An alternative way of soil sampling for detection of field infestations with Globodera spp.

Field infestations with potato cyst nematodes (Globodera rostochiensis and G. pallida) cause yield losses in potato but they often stay undetected when cyst numbers in the soil are low. Although both Globodera species have a quarantine status, Belgian authorities now allow sampling by farmers or traders as part of a self-checking system (auto control) without the requirement for official notification of individual fields. Many farmers are still not aware of the presence of these cysts in all or part of their fields. One of the reasons why they remain reluctant to evaluate their field status is that taking soil samples in the field is cumbersome. We investigated if an alternative, easier, sampling method whereby soil is taken from harvested potato lots is as reliable as field sampling, or perhaps even more sensitive. The field sampling methods consisted of gathering 1500 ml soil compiled from at least 100 soil cores per ha (official EU method) and from 11 samples of 600 ml taken from 40 soil cores per ha (AMI-100 method), with samples taken in a grid pattern for both methods. During three cropping seasons, data were collected from slightly to medium-infested potato fields cropped with resistant or susceptible potato varieties. The numbers of cysts in the field prior to potato planting and at harvest were compared to numbers, content and species of cysts found in soil taken during the harvesting process. In addition, more than 700 samples consisting of tare soil or adhering soil from harvested potato lots were checked for cysts and when these were found, the corresponding fields were sampled. We found that sampling of adherent soil or tare soil was as good at detecting cysts with eggs as the field sampling methods, especially after growing a susceptible potato variety. However, after growing a resistant cultivar the number of empty cysts retrieved in the samples of tare soil was in some cases lower than in the field samples. A major prerequisite of this alternative sampling method is good traceability: it is paramount to keep track of (the part of) the field that corresponds with the sample of tare soil. Next to the sampling, we also investigated methods to distinguish old cyst from new cysts retrieved from a same sample. Cyst color, cyst content (number of eggs and juveniles) and cyst density of young and old cysts were compared. An adapted way of extracting cysts from samples to separate young and old cysts based on their density and content was developed. This method still needs testing with field samples taken before and after potato cropping to see if it is practical and applicable to separate new cysts from old ones. This technique would allow rapid detection of deviant populations (different species or virulent populations).


Key words: Globodera rostochiensis, G. pallida, sampling, detection, tare soil