Diagnosis of Phytophthora infections in raspberry and strawberry plants by ELISA tests

Plant diseases caused by Phytophthora spp. may be reproducibly diagnosed by DAS ELISA techniques, but this type of analysis has long been hampered by the presence of phenolic and related compounds in plants to be tested, not least in strawberry and raspberry plants. The compounds will interfere with the ELISA test procedure, leading to high non‐specific optical density readings. To overcome this, a series of experiments was performed. Phenolic and related compounds in the samples were first absorbed to polymers during antigen extraction and thereafter separated by filtration at slow rate. To inhibit non‐specific binding of enzyme‐conjugated antibodies, the plastic wells were preincubated with non‐sensitized wells, equally high background values of optical density were seen under untreated conditions and after the use of polymeric adsorbents. A marked reduction in optical density was, however, seen after blocking with non‐fat dry milk, but the optimal conditions for all concentrations ofantigen were seen first after combined pre‐treatment with polymeric adsorbents and non‐fat dry milk. When the threshold absorbance for positive detection was calculated, the low optical density values from healthy plants, at all antigen concentrations studied, contributed to an excellent discrimination between samples from diseased and healthy plants.