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SAMPLING STRATEGIES |
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CONTENTS Timing of Sampling |
Nematode samples
may be taken in different ways depending on the objectives at the time. When samples are
taken for diagnosis of a problem, a single sample should be taken from an affected area
(or a composite of samples from several affected areas) and a separate sample should be
taken from an adjacent normal area. In most cases, nematode damage occurs in patches.
Where damage is severe, the center of the patch may consist of a thin stand of stunted
plants which may no longer support a high population density. In these instances, it is
recommended that samples be taken midway between the center of the patch and the patch
edge since this often represents the area of most intense nematode activity. Nematode
species and densities recovered from in and outside the patch are then compared to
determine if there appears to be a correlation between nematodes and plant appearance
and/or if the nematodes recovered are known to cause the observed symptoms. Sometimes it
is difficult to establish that nematodes are the cause of a problem but it is relatively
easy to establish that they are not the cause if they are not present in the affected
site. Because many soil conditions such as nonoptimal pH, nutrient level, standing water
etc., can cause patches of stunted plants, it is important that nematodes are confirmed to
be the cause or the money spent on nematode management will not realize any return. When sampling for routine nematode management, the population within the entire field needs to be determined. Since nematodes are distributed in patches, a large number of subsamples (i.e. cores which are combined together to make a single sample for analysis) is need to be taken to adequately represent the area being sampled. Undersampling risks the possibility that high population areas are missed and, thus, no management is recommended. Conversely, unrepresentative samples may recover very high populations from a patch and suggest a more expensive management plan than would be recommended if the field had been adequately sampled. Optimal sampling strategies are regulated by the number of samples taken from a field and sent to a laboratory for analysis and the number of subsamples (cores etc.) that make up each sample. Taking more subsamples involves only additional sampling effort while increasing the number of samples requires additional fees for each sample analyzed. Therefore, it is more cost effective to improve sampling precision by taking more cores/sample than by taking more samples/field. However, there are limits to the size of an area that can be adequately represented with a single sample. Current conventions suggest that 10 cores are required to represent an area one acre in size, 20-30 cores are necessary for an area up to five acres, and up to 100 cores may be necessary for areas as large as 10 acres. Representing an area larger than 10 acres with a single sample is not recommended. Sampling large areas can be facilitated by dividing the field into a number of different 5-10 acre areas and sampling selected areas that are representative of the field. Another useful strategy is sample stratification. If the field has different features such as different crop history, different variety of mint, different planting date, soil type, slope position etc., these areas should be represented by separate samples. Stratifying the sampling effort can provide very useful information because nematode populations may be different in these different field conditions. This may delineate some areas which need to be treated and other areas which do not require treatment. However, it can be a waste of effort to stratify into smaller areas than can be managed separately. For example, if a commercial applicator is restricted to either treating or not treating a minimum of five acres to make it worth his effort or because of the logistics of application, it is not necessary to know the nematode population in each individual acre of that section. |
