Investigating the importance of latent infection in causing potato tuber breakdown during storage and transit

A number of important potato diseases can remain asymptomatic after harvest with symptoms only developing during storage or transit to an importing country.  These latent infections can be particularly problematic to the seed potato industry.

Molecular analysis of pathogen DNA demonstrated that a range of pathogens responsible for surface blemish and rot diseases could be detected on tubers in the absence of disease symptoms. This confirmation of latent infection on seed potatoes indicates the potential threat that crops destined for export could be faced with if conditions during transit and subsequent delivery to, and handling and storage by, the end customer are not controlled.

However, seed tuber quality showed little deterioration in the crops assessed over three years of the project. In all cases the storage conditions during transit appear to have been maintained at the prescribed levels indicating no problems with the haulage providers’ facilities during export.

Occasional increases in disease incidence were recorded for blemish diseases that are already known to increase during storage.  Increases in DNA levels of pathogens were also observed but these weren’t consistently associated with increased disease incidence.

The most consistent changes in pathogen DNA levels were observed for Pectobacterium atrosepticum. Compared to tubers assessed after harvest, DNA levels generally increased in samples that had been stored in the UK or had been exported with levels typically highest in the exported samples. Stocks of a higher seed health pose a reduced risk of increasing P. atrosepticum DNA levels during the export process.

Conclusions

Overall, if tubers are stored under recommended conditions during transit then the risks of latent pathogen infections resulting in disease expression are minimal. Storage conditions on arrival in the importing country appear much more variable. Therefore, exporters may be advised to request that the official inspection of consignments is performed once the tubers arrive in the importing country to prevent losses after the transit process.

Maintaining the highest levels of seed health prior to export remains a key requirement to limit the risk of tuber quality deterioration during export

This project aimed to investigate the effect of transit and post-export conditions on disease development for several important potato pathogens. Molecular diagnostic techniques were used to monitor pathogen DNA levels and the presence of surface blemish diseases was visually assessed (for powdery scab, common scab, silver scurf, black scurf, black dot, skin spot and Verticillium). Internal rots were scored as soft rots, dry rots or gangrene-like symptoms. In the final year of the project the focus was on Pectobacterium atrosepticum and soft rot.

Transit and post export conditions were monitored using data loggers, including a prototype logger that was able to monitor light, movement, CO₂ and provide some indication of total VOC (volatile organic compounds) in addition to the standard temperature, relative humidity and dew point measurements.