Microbiological factors affecting the colonisation of soil aggregates by Fusarium oxysporum f sp raphani

To elucidate factors that affect the colonisation of soil aggregates by Fusarium oxysporum f. sp. raphani PEG-4 (hereafter PEG-4), microcosms were designed which comprised a row of three soil aggregates 4-6 mm dia. placed 2 mm apart on silicone rubber podia. The central aggregate was sterile and inoculated with PEG-4. The lateral aggregates were non-sterile or treated in various ways to affect their microbiological status. Following incubation, propagule populations of PEG-4 in the lateral aggregates were enumerated. The degree to which PEG-4 colonised non-sterile aggregates was less than 1% of the control (sterile aggregates). When non-sterile aggregates were dried to -200 kPa or antibiotics were added to them to inhibit bacteria, colonisation by PEG-4 did not differ greatly from the control, suggesting that the suppressing factors might be of fungal origin. That PEG-4 colonised chloroform-fumigated aggregates, where there was a relative abundance of bacteria, supported this hypothesis. In addition, sterile aggregates which had been colonised by a range of soil fungi generally inhibited PEG-4 to a greater extent than aggregates which had been colonised by single strains of bacteria. The fungal strains that showed the greatest resistance to the establishment of PEG-4 were closely related to it (F. oxysporum f. sp. raphani, F. nivale, F. oxysporum f. sp. cucumerinum). An antibiotic-producing bacteria, Burkholdier cepacia MRT11, was also effective at inhibiting PEG-4. Outcomes of interactions between PEG-4 and the other microbes on agar plates were poorly correlated with the outcomes in the aggregate system. We suggest that such aggregate-based systems may be useful in screening putative biocontrol agents since they are soil-based and effects can be readily assessed. Copyright (C) 1996 Elsevier Science Ltd