Nematode management starts with knowledge

Nematodes are commonly referred to as roundworms. Nematodes are microscopically small and mostly transparent and since they are so little, they can usually not be observed with the naked eye. A microscope is needed to identify many of nematode species. The biggest parasitic nematode is about 11 mm 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, Dr Fourie pointed out.

There are different groups of nematodes. Some, the 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 females of other species such as lesion nematodes remain slender worms.

According to Dr Fourie, three of most important nematode species that have a major impact on agriculture in South Africa and  that farmers should be aware of, include the Root-knot nematode (Meliodogyne spp.), the Cyst nematode (Globodera spp. & Heterodera spp.) and the Lesion nematode (Pratylenchus spp.).

She pointed out that the impact of nematodes are still very much underestimated because the non-specific above-ground symptoms are often confused with water stress or water logging, nutrient deficiencies, or secondary fungal or bacterial infections.

“What exacerbate 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 discolouration of leaves and have a negative impact on root development. The problem can easily be mistaken for something else, so plant analysis is necessary to identify the cause,” Fourie said.

She also pointed out that increases in soil temperature and the presence of moisture can 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 rain fall area. She advised that farmers should start monitoring for nematodes now.

She explained that plant parasitic nematodes survive in soil with a wide range of pH values from 3 to 10. “These unique organisms have highly specialised survival strategies that enable them to survive in extreme and adverse conditions.”

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 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 poor growing plants in their fields. Due to the small size of these organisms, they cannot move very far on their own, therefore causing damage in small patches.

“These damages 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.

Farmers need to look out for other signs such as:

• Yellowing of leaves
• Stunting of plants
• Wilting of plants

“If a farmer sees any of these signs, plants need to be removed from the soil and the farmer needs to look for the 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 and other underground plant parts 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 lesions, she warned, 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, remains a great concern to researchers and producers alike since it has a direct negative impact on crop production,” she said.