Counting nematodes? Proper sampling and handling is key! (Part 1)

If you’ve ever sampled for nematodes in soil, you’ve probably (hopefully!) read the guidelines for sampling and handling of samples.  But what if things don’t go according to plan and they didn’t get refrigerated right away or the person you gave the sampling job to didn’t quite follow your instructions?  Are the samples ruined?  What can you get away with and what will result in completely inaccurate results?  When you’ve gone to the trouble of sampling and are about to invest in paying a lab for nematode counts, this is something you need to know.

OMAFRA staff tested some scenarios in the spring of 2011 to look at some of the ways nematode samples can go wrong and find out what impact to expect.

Four sites that were in a processing tomato rotation and were known to be infested with plant parasitic nematodes were selected.  The recommended sampling and handling treatment was based on OMAFRA Factsheet 06-099, Sampling Soil and Roots for Plant Parasitic Nematodes and discussions with experienced laboratory personnel.  Treatments followed the recommended protocols, except for the differences listed.

Recommended protocol – key points:

  • Sample approximately 8” deep, and discard the top 1-2” of the soil core.
  • For row crops, sample in the row, so that samples contain feeder roots.
  • Take 10-20 soil cores/acre. Ideally each sample should represent no more than about 6 acres.
  • Mix the soil cores thoroughly, but gently. Take a subsample of about 1 litre of soil for shipment to the lab.
  • Place soil samples in a cooler with ice. Keep cool (not frozen) until delivery. Samples should not be subjected to sudden temperature changes.
  • Deliver samples to the lab as soon as possible — no longer than 3-4 days.

Sampling

Part 1 of this article will focus on the impacts of sampling.  See part 2 for what we found when we handled the samples improperly.

Sampling Treatments:

  1. Recommended protocol.  Stored in cooler or refrigerator. Delivered to lab within 1 day.
  2. Recommended protocol, except sample depth of 0-6” instead of 2-8”. Delivered to lab within 1 day.
  3. Recommended protocol except soil cores not broken up or mixed before taking subsample out for lab.  Delivered to lab within 1 day.
  4. Recommended protocol except sampled within a ½ acre square instead of a full 6 acre area. Delivered to lab within 1 day.

Results:

Results are shown as percentages, where the average count (4 sites) from the recommended protocol is shown as 100% and the average count (4 sites) of each treatment is shown as a percent of the recommended count.

Root lesion Cyst Spiral Root Knot
Recommended 100% 100% 100% 100%
Wrong depth (0-6″) 58% 14% 113% 180%
Poor mixing 75% 35% 138%
Grid 125% 128% 100%

We would have expected lower nematode numbers when sampling the 0-6” depth compared to the 2-8” depth, but that didn’t hold true for all species in these samples.  At the time of sampling, the top layer of soil had not become extremely hot and dry, so it could have still supported some nematodes.  Nevertheless, established nematode thresholds are based on sampling to 8”, but removing the top 1-2”.  Also, to compare nematodes counts over time, it is important to be consistent in your sampling.  Consistency in depth, sampling locations in the field, time of year, and laboratory are all important.  And of course use consistently proper handling, as we will see in part 2 of this article, next issue.

Nematode populations are notoriously variable across a field, even in the distance between two plants.  When you consider that some soil cores might contain no nematodes and some might be crawling with hundreds of them, it becomes clear that thorough mixing of the soil cores before taking out the sample for the lab is very important!  You can actually kill nematodes with overly vigourous mixing, they tell me, so gentle (but thorough) mixing is recommended.  Isn’t it amazing that nematodes would be so easy to kill once you get them in a pail!  In the test, you can see that we got very different results from grabbing a sample without mixing, compared to the same sample after thorough mixing.

In the final treatment, we simulated grid sampling (taking sample from within a half acre square) compared to taking a sample from across approximately six acres, the maximum recommended area to include in one sample.  The differences we found seem small, but this is after averaging four sites.  When we look at individual sites, we see differences like this (using root lesion nematode as an example):

Site A Site B Site C Site D
0.5 acre sample 2080 260 480 200
6 acre sample 280 260 1800 80

Not all crops will support the expense of detailed grid sampling for nematodes, but be aware that there may be clusters of high nematode populations within a field that may not be picked up when a larger area is included in the sample.

Continue to Part 2

Leave a Reply