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Assays for Determining Resistance and Susceptibility of Onion Cultivars to the Pink Root Disease. W. M. Hess, Department of Botany and Range Science, Brigham Young University, Provo, UT 84602; D. J. Weber, Department of Botany and Range Science, Brigham Young University, Provo, UT 84602. Phytopathology 78:115-117. Accepted for publication 1 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-115.

Biological assays using the pathogen Pyrenochaeta terrestris, crude exudates of the fungus, and semipurified preparations of toxic products of the fungal exudates were developed to evaluate resistance and susceptibility of seedlings of various cultivars of onion to the pink root disease. One bioassay involved inoculating onion seedlings in petri dishes with the fungus. Indications of pathogenicity of the fungal isolates were obtained within 7 days. Some indications of resistance of onion cultivars were obtained by this method, but the sensitivity was limited. A bioassay with a crude fungal exudate permitted determination of susceptibility of onion seedlings within 5 to 7 days. The disadvantage of the crude extract assay was that contaminants on seeds of certain cultivars grew rapidly in the carbohydrate material in the crude extract and made interpretation of the results difficult. A bioassay using a semipurified fungal toxin was the most reliable, and evaluation of the susceptibility of onion seedlings was possible in 5 to 7 days without serious problems from contaminants. An efficient procedure for mass production and preparation of the semipurified toxin was developed.