SCN: One cyst is the threshold for action
Fig. 1. Soybean cyst nematode density map for South Dakota east river counties. SCN density for each county was obtained by averaging the SCN detected in soil samples submitted to SDSU Plant Diagnostic Clinic since 2003. The soybean cyst nematode (SCN) continues to expand in South Dakota. Since its first discovery in Union county in 1995, it has expanded to 28 other counties (Fig. 1). What is eminent from the map is the relative risk for SCN in fields in counties toward the eastern side of the state. Soybean growers especially in high SCN density counties (dark brown) need to monitor SCN in their fields regularly and apply management practices to start reducing SCN numbers.
SCN is a silent yield robber, causing significant yield losses without displaying obvious symptoms. The challenge with SCN management is that once SCN is introduced in a field, it is not possible to eliminate it from the field. However, SCN can be managed to keep SCN population below injury level. The question then becomes: What is the threshold for SCN to initiate a management program? The answer is just one soybean cyst! By the time one cyst is seen on the roots or in soil, it is probable that more cysts are already occurring in the field. SCN is highly variable in the soil and difficult to accurately quantify. Moreover, one cyst can contain up to 500 eggs and in one soybean growing season, 2-3 cycles can take place thereby initiating the population build-up in the soil. Hence it is not the question of how many, but rather if any SCN are present in the field.
Soil sampling and testing for SCN should be done for two purposes: 1) to determine if SCN exist in the soil and 2) to determine the SCN population in the soil. When sampling to determine if SCN exists in the field, sampling can be done anytime. If sampling is being done to determine the effect of a management practice (e.g. resistant variety, crop rotation, cultivar rotation) on the numbers of SCN, then the best time to sample is shortly after soybean harvest. Sampling for detection is important so that a management program can be initiated if SCN is found, while sampling to monitor a SCN population is important to evaluate the success of the management program in bringing down the SCN numbers. For instance, a high SCN number at the end of season when a SCN-resistant cultivar was planted may indicate an SCN population that overcomes this given resistance has developed and therefore requiring a change in cultivar selection. In the case of still high SCN numbers after non SCN host crop rotation; this may necessitate longer rotations away from soybeans to bring down the SCN numbers.