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August 26, 2013

Nematodes Becoming a Major Pest of Brazilian Soybeans

The microscopic worms that live in the soil, known as nematodes, have long been the number one "yield robbing" pest in American soybean fields and they are quickly becoming a major problem for Brazilian soybean farmers as well. In a recent survey conducted in Mato Grosso where they examined soil samples from 332 farms, nematodes were found in 82% of the samples with root-lesion nematodes (Pratylenchus brachyurus) being the prevalent species. Root-lesion nematodes are the principal species that affects coffee in Brazil and in the U.S., they are only found in the far southern U.S. and in Florida citrus groves.

These parasitic worms live on the roots of soybeans consuming plant nutrients and injecting toxins into the roots. In cases of severe infestations, the soybean plant can be killed. It is estimated that nematodes reduce the nationwide soybean yield in the U.S. by 4-5 bu/ac. In Brazil, nematodes and soybean rust are considered the two main threats to soybean production.

There are four main species of nematodes that affect soybeans so it is critical for farmers to know which species are in their fields in order to choose the correct soybean varieties because some varieties are resistant to one or more species and not to others.

Planting resistant varieties of soybeans does not solve the problem because in the long run the nematodes can mutate to new races and overcome the resistance. The most practical way to control nematode populations is to rotate to a non-host crop for an entire growing season. For most farmers that would mean rotating from corn to soybeans every other growing season or to a non-grain crop that is resistant to nematodes. It's very hard to convince farmers to rotate to a non-grain crop for a year due to the lost income.

For farmers in Mato Grosso though it is more complicated. If they plant a first crop of soybeans followed by a second crop of corn during the same growing season, that type of rotation does not impact nematode populations. Having soybeans in the field for half of each growing season allows nematode populations to remain high. The best way to reduce populations would be to not plant any soybeans for an entire growing season, but the probability of lost income makes this an unattractive option.