PCN sampling

Press, 13 June 1986, Page 12

PCN sampling

To illustrate the difficulties of estimating nematode populations in a paddock, imagine how accurately you could estimate the whole human population of Canterbury by taking a few head counts in some spots around the province. You might hit the Square on a Friday night or more likely just an empty space in the middle of a pine plantation. Sampling for potato cyst nematode distribution is many times more difficult than this. Mr Gavin Harper, of the Plant Protection Centre, M.A.F. (Lincoln), says the present soil sampling method may often under-estimate PCN populations. The basic reason for this difficulty is both human and PCN populations are highly aggregated. Discrete populations are separated by large areas of empty spaces. The amount of space between populations, e.g. between Ashburton and Timaru, needs to be carefully defined before the human densities over this area can be calculated. To sample nematodes a 100 sq m sampling grid is located over the last known infestation, or lens, in a paddock and 100 subsamples dug, bulked and sent to the laboratory for evaluation. A similar method of sampling can be applied to Canterbury’s human population if the province is imagined to be a lens — the South Island being the paddock. To sample this large area, a proportionately larger “soil corer,” about Ikm in diameter would be re-

quired. If the routine 100 subsamples are taken the estimated total human population for the province is 50,000-100,000. However, information from the 1981 census puts Canterbury’s real population at 336,846 people. If enough samples were taken the sample estimate would eventually equal the census figure. This example illustrates that this type of sampling system, because it can not take into account the empty spaces between discrete subpopulations, tends to significantly underestimate the real provincial population. Individual nematodes, however, are many times more difficult to locate in the soil profile than people are across Canterbury. Therefore the PCN sampling system is mostly a method of detecting only the presence of nematodes.

Potato growers are reminded that before permission is granted to plant potatoes in an infected paddock it is necessary to apply to the M.A.F. to take a pre-plant soil sample from the last identified infestation. The end of June is the expiry date for submission of these samples.