Harmful or plant-parasitic nematodes are difficult to control, let alone eradicate. The best – if not only –way to protect your crop, is to know what you are dealing with before you plant.
Above-ground signs are not always enough to reliably identify the presence of harmful nematodes and determine the extent of the problem. In addition, prevention is always better than cure, especially in the case of soilborne diseases and pests that attack seedlings before they have developed the mechanisms to protect themselves.
Experience has taught Prof Driekie Fourie, world-renowned nematologist, technical product lead at Syngenta Seedcare in South Africa and extraordinary professor at the North-West University (NWU), that too many producers ignore nematode testing. They either think it is too expensive or too much effort and time-consuming, or they don’t believe they have a problem. ‘The only way to really know is to measure,’ she says, ‘particularly in this era of integrated pest management (IPM). The tests are indeed not cheap because they are done by trained staff using specialised equipment, but the cost is lower or at least on par with what you pay for fertiliser analyses and soil mapping. The same samples can also be used to determine (at an additional fee) the beneficial nematode population, which is an indicator of soil health.’
How to do nematode sampling
There are a few golden rules to observe, but the process is simple:
- Take the samples approximately six to eight weeks before harvesting (depending on the crop), when the crop is still active and the nematodes feeding. An approximate indication for example: If you plan to harvest at the end of June, take samples at the end of April to the middle of May. Soil samples taken in a bare field are of little or no value in the battle against nematodes. The nematode population in the current season is a reliable indicator of what you can expect in the next season, which enables you to take the necessary precautions, says Prof Fourie. ‘Cold winters are highly effective in controlling nematode populations, but we don’t really get winters anymore that are cold enough. You can, therefore, count on the results you get in autumn.’
- Do not take samples after heavy rains. Not only is it difficult and messy to dig in a waterlogged field, but the results are not entirely reliable.
- Take samples of crop roots (or other plant material, for example tubers) and soil. Endoparasites occur in the roots, while exoparasites live in the soil; both are necessary for a comprehensive picture of the rhizosphere.
- One sample is not enough to provide useful results given the variety of factors that influence nematode pressure. A simple guideline is to divide your field in four blocks or segments and take ten samples each from a spot with unhealthy plants and ten from a spot with healthy plants. A handful (200 to 300 g) of soil and a substantial piece of root/other below plant parts are all that are needed. You can then combine the healthy and unhealthy samples taken in the same block into two of each category (healthy and unhealthy), leaving you with a total of four samples per block and 16 per field. This approach makes nematode testing more cost effective.
- Place the combined samples in plastic bags and mark each one very clearly and carefully, e.g., Block A (unhealthy plants) and Block B (healthy plants) and the date.
- Immediately put the marked samples in insulated containers to keep them cool (not too cold or frozen) and out of the sun. Handle the samples with care, as certain nematode groups are easily damaged during handling.
- It is important to deliver the samples to your nearest laboratory as soon as possible – the fresher the better, but no longer than two days after sampling.
South Africa has several reliable nematode laboratories that offer diagnostic services. Locations include Potchefstroom, Cape Town, Pretoria, Nelspruit and Upington. Be sure to use a laboratory managed by nematologists who are registered with the South African Council for Natural Scientific Professions (SACNASP). ‘The scientists and support staff at these laboratories have been trained properly and do the right tests, giving you peace of mind that the results can be trusted,’ says Prof Fourie.
‘Harmful nematodes are a major issue in sandy soils and for crops such as sunflower and soybeans, vegetables (specifically potatoes), as well as vineyards and deciduous fruit orchards,’ notes Prof Fourie. The same applies to industrial and floricultural crops. ‘We are also seeing serious nematode damage to crops in clay soils.
The message is therefore that growers should use all the tools available to understand what they are faced with before they invest in any IPM strategy such as crop rotation, crop protection products (including seed treatments), and other approaches.’ Prof Fourie emphasises that seed treatments are the first line of defence against harmful nematodes that often join forces with soilborne diseases to inflict severe damage on crops.
