Root-knot nematode (Meloidogyne spp.) is a pathogen that causes significant yield loss in soya bean production in South Africa.
The root-knot nematode’s life cycle is approximately 30 days, which means a large number of eggs can be produced by the female, which lives for three months. She will penetrate the root and migrate in search of a favourable area to feed on the tissue, where she will remain for the rest of her life cycle and form an egg parcel of approximately 1 000 eggs. These egg parcels are known as galls.
Root-knot nematode populations in the soil can increase exponentially over a short period of time, because of the advanced ability of the female to produce so many eggs. These nematodes can also change sex, depending on environmental conditions.
The hatching of eggs is not dependent on root secretions and it has been found that not all eggs will hatch under favourable environmental conditions. A percentage will therefore remain dormant in the soil and can live there for up to one year.
The prevalence of this nematode is widespread across all soya bean production areas in South Africa and is increasingly reaching economic threshold values in more areas. The presence of root-knot nematodes can drastically decrease yields without any visible symptoms of infestation, and they spread very easily.
The root-knot test
Surface symptoms of nematode infestation on crops are easily confused with typical environmental stress symptoms such as drought, nutrient deficiency or chemical damage. Knots or galls will, however, be visible on the roots if root-knot nematode infestation is very high – hence the name ‘root-knot’.
Underground symptoms of nematode infestation are noticeable in the root and the farmer can thus be sure of the cause of crop losses. Root-knot nematodes tend to occur in patches, which makes it easier to identify infestation.
However, many farmers are still doubtful as to whether root-knot nematodes are present in their soya bean fields and whether nematodes are harmful to their crops. There is an easy way to test for nematodes. This test can be done as early as the second-leaf stage in soya beans. All that is needed is a shovel, the possibly infected soya bean plants, food colouring, water, a watch, a container, a measuring cup and a teaspoon.
- Dig out the root with the shovel and shake off the soil around the roots (fine roots easily remain in the soil when the soya bean plant is pulled out). Keep the leaves on the plant.
- Rinse all the soil from the roots with clean water.
- Mix a teaspoon of food colouring with 500ml water.
- Immerse the plant’s root section in the coloured water and leave for 20 minutes.
- Remove the plant from the coloured water and rinse again with clean water.
- Place the soya bean root on a clean, white surface so you can clearly see if knots or galls are present.
- Use your smartphone’s camera to zoom in if necessary.
The difference between root knot nematode galls and bacterial nodules is that a nodule can be broken off and crushed, while root knot nematode galls form part of the root and the whole root will split down the middle when the knots or galls are crushed.
Tolerant vs susceptible varieties
Root-knot nematodes are sedentary endo-parasitic nematodes which can increase rapidly and cause serious problems. There are, however, different control measurements that can be applied.
Chemical control works if properly implemented, so that resistance does not develop. The implementation of crop rotation with the right crops and various biological control methods are other options.
It is important to understand that the implementation of any measure cannot be effective if it is not combined with a tolerant variety. Plants contain a genetically transferable ability to exhibit natural tolerance to nematode infestation, and if a susceptible cultivar is planted, the nematode population will continue to increase.
If a more tolerant variety is planted, infestation may be decreased or restricted to a minimum. Cultivar choice is therefore a major determinant and plays a significant role in combating crop losses caused by root-knot nematodes.
If the root-knot nematode infestation is very high, the plants will die before the producer can harvest if no control methods are applied and the plants are highly susceptible. – PR Janse van Rensburg, Pioneer
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