However, if the report is negative, one must consider if resampling in a different manner or at a different time of the year might have yielded or will yield different results.
As discussed under the topic of sampling, for those instances where the population cycling of nematodes on perennial crops has been examined, it can be observed that there are times of the year when one will be more likely to detect nematodes than others. If the sample was taken at a time when nematodes are known to be at low levels, then resampling at a different time might produce different results.
For annual crops, nematodes are typically highest at or shortly before harvest.
If a field remains fallow for a period of time, numbers of nematodes will start to decrease and may drop below the detection ability of the extraction technique used. In this case, a grower might rely on past experience with a particular field and still utilize some type of nematode management prior to planting a susceptible crop.
It seems to be easier to sample and to extract nematodes from lighter (sandier) rather than heavier (large amounts of silt and clay) soils. Heavier soils are more difficult to penetrate with sampling equipment when dry and more likely to stick to the sampling equipment when wet.
Consider the depth at which the sample was taken, keeping in mind that root parasites are likely to be most prevalent at the rooting depth of the crop.
As reviewed in the discussion on sampling, the smaller the area covered by a sample composed of a number of subsamples, the more representative the sample will be. Choosing the size of the area to represented by a single sample is determined by time and cost to take and process the sample and practicality. If the area covered by the sample is 5 acres or less, most field nematologists feel a multi core sample is fairly representative of what is present or can be recovered at the time of sampling. As the area covered by a single sample increases, confidence in interpreting the results of a negative sample decreases. As the number of subsamples in a sample increases, confidence in the results increases.
If a sample does not yield plant parasitic nematodes when expected, or yields no nematodes at all, the storage conditions and length of time from the time of sampling to processing should be reviewed. If samples become too warm during shipment as could happen if left in a vehicle, or shipped by public or private carriers, nematodes could be killed and not recovered during processing.