Background

When exporting plants and plant products to countries outside the European Union, each batch must be officially inspected and found free of harmful organisms. In addition, the batch must meet the additional requirements of the destination country. Only then can an official phytosanitary certificate be issued. For example, in China, each batch of lily bulbs must be officially declared free of:

- the Arabis mosaic virus (Nepovirus arabis) and
- the strawberry latent ringspot virus (Stralarivirus fragariae).

The Netherlands and China have established the "China Protocol" for this purpose. According to this protocol, a batch of lily bulbs meets the aforementioned Chinese requirements if the propagation material has been tested and found free of both viruses prior to cultivation of the final product. The export batch itself then no longer needs to be tested. This assumes that no new contamination or spread of these viruses from the soil occurs during the cultivation of lily bulbs.

Random sample in addition to planting material test
In addition to the inspection of the planting material, a random sample of 75 harvested lots of lily bulbs is officially sampled each year and tested for the presence of the two viruses.

The director of the NPPO has asked the Bureau for Risk Assessment & Research (BuRO) to investigate whether the approach in the China Protocol is sufficient to declare that export consignments of lily bulbs are free of both viruses.

Approach
BuRO conducted research based on the results of the planting stock test and the monitoring test from 2018-2021. The fraction of infected lots in the monitoring test is significantly higher for both viruses than in the planting stock test (2.5 to 4 times higher). BuRO suspects that the observed differences in infection levels are caused by the low analytical sensitivity of the ELISA test used, which causes bulbs with a low virus concentration in the planting stock test to be incorrectly considered virus-free.

Answer to the question: plant material testing alone is not sufficient
The results of the monitoring test show that the current approach in the China Protocol is insufficient to declare that export consignments of lily bulbs are free of the two viruses.