Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
The soybean cyst nematode (Heterodera glycines) is the most limiting biotic factor of soybean production in Delaware. In 1993 and 1994 a major effort was made to survey the soybean acreage for the soybean cyst nematode (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 that were race tested were race 3 and 30% race 1 and the remainder 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, only seem to be present when a susceptible variety is grown or high egg numbers combined with dry weather at planting occurs when a resistant variety is planted. During the 2008 growing season a small number of soybean fields had stunted plants, chlorosis, and SCN was present on the roots. All of these fields were planted with a Round-Up Ready variety with resistance to SCN. The difference in 2008 was that it was dry from planting through the first thirty days after planting. High SCN egg numbers and dry weather early are known to be very detrimental to early soybean growth and can produce stunting, chlorosis and yield loss.
Within the last 5 years there were 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 varying amounts of 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 had been conducted in Delaware since last survey in 1993 and 1994.
The results of the 2009 survey were very informative. No SCN were found in New Castle County even though the previously reported areas were sampled intentionally. In Kent county most of the infested fields were south of Dover. 41% of the samples in Kent County were infested. It was no surprise that all but one sample in Sussex County was infested, resulting in 96% of the samples taken in Sussex were infested. Fifteen samples (43% of the 35 samples that had SCN) were sent to the University of Missouri Extension Nematology Laboratory for race and HG typing. The most significant finding was that no race 3 populations were identified. Race 1 was the most prevalent (47%) followed by race 5 (33%) and two race 2 populations (20%). These results confirmed our suspicions that the race structure in DE has indeed shifted.
Mature cyst of the soybean cyst nematodes with 200-250 eggs.
Since more than 90% of the current soybean acreage infested with SCN in Delaware is planted with glyphosate resistant soybeans with SCN resistance from PI88788, it was significant that 100% of those SCN populations could reproduce on PI88788 from 40- 80% (average 67%) compared to a susceptible variety. Thirteen of the populations from the 1993-4 survey had an average female index (FI) on PI88788 of 24%. This means that the current SCN populations that we sampled will reproduce and increase on varieties with PI88788 as the source of resistance to SCN — although more slowly than on a susceptible variety. At what level they will produce on these current varieties depends on the population of SCN present. Will you notice any symptoms on the crop? It will depend on the growing season and the initial number of SCN eggs present in the field.
Managing SCN requires knowing if fields have SCN and how many eggs are present. Growers with increasing egg counts in fields planted with SCN resistant varieties will need to rotate to a non-host crop like corn or vegetables (except snap beans, which are also a host). Soil sampling in the fall after harvest is probably the best time to monitor SCN populations but samples can be taken anytime the soil is not flooded or frozen. When soybeans are grown, do not plant the same SCN resistant varieties in the same field, rotate varieties as well as crops. Although most varieties have resistance from PI88788, there are 4 major genes for resistance and several minor ones. Varieties with resistance from that source can vary in the resistance genes they carry. So there are differences between varieties that have the same source of resistance. Planting high yielding varieties with resistance to SCN when SCN egg counts are low to medium can be expected to perform well under our conditions. The problem is that the nematode populations can be increasing on some of these varieties depending on the SCN population in the field and the crop’s full yield potential may not be met.
Beat the Pest-Take the Test
Soil sampling is highly recommended if you are planting soybeans frequently. Irrigation can mask SCN damage so irrigated fields should be sampled as well to make sure that populations are not increasing.
A workshop on soybean cyst nematode is planned for August 3, 2010.