WW-03935 Revised 2001
SCN field populations vary in their abilities to successfully develop and reproduce on a set of four differential soybean lines that differ genetically in their resistance to SCN. These different populations are referred to as SCN races and are given number designations. There are currently 16 possible reaction combinations and, thus, 16 potential SCN races. To date 12 different races have been reported in the United States. Although race 3 is the most common in Minnesota, races 1, 4, 5, 6, and 14 have been detected, also.
Most, if not all, SCN resistant soybean varieties available in the state are resistant to race 3 and some may be resistant to other races. Resistant soybeans are not immune to SCN. Some nematodes can be expected to develop to maturity and reproduce on a resistant variety. The current race scheme for SCN classification may have limited practicality in a production field since all SCN in a field are unlikely to be genetically similar. Some individuals in the field will overcome the particular source of resistance planted. These individuals will be selected for and will increase in percentage of the total SCN population with continued use of the same source of resistance in a field. If varieties with a specific source of resistance are used continually in the same field, that source of resistance will eventually become ineffective at preventing yield loss due to the nematode.
Healthy, newly hatched SCN J2 can move only a few inches in the soil. However, soil with cysts containing eggs can move long distances within a field or between fields by any means that moves soil. Soil movement in runoff or flood water, on wildlife, and as windborne dust are examples of natural mechanisms that spread SCN. Since eggs enclosed within the nematode cyst can survive passage through a birdís digestive system, migrating birds may spread SCN over long distances. Human activities that move soil between fields on equipment, tools, and vehicles are the primary means by which SCN spreads. Seed lots contaminated with soil peds infested with SCN are another way SCN can move long distances.
In Minnesota, SCN has been reported throughout soybean-producing areas in the southern and central regions of the state (Figure 1). It appears that colder climatic conditions will not prevent the establishment of the nematode in the Red River Valley where soybean production is increasing.
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