Charles Overstreet, Xavier, Deborah
Galling from Southern root-knot nematode evident on soybean roots.
Soybeans share a common problem with most of the agronomic crops in our state. Most of the plant-parasitic nematodes can attack a wide host range and include soybeans as a favorite plant to attack. The Southern root-knot, reniform, and soybean cyst nematodes are generally considered our most important nematode pests on soybeans. Each of these nematodes can cause serious losses to soybeans. Stunting, yellowing, early death, and low yields are common symptoms of nematode injury.
Soybean cyst nematode (SCN) may nationwide be the most important nematode on soybeans but in Louisiana it has declined during the past 20 years. This pest is now relatively scarce in our state. However, it does have serious potential for damage and is being constantly monitored for outbreaks or resurgence. Symptoms of SCN have been stunting, stand loss, and low yield. Because many of the current varieties seem to show very slight foliar symptoms, watch out for yield reductions or areas not performing like they should. Rotation with other crops such as corn, cotton, milo, sugarcane, or rice has been very effective in breaking up the life cycle of this nematode which seems to favor only soybeans. Since we are currently having such minor problems with the soybean cyst nematode, variety selection may not be very critical. However, watch out for problem areas in the future.
The Southern root-knot nematode is one of most damaging nematodes in soybeans. Although this nematode is largely confined to sandy soils, it is very capable of causing devastation where it occurs. Look for seriously stunted plants early or early death during bloom and pod fill. Drought really enables this nematode to cause serious damage. The galls are a distinctive feature of this nematode and make it easy to recognize. The damage from this nematode is usually associated with spots are areas within a field and rarely occurs across the entire field.
The reniform nematode is probably one of the most widely spread nematodes in our state. Although it does seem to be the most problem on cotton, soybeans are certainly an excellent host for this nematode. As less acres of cotton are grown and more soybeans going into these areas, producers really need to keep a close watch on the development of reniform problems. Symptoms are not always clear cut with reniform in soybeans. Although you can get stunting, the most common symptoms are some yellowing and loss of yield. This nematode does seem to cause much greater damage during drought stress and in years with adequate moisture, little damage may be observed. This nematode is so widespread throughout the state that substantial problems are likely to be encountered as soybean acreage begins to increase.
Rotation: Crop rotation has been shown to be an excellent method of reducing damage. There are several crops that are typically used in rotations with soybean including corn, cotton, grain sorghum, wheat, sugarcane, rice, and sweetpotato. Corn does have some problems with the Southern root-knot nematode. Most corn varieties are excellent hosts for this nematode and do little to help with reducing the populations. Corn may actually be damaged by the Southern root-knot nematode. Don’t look for large galls on the root system since galls are very small and not very evident. Rice and peanut would be excellent rotational crops to use against either reniform or Southern root-knot nematode. You can expect to find very little reproduction on these hosts. Although rice is susceptible to root-knot nematode, the flooded fields where it is grown are not suitable for this nematode. Grain sorghum varieties have been shown to be highly variable with Southern root-knot nematode with most varieties being fairly susceptible. Sugarcane, corn, and grain sorghum are effective against the reniform nematode. Sweetpotato is a favorite host of both the reniform and root-knot nematode. Wheat doesn’t seem to impact Southern root-knot nematode very much even though it is considered a fair host. The soil temperatures during our winters are not very conducive for nematode development. I do recommend planting wheat closer to the end of the planting time in fields that may have high levels of root-knot nematode. The soils are cooler then and less likely to favor the development of root-knot nematode on wheat.
Resistance: The use of resistant varieties has proven to be great management option in soybeans. There are a few varieties that have good resistance to Southern root-knot nematode but most appear to be susceptible. Soybean does produce very large galls on the root system. If you have seen galls in the past or seeing them in the future, then that variety is generally very susceptible. Soybean varieties can have resistance against the reniform nematode. Unfortunately, little emphasis has been placed on breeding for resistance against this nematode in soybeans. There are only a few soybean varieties with resistance against reniform nematode that are on our recommended list. Most varieties are not listed from the various seed companies as to whether or not they have any resistance against this nematode. Most of the soybean varieties that are recommended for Louisiana have some resistance against SCN. There are several races of this nematode. Problems will occur when soybeans are grown continuously and varieties with the same type of resistance are used.
Nematicides: There are a number of seed treatment nematicides available to use on soybeans including Avicta Complete Bean, Poncho Votivo, and ILeVO. The fumigant Telone II can also be applied but should be used in a site-specific manner. The fumigant should only be used to treat the parts of a field where the nematodes are causing serious injury.
Identification of the Problem
Root-knot nematode tends to occur in the sandy soils, reniform tends to occur in sands to silt loams, and soybean cyst is not very particular but most damage has been in lighter soils. If you don’t know what type of nematode you have, soil samples for nematode analysis can be taken just about the entire year with fairly good confidence in identifying the type present. Late spring may be the least desirable time for root-knot but fine for the other nematodes. Fall is always the best time to collect nematode samples.
Area in a soybean field showing severe stunting and early plant death from the Southern root-knot nematode.
Large areas in a soybean field showing premature death from root-knot nematode.