Comparison of Serological and Culture Plate Methods for Detecting Species of Phytophthora, Pythium, and Rhizoctonia in Ornamental Plants. J. D. MacDonald, Associate Professor, Department of Plant Pathology, University of California, Davis 95616. J. Stites, and J. Kabashima. Research Associate, Department of Plant Pathology, University of California, Davis 95616; and Farm Advisor, University of California Cooperative Extension, Anaheim 92805. Plant Dis. 74:655-659. Accepted for publication 17 February 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0655.

Commercially available ELISA (enzyme-linked immunosorbant assay) kits (in multiwell format) were compared to standard culture plating for their ability to detect Phytophthora, Pythium, and Rhizoctonia in a variety of ornamental plants exhibiting symptoms of root disease. When the pathogens were present in roots at levels that yielded ELISA reactions >0.3 absorbance units above the test threshold, there was good agreement between the two methods, particularly for detection of Phytophthora spp. Agreement between methods was less when the pathogens were present at apparent low levels, an effect attributed largely to sampling error. In greenhouse experiments, there was no significant difference between ELISA and culture plating in detection of P. cryptogea in inoculated sage roots. When extracts from infected chrysanthemum roots were mixed at various ratios with extracts from healthy roots, P. cryptogea could be reliably detected (P = 0.05) when the amount of infected tissue in a sample was as low as 1%. While meaningful thresholds still need to be established, the tests appear to be a promising method with which growers could rapidly diagnose disease problems.