Detection of spore balls of Spongospora subterranea on potato tubers by enzyme-linked immunosorbent assay

A rabbit was immunized with a homogenate of spore balls (cystosori) of Spongospora subterranea which also contained some potato tuber debris. The resultant polyclonal antiserum contained antibodies which reacted against extracts of spore balls and healthy tubers. Antibodies to the tuber debris were removed by incubating the blood serum with an extract from a S. subterranea -free tuber. In enzyme-linked immunosorbent assay (ELISA), the γ-globulin fraction of the absorbed antiserum reacted with dilute tuber extracts containing the equivalent of as little as 0·08 spore balls per ml. The reaction with spore balls was inhibited in high concentrations of tuber sap. Extracts of peel from symptomless tubers which had been stored in contact with scabbed tubers also reacted with the γ-globulin, presumably because the symptomless tubers became contaminated with spore balls. However, spore balls in soil were only weakly detected when high numbers were present. The γ-globulin did not react with the plasmodial stage of the pathogen, or with 15 other micro-organisms tested, including resting spores of Plasmodiophora brassicae.