Based on this PRA, Clavibacter nebraskensis was added to the EPPO A1 List of pests recommended for regulation as quarantine pests in 2026. Measures for seeds and grain of Zea mays are recommended.

Main conclusions: Clavibacter nebraskensis is a seed-transmitted bacterium occurring in North America and South Africa whose main host is maize (Zea mays). Other natural hosts are Setaria viridis and shattercane (Sorghum bicolor) and many experimental hosts have been found through inoculation, but the role of these as alternative hosts is not clear. The primary phytosanitary risk of C. nebraskensis is associated with maize. The main source of inoculum where the pathogen is present are infested crop debris, although maize seeds may play a role in introducing the disease into new areas. No completely resistant maize cultivars exist, and susceptibility amongst cultivars varies. Some maize types (such as popcorn or sweet corn) appear to be more susceptible, possibly because selection of resistant cultivars is less likely due to a narrower genetic pool.

Seeds (for planting) of Zea mays was identified as the most likely entry pathway, with a moderate likelihood and moderate uncertainty. The EWG noted in particular that some commercial seeds (for sale to growers) are imported into the EPPO region from countries where the pest occurs. Successful entry with grain of Zea mays (kernels for processing or consumption, but not for planting) requires a chain of unlikely events for transfer to a host to occur, although the risk of transfer is higher when associated with husks or maize plant debris. The likelihood of entry on grain was rated as low with a moderate uncertainty. 

The likelihood of establishment was rated as very likely with a moderate uncertainty. Maize is widely cultivated in many EPPO countries and in its current distribution, C. nebraskensis occurs under a wide range of climatic conditions. Subsequently, climate was generally not considered to be a limiting factor for establishment in areas of the EPPO region where maize is cultivated. The pathogen is known to occur under certain Köppen Geiger climate types that also occur in the EPPO region (BSk, Cfa, Dfa, Dfb – see section 9.1). These areas of the EPPO region were considered to be suitable for establishment with a higher degree of certainty than areas of the EPPO region which cultivate maize under climate types from which the pest has not been reported. The EWG considered that the extent of maize cultivation and agricultural practices in maize cultivation may have a greater effect on establishment than climate. Establishment is most likely to occur if the pathogen enters an area where there is a high density of maize cultivation and where agricultural practices such as continuous maize production (without crop rotation) and/or reduced or no-tillage are common. Although it is not possible to precisely delimit where favourable agricultural practices are widely used, they are known to occur in some areas, which would increase the establishment potential.

The magnitude of spread was rated as moderate with a moderate uncertainty. Spread would occur naturally and through human-assisted means (such as infected seed or grain, green forage, silage, debris, contamination of machinery). It is noted that extreme weather events (strong winds, hail, and storms) move infested crop debris and damage plants promoting disease development, which have an important role in the U.S. Midwest, may not happen at a comparable scale and frequency in the EPPO region, although their frequency is likely to increase as a result of climate change. In the EPPO region, like in South Africa, C. nebraskensis may not be detected immediately, allowing for significant spread and multiple outbreaks before detection.

Overall, impact in the current area of distribution was rated as moderate with a moderate uncertainty. In the past in the USA, initial outbreaks and reemergence events have had a high impact in terms of crop losses. Currently, direct economic losses are relatively low in the USA, where management practices are in place, although there is higher impact on a local scale in certain areas due to the cultivation of certain maize types that are more susceptible, or to the occurrence of severe weather events. In South Africa where the pathogen was first found in 2024, the disease has to date affected trade and can disrupt production.

For the potential economic impacts, two scenarios, optimistic and pessimistic[1], were assessed. Under the pessimistic scenario, the magnitude of impact in the area of potential establishment was rated as high with a moderate uncertainty. Under this scenario impacts were expected to be similar to US ‘outbreak’ years or specific severe cases. Under the optimistic scenario, the magnitude of potential impact was rated as moderate with a moderate uncertainty. Under this scenario yield losses exceeding normal variation were expected and some targeted measures would likely be necessary.