Sampling and PCR method for detecting pathogenic Fusarium oxysporum strains in onion harvest

Fusarium basal rot is a worldwide disease problem in onions, and causes substantial losses in onion production, both during the growing season and in the storage. To minimize the post-harvest losses, a protocol for screening of latent infections with pathogenic Fusarium oxysporum strains from harvested onions was developed. This protocol is based on a dual PCR test with primers specific for the fungal species and new SIX3 primers specific for the onion-pathogenic F. oxysporum strains. A pooled sample containing pieces from 50 harvested symptomless onions was prepared for the dual PCR using microwave disruption of the filamentous Fusarium fungi and Whatman FTA^TM filter paper matrix technology, or as a reference protocol, by extracting DNA with a commercial kit. The two sample preparation protocols gave consistent results with the tested onion samples. Detection limit of the dual PCR protocol was 100 pg of F. oxysporum DNA, in a mixture with onion DNA, when the FTA card was applied. The new protocol reported here is simple and sensitive enough for routine testing, enabling the detection of latent infections in harvest lots even at the infection levels under 10%. SIGNIFICANCE AND IMPACT OF THE STUDY: Fusarium basal rot causes serious problems in onion production. To minimize post-harvest losses, a simple protocol based on FTA^TM technology and a dual PCR test with Fusarium oxysporum species-specific and pathogenicity-specific primers was developed. By testing pooled onion samples using this method, latent infections with F. oxysporum can be screened from a representative sample of the harvest. This screening method could be a useful tool to manage the post-harvest losses caused by latent infections with F. oxysporum and, with modification of the PCR protocol, with other Fusarium species pathogenic to onion.