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Serological Detection of Phomopsis longicolla in Soybean Seeds. M. L. Gleason, Department of Plant Pathology, University of Kentucky, Lexington 40546, Present address: Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011; S. A. Ghabrial, and R. S. Ferriss. Department of Plant Pathology, University of Kentucky, Lexington 40546. Phytopathology 77:371-375. Accepted for publication 30 July 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-371.

Antiserum to freeze-dried powdered mycelium of Phomopsis longicolla, a soybean seed decay pathogen, was used in an indirect ELISA and a modified immunoblot assay for detecting seedborne infections. Antigen of P. longicolla was detected by indirect ELISA in as little as 250 ng of freeze-dried mycelium per milliliter of extract. The antiserum reacted strongly with mycelial preparations of Diaporthe phaseolorum var. sojae and D. p. var. caulivora but showed comparatively little or no reaction when tested against seven other seedborne fungi. Extracts of whole seeds, but not of seed coats, produced a nonspecific background reaction that obscured the specific reaction. P. longicolla could be detected in individual seed coats of symptomless infected seeds. A single infected seed coat in 20 could be readily detected by indirect ELISA of extracts of seed coats. A modified immunoblot assay, designated the seed immunoblot assay (SIBA), was developed to overcome problems with nonspecific interference in indirect ELISA. Mycelium of P. longicolla growing onto nitrocellulose paper from infected soybean seeds produced a conspicuous colored blotch after the paper was assayed. Results of SIBA for incidence of P. longicolla and D. p. var. sojae in halved seeds from 10 seed lots correlated (P<0.001) with agar plate bioassays of the corresponding seed halves but not with incidence of symptomatic seeds. Indirect ELISA absorbance values for bulked samples of seed coat halves from the same 10 seed lots correlated weakly (0.10 >P >0.05) with agar plate bioassays but strongly (P<0.01) with incidence of symptomatic seeds. Because SIBA detects only viable P. longicolla and ELISA does not discriminate between live and dead fungus, SIBA should be a better indicator of pathogen activity on seeds after planting. The two types of serological assays apparently measure different aspects of the disease, however, and both may be useful for evaluating soybean seed lot quality.