Effects of soil matric potential and bulk density on the growth of Fusarium oxysporum f. sp. raphani

The effects of soil dry bulk density and matric potential on the growth of the pathogenic fungus Fusarium oxysporum f. sp. raphani PEG-4 (PEG-4) in the absence or presence of selected soil microorganisms were investigated. These 'challenging' organisms included F. oxysporum f. sp. cucumerinum, a soil fungus Trichoderma viride and a Gram-positive bacterium Pimelobacter sp. The degree of growth of PEG-4 in sterile soils previously colonised by, or simultaneously inoculated with, the soil microorganisms was determined. Soil extracts were also used to test for the effects of a mixed population of soil microorganisms on the interactions. Soil bulk density, ranging from 1.1 to 1.5 Mg m-3, and matric potential (-1000 to -1 kPa) significantly affected the PEG-4 propagule numbers: PEG-4 appeared to form fewer propagules in more compacted soil or soil at lower matric potential (i.e. drier soil). The potential for the prior colonisers to explore the soil and inhibit the growth of PEG-4 is discussed in relation to the soil structure and associated moisture conditions.