Nematode problems in soybeans increasing

As more soybeans are planted on the sandy water table soils in the Free State and parts of the North West, the nematode problem is growing.

Nematodes thrived this year in the heat and drought conditions in the summer rainfall region, and much of the damage to soybeans and maize in the western parts can be attributed to these worms.

Chris Schoonwinkel, a farmer between Wesselsbron and Hoopstad, observes that nematodes played a significant role in yield losses, which could not be attributed to drought alone. “If there is no proper root system, the plant cannot produce.”

He says this was particularly a problem in districts such as Bothaville, Wesselsbron, Hoopstad and Bultfontein. He believes more research is needed on cultivars and their resistance to nematodes. “There has been good research on maize cultivars and their susceptibility to different diseases, but we don’t have such information for soybeans.

“This is a bigger problem than we think, and more research is needed. In Hoopstad, we are now trying to see for ourselves what the results were among different farmers and their cultivars, but we need scientific research.”

Drought and heat create problems

Sheila Storey, nematologist and owner of Nemlab laboratories, says the water table soils are not the problem, but any sandy soil with ground temperatures around 25°C creates the ideal conditions for nematodes to thrive. She says maize is also an excellent host and should receive more attention.

“In drought conditions, when plants are under stress, they cannot overcome nematodes. If the roots are weak, the nematodes will take over. There has already been a survey on nematodes in soybeans, but more research is needed.”

She says there is also a type of nematode known as the cyst nematode, which is a big problem in Brazil. It has a hard shell and can lie in the soil for 20 years until it finds soybeans as a host.

“We don’t have it here yet, but there are too many feet moving between us and Brazil. It might already be in the soil here or it is going to come, and then we will have to seriously look at resistant cultivars.”

Prof Driekie Fourie, a world-renowned nematologist and Syngenta’s head of technical products for Africa and the Middle East, warns that the impact of plant-parasitic nematodes on root health is underestimated and that very little information about resistant cultivars is available in South Africa.

Test your soil

In a recent Grain SA article, Fourie states that to combat nematodes, without even thinking about eradication, soil tests are of the utmost importance. “Above-ground indicators are not always sufficient to determine the presence of nematodes and the extent of the problem. The only way to really know is to measure, and prevention is better than cure.”

She says too many farmers fail to test because they think it is too expensive and time-consuming or they believe they do not have a problem. She recommends taking soil samples about six to eight weeks before harvest time while nematodes are still active. The population in the current season will be a good indication of what can be expected in the next season.

A handful of soil samples should be taken from roots and soil at various places where plants look healthy and unhealthy, placed in plastic bags and tested as soon as possible (within two days) at a registered laboratory in Potchefstroom, Cape Town, Pretoria, Nelspruit or Upington.

Jozeph du Plessis, Grain SA’s regional chairperson for the Schweizer-Reneke district, who farms with cotton, soybeans and maize, says in his area and further north, the drought caused significant damage, but nematodes were not a problem. Trials they conducted with nematicides did not indicate the presence of the pest.

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