Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
It has been a hot week. This stress along with the transition into September we often begin to see areas of the field that are dropping leaves early. Often these patches may have Soybean Cyst or Root-knot nematodes present. Soybean Cyst Nematode consistently ranks as the most yield limiting pathogen of soybeans across the US, with root-knot nematodes not far behind in the southern to mid-Atlantic regions of the US. Soybean cyst, root-knot, and other nematodes are often silent yield robbers, being present in the field without noticeable aboveground symptoms through most of the season. If symptoms do occur, they can resemble production challenges like nutrient deficiency, soil compaction, drought stress, or other diseases. Due to the lack of consistent or obvious aboveground symptoms, it is very common for nematodes to go unknown until severe infestation develops. Soybean roots can be scouted for SCN females early in the season (see June 23 article) and by this time in the season, root knot nematodes are very notable when root systems are dug from the ground (Figure 1). While females on the roots confirm the presence of SCN and galls confirm RKN, these sightings do not provide information on the level of infestation. Soil samples are the best method to assess overall populations across the field. Soil sampling can be conducted at any time, but fall samples provide a great snapshot of end of season populations and samples can be collected when already out for routine fertility sampling. Today we will discuss the steps to collect soil samples.
Figure 1. Soybean plant with significant galling from root-knot nematode
What type of sampling?
- Predictive: Since nematodes are widespread across the region, if you have never looked for nematodes before, you may have interest knowing nematode estimates within your field. For this type of sampling, you may sub-divide the field into several sections based on soil type, yield pattern, crop rotation etc.
Although the objective of detection seems simple, careful sampling is needed for accurate detection, especially when populations are low. A negative result does not necessarily prove absence of nematodes, but it indicates that population levels are below the detection level. The best time for preventative sampling is towards the end of the growing season or immediately after crop harvest when population densities are near their maximum levels. I tend to target most of my sampling late-August through September.
- Diagnostic: If you have a problematic patch or patches within the field showing abnormal symptoms that cannot be explained by other causes, you may want to collect a diagnostic sample. Diagnostic sampling can also be conducted to investigate the cause of observable yield decline over time. When collecting diagnostic samples, soil should be collected near the root. When submitting diagnostic samples, it is helpful to also include soil samples from adjacent, healthy-appearing plants/areas nearest to the most severely affected plants/areas. Numbers of diagnostic samples to be collected vary with field size and type of problem suspected. If the severity of the symptoms varies in the field, include samples representing various severity categories.
When to sample?
Nematodes can be sampled in the fall in soybean stubble or the non-host crop that will be planted into soybeans the following season, in spring before the soybean crop, or in season from the soybean crop root zone. Of these options, the optimal time to sample for nematodes in soybean is early fall, usually before harvest in preparation for the following season/crop. When collecting nematode samples, the soil should be moist, but not excessively wet or frozen.
Where to sample?
For preventive sampling, collect random samples in a zigzag or “w” pattern across the field (Figure 2). For diagnostic sampling, collect samples from problem areas and healthy areas (Figure 3). Soil samples should be collected from the plant root zone, 6-8” deep. If possible, include some roots along with the soil sample. Areas of the field that tend to be higher risk for nematodes include: near a field entrance, areas that have been flooded, areas with pH greater than 7, areas where yield has historically been lower, areas where weed control is not as good.
Figure 2. Sampling pattern for predictive samples
Figure 3. Sampling pattern for diagnostic samples; take on the edges of hotspots (red) after taking a separate sample from a nearby healthy area (blue X)
How to sample?
A soil probe, trowel, or shovel may be used for sampling. Within each section, collect 10-25 subsamples and mix well to make one representative sample/bag per field. Take the initial sample from a healthy area and then sample symptomatic areas. Clean sampling tools when moving between different symptomatic fields. Use re-sealable, clean, convenient sized plastic bags. I like to use quart-sized bags filling about ¾, so they still comfortably zip. Label each bag clearly (using your ID system) and provide a log of the ID numbers on a separate form, for reference.
Keep samples in shade (or preferably a cooler) while in the field and during transport. Do not expose soil samples to high temperatures/direct sunlight, do not let the samples dry out (make sure bags are sealed tightly); do not leave in a car trunk, or other area that may heat excessively; do not put the soil samples in a freezer; and do not add water. During collection, storage, and transportation, samples should be kept cool (ideally, 50 to 55oF). Nematode samples can be processed at the NCDA&CS Nematode Assay Service or Virginia Tech.
What to do next?
If you find out that you have elevated nematode populations, control options include host resistance, crop rotation, and use of seed treatment nematicides. For many years, SCN nematode populations were well managed through a single source of resistance, PI88788. Over the past few decades, we have seen a break down in this resistance and SCN are reproducing at far higher rates than they should. While the PI88788 resistance gene still accounts for over 95% of soybean acreage, there are new resistance genes coming out on the market. If high levels of SCN are present, crop rotation is another tool to reduce populations. Corn and wheat are both non-host options for SCN. Root-knot nematode can also be managed by selecting varieties with varying levels of RKN resistance, but host rotation can be more challenging due to the wide host range of RKN. Seed treatments are another control option. We have recently published a paper highlighting some of the results from seed treatment trials. In our trials and trials across the US, we see that response from seed treatments varies year to year and across field sites. On average, yield increases of 2-5 bu/a may be observed when using seed treatment compared to non-treated seed. If you are interested in learning more about SCN, you can also check out thescncoalition.com to talk todes.