Why Do SCN Populations Fluctuate?

Variability in SCN numbers within a field at a single point in time depends on many of the same factors that affect seasonal changes. 

Seasonal changes in numbers of SCN in a field are dependent on
    • temperature
    • moisture
    • row spacing
    • host suitability
    • overwintering survival
    • crop 
    • soil type
The numbers are extremely variable. If two samples are taken from different places in a single field, one may have very high numbers of SCN while the other may have none. This is why hot spots (see aerial photo) develop in heavily infested fields. The presence of a hot spot does not mean that SCN is absent from the apparently "healthy" area.

 Anything that moves soil will move SCN with it: wind, water, migratory birds, tillage and harvest equipment, and soil peds in seed stocks. Once introduced into a field, SCN may take about 10 years to build up to a damaging level, depending on how often susceptible soybean is grown.

In this aerial photograph of a heavily infested field in Minnesota, areas where SCN has severely damaged plants ("hot spots") can easily be seen. Note that the patches are elongated in the direction of tillage.

SCN distribution within a field depends on soil type, moisture (drainage patterns), tillage, soil fertility, and other factors. It is not uniform across the field. Sampling only one spot in a field can result in misleading test results. Area shown is 112 acre.

When you buy seed that has not been cleaned in modern equipment, the soil peds in the bag may contain viable cysts.

Host suitability refers to the relative susceptibility of the plant. SCN-susceptible soybean is a highly suitable host plant, on which SCN can develop and increase to high levels. The result is decreased seed yield. In contrast, alfalfa is not a suitable host, and SCN develops and reproduces poorly. The crop suffers no yield loss. The host suitability of SCN-resistant soybean depends on the SCN race present in the field.

Overwinter survival refers to the ability of SCN to remain dormant when temperatures are low and host plants are not available. The number of infective juveniles in the soil at planting depends on the number of eggs produced in the previous season and on the eggs overwinter survival rate. This rate is higher in northern states than it is in the south, because the lower winter soil temperatures in the north inhibit hatching. Overwinter survival rates can be as low as 1 0% in the deep south and as high as 1 00% in northern Missouri.

The only definite way to confirm the presence of SCN, determine the level of infestation in a field, or determine the race, is to submit a soil sample to a qualified laboratory for analysis. The results you obtain can be used for soybean variety selection or to select a crop rotation sequence that will reduce or keep SCN numbers below the damage threshold.

Soybean yield loss depends on the number of SCN present at planting. At the action threshold, a long-term strategy should be planned to reduce future losses. At the damage threshold, yield loss should be expected if a susceptible variety is planted.

 Damage thresholds for SCN depend on regional and local conditions, so local sources should be consulted for this information. It is important to distinguish between a damage threshold and an action threshold, the infestation level at which some action should be taken to prevent future losses. The action threshold for SCN is the same as the detection threshold: if there is any infestation at all, a long-term SCN management strategy should be adopted.

Soybean yield loss due to SCN depends on the number present at planting and on the environment: temperature, moisture, soil type, and other diseases, etc.