Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
The soybean cyst nematode (SCN) is the most limiting biotic factor of soybean production in Delaware. In the mid 1990s a major effort was made to survey the soybean acreage for SCN and determine the race composition of the SCN populations present at the time. The Delaware Soybean Board funded this project and the results demonstrated that roughly 60% of the populations tested were race 3, 30% were race 1 and the remainder were a mix of races 5, 7, and 9. Since that time Round-Up Ready soybeans were introduced with a single source of resistance to SCN derived from a soybean plant introduction referred to as PI88788. At the time of the first survey we demonstrated a significant yield reduction in one variety trial where race 3 resistant soybeans were planted in a field known to be infested with race 1. This was the first indication that not all race 1 populations could be controlled with a race 3 or 3,14 resistant soybean variety. For the past 10, years SCN has not been identified as causing much yield loss because symptoms that were seen previously, namely severe stunting and chlorosis, now only seem to be present when a susceptible variety is grown. This past season a small number of fields had stunted soybeans, chlorosis, and soybean cyst nematode was present on the roots. All of these fields were planted with a Round-Up Ready variety with resistance to SCN. The difference last season is that it was dry from planting through the first 30 days. High SCN egg numbers and early dry weather are known to be very detrimental to early soybean growth and can produce stunting, chlorosis and yield loss. It is time to determine what SCN egg numbers are present in soybean fields 12 years after the initial survey was conducted and what the race composition of infested fields might be. Within the last 5 years there are indications that race 3 is no longer the predominant race. A small set of samples tested here and those sent to other institutions have tested as race 1. Since the majority of resistance in Round-Up Ready soybeans is from PI88788, which allows reproduction of race 1 populations, these varieties may have reduced effectiveness in suppressing current SCN populations. Other control measures may be needed if the current population structure is no longer predominately race 3. No surveys of SCN have been conducted in Delaware since 1996.
The Delaware Soybean Board approved the SCN Survey Project that will provide this valuable information. We are asking that soybean growers that have had SCN in the past or that have soybean fields that have not been producing as expected to have your field sampled for SCN. The field can be in a crop presently as long as we can take a sample in a zig-zag pattern. Fields that have been in continuous soybean production or rotated with corn or other crops are good candidates for the survey. Sampling fields this spring before any tillage is done will give us a good start on the project, which will continue into the fall as well. If we find SCN infested fields during the growing season, these will be added also. We are hoping to sample at least 30 fields in Sussex, 20 in Kent and 10 in New Castle. A percentage of the fields will be typed for race, or HG type as we call it now. If you are interested in participating in the survey please give me a call (302) 831-4865 or email bobmul@udel.edu or contact your county agent and we will take the sample.