Detection of Pythium ultimum with a Species-Specific Monoclonal Antibody. G. Y. Yuen, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. M. L. Craig, and F. Avila. Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. Plant Dis. 77:692-698. Accepted for publication 11 February 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0692.

We obtained four monoclonal antibodies to Pythium ultimum using a crude preparation of P. ultimum cell wall material as the immunogen. Initial selection was based on positive reactions with a culture of P. ultimum and negative reactions with one isolate each of P. irregulare and Phytophthora cinnamomi in indirect enzyme-linked immunosorbent assay (ELISA). The antibodies were screened further against 21 isolates of P. ultimum (including 11 asexual strains), 39 isolates of 16 other Pythium spp., and 24 isolates of 12 other genera of soil fungi. Antibody E5, the most specific to P. ultimum, was highly reactive to all P. ultimum isolates and had low or no reactivity to other Pythium spp. The other three antibodies exhibited cross-reactivity to various other species of Pythium. Two serogroups were distinguished within P. ultimum on the basis of reactivity to these three antibodies. None of the four antibodies reacted with other genera of fungi. A protocol was developed for the rapid identification of P. ultimum using E5, in which raw extracts from 2- to 3-day-old fungal cultures grown in liquid media are tested by ELISA. This system identified P. ultimum isolates among 246 cultures of pythiums isolated from sugar beet roots and soil with greater than 99% accuracy. An antigen competition assay, also using E5, was developed for detecting P. ultimum in roots. It was effective in detecting the fungus in sugar beet seedling roots that contained more than two infections per 10 cm of root.