Serological detection of latent infection with Phytophthora infestans in potato stems

The fungus Phytophthora infestans was detected in potato crops before the o ccurrence of the first symptoms by using an indirect ELISA. Potato shoots o f 5- to 9-week old plants were used for the analysis. The pathogen was foun d up to 39 days before disease outbreak in the field. The percentage of lat ent 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 arti ficially infected seed tubers did not exceed the number recorded from norma l, obviously healthy seed tubers. No agreement was noticed between the resu lts of the ELISA and the symptoms later appearing on single plants in the f ield. After heavy rainfall early in the season, latent infection was freque ntly detected in several neighbouring plants in a field row. For this reaso n, the spread of the disease caused by zoospores transported in soil water was taken into consideration.