to significant yield reductions. Due to their unspecific symptoms, they are often not detected as the source of the problem. Some of the nematodes form galls around the roots and can thus be more easily recognized than endo-parasitic species. Repetitive cultivation of suitable host plants in the same fields can result in an increase of the nematode population to such a level that severe damage to crops occurs. Particularly in vegetable monocrops without rotation, complete crop failure is not uncommon. Nematodes are problematic in cultivation systems which are based on perennial crops or which are highly intensive, where crop rotation is difficult to practise.

Control measures

The most sustainable approach to nematode control involves integrating several strategies, including the use of crop rotation, soil solarization, cover crops, nematode-controlling plants, and the use of plant varieties resistant to nematode damage. These methods work best in the context of a healthy soil environment with sufficient organic matter to support diverse populations of microorganisms. A balanced ecosystem in the soil, with a wide variety of biological nematode control tactics, will help keep nematode pest populations in check.

Cultural methods

• Seedlings can be dipped in garlic extracts before transplantation.

• Dried leaves of garlic can also be incorporated into the soil.

• Most nematode species can be significantly reduced by ploughing in chitinous materials such as crushed shells of crustaceas (shrimp, crab, etc.). This is effective because several species of fungi which 'feed' on chitin also attack chitin-containing nematode eggs and nematodes. Increasing the amount of chitin in the soil will increase the population of these fungi, which will move on to nematodes when the crushed shell is gone.

Nematode - controlling plants

Marigold

Tagetes spp., Fam. Compositae

T. patula, according to the present state of knowledge, can suppress the widest range of nematodes, followed by T. erecta. Another nematode-controlling species is T. minuta.

Plant parts with nematode-controlling properties

Roots

Mode of action

Acts as trap crop, reduces reproduction rate

Target nematodes

Meloidogyne spp. (root-knot nematodes)

M. incognita is one important species of nematodes due to its extraor-

dinary number of host plants, including most vegetables but also tea and

cotton.

Vegetables, however, differ in the degree of susceptibility, tolerance and

reproduction rate. However, not all nematodes can be controlled by enemy

plants.

Pratylenchus spp.

Pratylenchus penetrans is the species most common of this genus of

nematodes. In the tropics it is widely distributed in the higher altitudes.

It attacks a wide range of host crops.

Control mechanism

Research results provided evidence that the marigold species T. erecta, T. patula and T. minuta reduce the population of root knot nematodes to very low levels in 42 to 70 days. The root knot larvae failed to penetrate the marigold roots in appreciable numbers and those that entered did not develop beyond the infective second-stage larval form. The suppressing activity of marigold on nematodes is based on the presence of terthiophenes, a group of chemicals, in its roots. These are oxidized when the nematodes penetrate the roots. This causes the release of a 'singulett-oxygen' which blocks the metabolism of the nematodes. Besides this explanation, others are presently being explored.

Marigold as cover crop

In greenhouse experiments testing the control effect of marigold as cover crop under roses and cucumbers, 1 g and 0.5 g seeds of marigold per m2 have been sown. Infestation was checked after 4 and 7 months. The results showed that marigold was able to reduce the population of the nematode Pratylenchus spp. by 75% and of Paratylenchus spp. by 100%.

Marigold in crop rotation

Comparing the effectiveness between cover crop and crop rotation, LUNG et al. concluded that the application as monoculture in a crop rotation system produces a higher suppressive effect on the above nematodes. They also suggest that a density of 1 g seed/m2 land is superior to lower cover densities because the higher density suppresses also weeds some of which can also be hosts for the nematodes.

It has also been observed that the suppressive effect of 70–85% on the nematode Pratylenchus spp. continues over several subsequent cropping seasons of host plants after a 3-month marigold rotation. The cultivation of host plants over three years after Tagetes patula did not lead to an essential increase of the nematode population density. In the greenhouse, Tagetes reduced the population density of Meloidogyne spp. nematodes by 95% after a cultivation time of two months.

Assuming that an effective control requires a density of 0.5–1.0 g/m2, then 5–10 kg of seeds per hectare are needed. The costs of cultivating marigold are only 5% of those of using the nematocide Basamide and 1% of soil heating/steaming. More experiments should be conducted to find out if this continued suppressing effect is higher if marigold is incorporated into the soil at the end of the rotation.

Assessment

A weakness of this method is the high amount of seeds required on larger areas. In areas where land is scarce, the occupation of the field with a trap crop for three months may be critical.

The printed version contains more information about the following themes:

Manures and compost

Wood ash

Blue-greenalgae

Soil solarization