Serological detection of latent infection with Phytophthora infestans in potato stems

The fungus Phytophthora infestans was detected in potato crops before the occurrence of the first symptoms by using an indirect ELISA. Potato shoots of 5- to 9-week old plants were used for the analysis. The pathogen was found up to 39 days before disease outbreak in the field. The percentage of latent infected stems detected by ELISA changed greatly between 0.27 and 10.8% in the two years of investigation. The number of invaded shoots from artificially infected seed tubers did not exceed the number recorded from normal, obviously healthy seed tubers. No agreement was noticed between the results of the ELISA and the symptoms later appearing on single plants in the field. After heavy rainfall early in the season, latent infection was frequently detected in several neighbouring plants in a field row. For this reason, the spread of the disease caused by zoospores transported in soil water was taken into consideration.