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.