FOR IMMEDIATE RELEASE
THURSDAY, AUG. 5, 2004
||Dr. Jack Imbriani, nematologist
Nematodes and manganese at root of soybean problems
WINTERVILLE — Yellow soybean fields are common across eastern North Carolina every year. Plant-parasitic nematodes and manganese deficiency are two of the known culprits. A good way to identify the exact cause and select the correct treatment is to use the N.C. Department of Agriculture & Consumer Services' agronomic testing services.
Three NCDA&CS laboratory tests help diagnose soybean problems: soil testing, plant tissue analysis and nematode assay. Each of which has a unique purpose. Therefore, the best approach is to submit all three types of samples for each problem field.
A soil test indicates how much manganese is present and, more importantly, whether it is available to the crop. Because manganese is a micronutrient, only very small amounts are needed for plant growth. In eastern North Carolina, however, high soil pH frequently interferes with manganese availability. Deficiencies usually occur when too much lime has been applied to soils.
Plant tissue analysis can pinpoint the nutrients a plant needs. It can reveal a single nutrient deficiency—like manganese due to high soil pH—or a deficiency involving many nutrients. Deficiencies of several nutrients may indicate a more general root problem, such as nematodes.
Plant-parasitic nematodes are microscopic worms that live in the soil and feed on roots. They are present in all fields in eastern North Carolina. A nematode assay can identify the species present, estimate their populations and indicate whether there is a threat to the crop being grown.
The presence of nematodes doesn't necessarily mean that they are a danger to the crop. A recent survey of yellow soybean fields in Craven, Lenoir, Pitt and Greene counties found nematodes in every field visited. However, only two-thirds of the samples had populations high enough to cause significant crop damage. This finding indicates that, in at least one-third of all fields surveyed, soybean yellowing has some cause other than nematodes.
If the problem is a manganese deficiency, treatment is relatively simple. A foliar application of manganese at a rate of a half-pound per acre should solve the problem. To correct a long-term manganese problem, follow soil test recommendations, which may suggest a broadcast application of manganese fertilizer to the soil.
If the problem is due to nematodes, treatment options are more complicated. The most prevalent nematode problem on soybeans in eastern North Carolina is soybean cyst (SCN). When this nematode is present, growers need to take steps to minimize damage to future soybean crops. Rotating crops and selecting resistant varieties are the two best options.
Use of resistant varieties is not foolproof. Resistance is only effective against certain populations, or races, of nematodes. For example, most Roundup Ready varieties have resistance only to soybean cyst races 1 and 14, which occur in about 20 percent of North Carolina's soybean fields. Therefore, these varieties are not effective against nematodes in 80 percent of the state's bean fields. Anyone who uses resistant varieties to manage soybean cyst nematodes should switch varieties often, even if they are resistant to the same SCN races. Soybean varieties differ in their tolerance of the races.
When resistance fails and cyst populations rise, it may be necessary to grow non-host crops for two years before growing soybeans again. Corn, tobacco, peanuts and grass crops are some good choices for rotation. Another possible strategy is to plant a short-season soybean variety as a second crop following small grain. Because short-season varieties mature more quickly than full-season ones, they are less likely to be damaged and less likely to prompt an increase in nematode populations.
Nematode assay, soil testing and tissue analysis from the NCDA&CS Agronomic Division are effective tools to diagnose soybean problems. For advice on collecting samples or understanding your report, contact your local NCDA&CS regional agronomist (visit www.ncagr.com/agronomi/rahome.htm to find the agronomist assigned to your county).