Rotating crops and planting resistant varieties are two options that may help farmers manage nematodes found in Southern soybeans.
By Charles Overstreet Nematologist LSU AgCenter
[dropcap]L[/dropcap]ike most of the crops that are grown in Louisiana, soybean has problems with plant-parasitic nematodes. The damage is often difficult to recognize and usually blamed on something else. There are three nematodes attributed to causing losses to soybean in Louisiana, including the soybean cyst nematode (SCN), Southern rootknot nematode and reniform nematode.
SCN, Southern root-knot, reniform
In Louisiana, SCN, which was once very common has virtually disappeared. In a recent survey of 165 soybean fields, SCN was detected in only very low levels in 1.2 percent of these fields. SCN is more likely to be better adapted to a more northern climate and cannot survive well under our growing conditions. It is very likely that parasites and predators of this nematode are active in our soils almost the entire year.
Southern root-knot is still a big problem in our sandy soils in Louisiana. This nematode is usually found in patchy patterns in coarse textured soils. One of the most common symptoms is stunting of plants in early summer, alerting producers that something may be wrong. Late-season damage may still be stunting, but there may be premature death of plants, which greatly reduces yield in these areas. This nematode does produce distinctive galls on the root system and can easily be identified in the fields.
The third major nematode is the reniform. Reniform nematode has often been associated with cotton or sweet potato. Both of these plants are excellent hosts for this pest. However, soybean is also considered an excellent host, and populations can build up very rapidly on this plant. Reniform tends to spread rapidly throughout a field and has a more uniform distribution than most other nematodes. Although it seems to reach the highest populations in fields that are more fine-textured, the highest populations seem to occur when clay content is between 10 to 20 percent in Louisiana.
This nematode is often difficult to recognize in the field because its uniform spread may impact the entire field. The nematode can cause stunting, but often plants appear normal. Yield losses of up to 40 percent have been reported from this nematode. Areas in a field with very high levels of reniform nematode tend to show drought stress earlier.
Most of our producers are doing a good job of rotating crops and avoiding planting soybeans in the same area each year. Although corn is one of our most important rotation crops for cotton and soybeans, it does only provide fair management of the root-knot nematode.
However, corn is an excellent rotation crop against reniform nematode, and one to two years of crop rotation can drastically reduce reniform. Unfortunately, reniform nematode increases rapidly on a susceptible crop and returns to high levels in a single growing season.
Resistance to nematode is still one of the best management options. Table 1 lists a number of varieties with at least moderate resistance against either the Southern rootknot or reniform nematode.
Although there are a great number of varieties with SCN resistance, these varieties are not included due to the absence recognized problems in Louisiana. Unfortunately, there are only a small number of varieties with resistance against the major nematodes that we have in our state.
Many of the fields that will be planted this year are on ground where cotton was grown in the past. If reniform nematode is thought to be present or has been identified in the past, you might want to try planting a resistant variety. However, it is still not clear exactly how damaging reniform nematode is to soybean since there are many varieties that vary considerably in susceptibility to this particular nematode.
This article appeared in LSU AgCenter’s March 2014 “Louisiana Crops Newsletter.”