SELECTION OF RHIZOBACTERIA FOR THE CONTROL OF PYTHIUM-ULTIMUM AND APHANOMYCES COCHLIOIDES ON SUGAR-BEET SEEDLINGS

Approximately 400 bacterial isolates from the roots of field-grown sug ar-beet seedlings were examined for their ability to inhibit the growt h of Pythium ultimum and Aphanomyces cochlioides in culture and to sup press disease caused by these pathogens when applied to pelleted sugar -beet seed in artificially infested soil. Biocontrol potential was ass essed in controlled environments, under optimal conditions for disease development and expression. A large proportion of the rhizobacteria i solates were antagonistic to the fungi in culture, but only c.1% were effective biocontrol agents. Isolates that inhibited growth in vitro w ere, in general, active against both fungal species, whereas in vivo, isolates that most suppressed P. ultimum were different from those mos t effective against A. cochlioides. Over all isolates with both pathog ens, there was no correlation between the degree of inhibition in cult ure and the extent of disease suppression. Effective biocontrol isolat es significantly improved seedling emergence and the proportion of hea lthy seedlings compared with untreated seed. However, under the high d isease pressure generated in these experiments, the level of protectio n fell short of that achieved with the fungicides currently in use on sugar-beet seed. Copyright (C) 1996 Elsevier Science Ltd.