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March 28, 2012

Nematodes Becoming Major Pest of Brazilian Soy, Similar to U.S.

Brazilian soybean farmers are beginning to battle with a pest long known by American farmers - nematodes. These microscopic worms are the number one "yield robber" of American soybeans and they are now becoming a major pest in Brazil as well. If left uncontrolled, yield losses can commonly be in the range of 20% to 30% and in the most severe cases, loses can be as high as 75%.

There are many different types of nematodes, but most share a taste for soybean roots. As the worms feed on the roots, the plant is less efficient in the uptake of water and nutrients, thus reducing the potential yield. The easiest way to confirm nematode damage is by the presence of galls which form on the roots of host plants. Nematodes do not need plant residue to survive because the galls, which contain eggs, can survive in the soil for up to eight years. The worms themselves do not move very far within the soil, but the galls can easily be transported throughout the field and from field to field by planters, tillage equipment, or even combines. In areas of heavy infestation, it is recommended that equipment be thoroughly cleaned before it is moved into a new field.

One of the most common types of nematodes in recent years in Brazil has been P. brachyurus, which not only feeds on the roots of soybeans, but also on the roots of corn, cotton, millet, sorghum, different types of grasses, and even noxious weeds. Their ability to feed on the roots of different plant species makes them a major threat to soybean production especially when corn or cotton is double cropped after soybeans. This allows the worm populations to continue expanding without interruption. The expansion of safrinha corn production in Mato Grosso over the last decade is believed to have made the nematode problem even worse in the state.

There are no economic treatments for nematodes and the best way to limit the amount of damage is to develop resistant varieties of soybeans and by practicing crop rotations. In heavily infested fields, it is highly recommended that a non-host crop be planted every few years. This elimination of a host plant reduces the food supply for the pest and helps to hold down their numbers. That in itself is a problem in places like central Brail where a monocrops of soybeans is generally followed by a monocrops of corn - both susceptible to the pest. Without breaking the cycle of susceptible hosts, worm populations continue to increase.

Even the development of resistant soybean varieties has had limited success in controlling the pest because the worm has the ability to mutate into new races that can overcome the resistance. As a result, the plant breeders and the worms are now locked into a perpetual "arms race". One new and novel approach in controlling the pest has been to plant a highly susceptible soybean variety every three or four. By giving the worms a preferred food source every so often, it reduces the chances of developing new races.

Nematodes are destined to become a chronic pest problem in Brazilian soybean fields just like they have become in the United States.