The hidden impact of nematode pests on South African growers

Prof Fourie’s latest research indicates that once nematodes are diagnosed in the soil, eradication becomes almost impossible. This leads not only to substantial yield losses; more importantly, it threatens sustainable crop production. “Approximately 12% of yield losses in South Africa are attributed to plant-parasitic nematodes, with highly infested fields experiencing losses of up to 60% or more,” explains Prof Fourie.

Nematodes also severely impact maize root health. The tiny, fresh, white roots are especially vulnerable when it comes to nematode infection and parasitism. Dicotyledonous crops, such as soybeans, are also highly susceptible to nematode infections.

There are two major nematode groups that cause the most significant problems in South Africa. The first is the root-knot nematode, which cause the yellowing, stunting and/or wilting of aerial plant parts in severely infested areas in fields. Female root-knot nematodes can produce up to 1 800 eggs at a time, as Fourie herself recorded from one egg mass produced on the roots of a susceptible soybean cultivar. The second group is lesion nematodes whose females produce one egg at a time but for a continuous period.

While certain cover crops, like clovers, can help manage nematode populations, they can also sustain them. Other popular cover crops, such as sunn hemp, will reduce root-knot nematode population densities but enable an increase in lesion nematode numbers.

Grain crop production in South Africa is especially hampered by endo- and ectoparasitic nematodes. Fourie mentions that ectoparasite population densities seem to be on the rise in conservation farming areas where she is involved in research projects.

The emergence of new nematode species, such as the highly pathogenic root-knot nematode Meloidogyne enterolobii, further highlights the need to conduct more research on these organisms. Understanding the correlation between nematode numbers and irrigation practices is crucial as the combination of cover crops that are good nematode hosts and warmer soils, caused by the covering of soil in conservation farming systems, favour nematode survival (nematodes perish in excessively cold temperatures). Moreover, surveys done in local grain crop production areas have linked higher levels of rainfall and irrigation to increased nematode pest populations.

While there have been improvements in our academic understanding of nematodes, there is still insufficient knowledge at the farm level.

“Growers need to be educated about nematodes and their potential impact on root and soil health,” says Prof Fourie. “Identifying and understanding the presence of new nematode species is crucial for effective management and research. By becoming more aware of and investing in research and management strategies, South African growers can mitigate the negative effects of nematodes and improve agricultural sustainability.”

The ongoing challenge posed by climate change that causes shifts in geographical row crop production, is a game changer that needs to be considered. Furthermore, price sensitivity around nematode management practices on the farm, especially in rain-fed areas, is a pressing concern. Current crop rotation systems may inadvertently promote nematode build-ups, as the predominant and economically important root-knot and lesion nematodes that occur widely in local crop fields have a wide host range.

The good news is that damage can be mitigated by using rotation crops that are nematode resistant or poor host genotypes in maize-based cropping systems. Research has shown promising results, with resistant soybean cultivars recording a 39% yield increase.

“The discovery of a global threat species, M. enterolobii, in the Highveld region of Mpumalanga and in other grain production areas highlights the need for ongoing research and vigilance,” says Prof Fourie. “It is important to adopt a holistic approach to address plant-parasitic nematodes, as they often pave the way for secondary infections by fungi. By combining knowledge, effective management strategies and collaboration, we can better tackle the challenges posed by nematodes and ensure the long-term sustainability of South African agriculture.”

Increasing the organic content of soils in grain production areas, planting cover crops that area resistant to or poor hosts of the target nematode pest, responsibly using chemical nematicides with favourable environmental profiles and no adverse effects to non-target organisms (animals, humans and beneficial organisms) and other strategies can already be implemented to enable producers to grow crops sustainably in soils with lower nematode pest densities. This is an important aspect to bear in mind, given that it is impossible to completely eradicate these unseen enemies with their superior survival characteristics.

Photo by Dylan de Jonge on Unsplash