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A Seedling Bioassay Chamber for Determining Bacterial Colonization and Antagonism on Plant Roots. P. S. Randhawa, Postdoctoral research associate, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83843, Present address: USDA, ARS, Horticultural Science Institute, Fruit Laboratory, Beltsville Agricultural Research Center, Beltsville, MD 20705; N. W. Schaad, professor, Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow 83843. Phytopathology 75:254-259. Accepted for publication 4 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-254.

A seedling bioassay is described for identifying bacteria or other microorganisms antagonistic to plant pathogens based upon competitive colonization of roots. A seedling bioassay (SBA) chamber is constructed by dividing a square petri plate into two compartments with a sterile glass rod held in place with a small amount of sterile molten water agar. Agar medium is added to one compartment and a perforated, absorbant paper pouch, folded to hold seeds and soil, is attached to a glass platform in the other compartment. The chamber is incubated vertically; the glass rod divider is horizontal, the solidified agar medium is in the compartment beneath it, and the paper pouch is in the compartment above it. Seeds infested with possible antagonists are germinated, the emerging radicles are inoculated with bacterial or fungal pathogens, and the germinating seeds are placed in the pouch in the upper compartment of the SBA chamber. After 48- 72 hr, roots emerge through the pouch, cross the glass rod partition, and begin growing down the surface of the agar. An appropriate selective agar medium can be used to detect root colonization and pathogen antagonism along the root. Roots can also be assayed for bacterial populations by excision, comminution in water, and dilution plating on agar media. For fungal pathogens, antagonism is expressed by prevention of symptoms in the seedlings. Of 122 bacteria obtained from washings of various seeds, 82 colonized collard roots and 20 of the colonizers prevented growth of Xanthomonas campestris pv. campestris. Similar results were obtained with Pseudomonas solanacearum on tomato roots. Relatively few bacteria that produced inhibition zones in agar colonized roots of seedlings. All bacterial strains identified as antagonists in SBA chamber were root colonizers, but all colonizers were not antagonists. The SBA chamber provides a simple, inexpensive method of screening bacteria for their ability to colonize roots and antagonize specific pathogens.