Selection of Antagonistic Fluorescent Pseudomonas spp. and their Root Colonization and Persistence following Treatment of Seed Potatoes

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Abstract
One hundred and twelve fluorescent Pseudomonas isolates from potato periderm were screened for antagonistic activity in vitro against 5 pathogenic and saprophytic fungi, 5 Gram‐positive bacteria including Streptomyces scabies and 4 Gram‐negative bacteria including Erwinia car. var. carotovora and Erwinia car. var. atroseptica. Fifty‐seven percent of the isolates showed wide spectrum inhibitory activity, mostly due to the production of fluorescent siderophores. Only 4 % of the isolates were very strong antagonists. By adding iron (Fe3+) to the test medium, the antagonistic activity drastically diminished but revealed some strong antagonists of which the antagonism was not affected. Apparently growth‐inhibiting substances other than siderophores were involved. Antibiotic‐resistant mutants of the antagonists were used to establish that the bacteria on the seed tubers migrated to the roots under different conditions. Drying of the treated potatoes gradually diminished the number of viable cells, but subsequent planting in moist soil led to substantial increases in the first three days. Initially high numbers of the antibiotic‐resistant antagonists on the roots tended to drop quickly in time, probably due to loss of resistance to the antibiotics in the selective medium.

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