Defining the biosecurity risk posed by soil found on sea freight

Soil associated with sea freight (shipping containers, flat racks and used machinery) arriving at New Zealand seaports was sampled for bacteria, fungi, nematodes, macro-invertebrates and plant seeds. Pseudomonads were selectively isolated, as several significant plant pathogens fall within this bacterial group. The mean and median sample weight collected from sea freight was found to be 417.3 and 152.7 g, respectively, with most recovered soil (73%) collected from the underside of shipping containers and flat rack containers. Likewise, for used machinery, most recovered soil (75%) was found under the machinery. Flat rack containers had significantly higher soil contamination compared to shipping containers and used machinery, but generally the counts and incidence of taxa were significantly lower compared to these other freight types. Viable bacteria, fungi, nematodes, seeds and arthropods were associated with the soil, with both counts g-1 and prevalence in samples varying with taxa, freight type, and location on the freight. Various regulated biosecurity organisms were recovered from the samples, including Aphelenchoides besseyi (rice white tip nematode), and seeds from genera such as Brachiaria, Cortaderia, Digitaria, Eragrostis and Sonchus. There were also live arthropod taxa that were not recorded as being present in New Zealand. No known plant pathogenic pseudomonads were identified through sequencing of the 16S ribosomal RNA gene. Shipping containers were found to be an important introduction pathway for exotic species, and therefore require careful monitoring and management. Comparisons of the incidence and mean number of organisms associated with soil on sea freight compared to a previous study with soil on footwear, generally showed that incidence and counts of many taxa were lower on sea freight, indicating that biosecurity risk can vary with pathway. However, prioritising one soil pathway over another according to the risks they present, and differentially allocating resources is problematic because the relative risk is dynamic, dictated by factors such as new pests or diseases entering the respective pathways.