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Nematodes are far more prevalent and a more serious problem than many farmers think. This was the important message conveyed by Dr Driekie Fourie, technical product lead for Syngenta Seedcare in Africa and the Middle East during a panel discussion with Yolandi Furniss, marketing lead, Syngenta Seedcare South Africa, at the International Seed Federation’s World Seed Congress. The conference was hosted by the South African National Seed Organisation and held recently in Cape Town.
Nematodes are commonly referred to as roundworms. Microscopically small and mostly transparent, they can usually not be observed with the naked eye: a microscope is needed to identify many of the nematode species.
Various groups
The biggest plant-parasitic nematode is about 11mm long but very thin, only a few micrometres in diameter, and it can be seen as a little thread in a petri dish when extracted in water, Fourie pointed out.
There are different groups of nematodes. Some, the plant-parasitic nematodes, are seen as pest organisms while others are as seen as beneficial as they play an important role in nutrient recycling in soils, and feed on other organisms.
Mature adult females in some species, such as the root-knot nematode, change to a swollen, pear-like shape, whereas the females of other species such as lesion nematodes remain slender worms.
According to Fourie, three of the most important nematode species that have a major impact on agriculture in South Africa and that farmers should be aware of, include root-knot nematodes (Meloidogyne spp.), the cyst nematodes (Globodera spp. and Heterodera spp.) and lesion nematodes (Pratylenchus spp.).
Impact is underestimated
She pointed out that the impact of nematodes was still very much underestimated because the non-specific above-ground symptoms are often confused with water stress or water logging, nutrient deficiencies, and secondary fungal or bacterial infections.
“What exacerbates the situation is that these various nematode species commonly simultaneously occur in crop fields, causing significant damage to the crops and subsequent yield losses. In high population densities, nematodes can cause discoloration of leaves and have a negative impact on root development,” Fourie said.
“The problem can easily be mistaken for something else, so plant analysis is necessary to identify the cause.”
Spring activity
She also pointed out that increases in soil temperature and the presence of moisture could lead to a resumption of activity after winter dormancy. This is especially the case at present, following the early winter rains experienced in many parts of the summer rainfall area. She advised that farmers should start monitoring for nematodes now.
Fourie explained that plant-parasitic nematodes survive in soil with a wide range of pH values, i.e. from 3 to 10. “These unique organisms have highly specialised survival strategies that enable them to survive in extreme and adverse conditions.”
Damage
Plant-parasitic nematodes can survive in a variety of soil types, although these pests most often cause the most damage in cereal crops grown in sandy soils.
As nematodes adversely affect the development of the root systems of plants, damaged or affected roots cannot effectively translocate water and nutrients to above-ground parts of the plant. Consequently, farmers will see areas of poorly growing plants in their fields. Due to the small size of these organisms, they cannot move very far on their own, therefore causing damage only in small patches.
“These areas of damage usually enlarge over seasons. This is due to the use of implements that spread the soil. Run-off water also plays a role in spreading nematodes, especially during heavy rainfall spells. Wind can also be a factor in spreading nematodes, especially in areas with very sandy soils,” she explained.
Signs of infestation
Farmers need to look out for other signs such as:
- Yellowing of leaves;
- Stunting of plants;
- Plants wilting; and
- Spots in fields where plants are not growing optimally.
“If a farmer sees any of these signs, plants need to be removed from the soil, and the farmer needs to look for symptoms on the parts that are beneath the soil surface,” she explained.
Infestations of the root-knot nematodes are fairly easy to recognise. Root-knot nematodes usually cause distinctive swellings, called galls, on the roots of affected plants. The characteristic galls produced on infected roots or other underground plant parts (e.g. tubers, rhizomes and pods) distinguish root-knot nematode damage from that caused by any other factor or disease.
Lesion nematodes can cause darkened necrotic lesions on roots or other parts of the plants that are below the soil surface. These are difficult to identify because they can also be caused by other plant diseases.
“Limited information with regard to the health status of soils and the prevalence of nematodes still remains a great concern to researchers and producers alike, since it has a direct negative impact on crop production,” she said.
Fortunately, more research is nowadays focusing on this topic.
Increased dissemination of this type of information will contribute to farmers’ knowledge of nematodes and plant health while providing strategies that can be used to improve soil-health status in local crop production areas.
