Relationship of Micro-Organisms to sporulation of Phytophthora cinnamomi rands

Abstract

The effect of soil extracts on sporulation of Phytophthora cinnamomi Rands has been investigated. Non-sterile soil extracts from different soil types and from soils under different plant species all supported sporulation, stimulatory activity being markedly reduced only after 1,000-fold dilution. Autumn soil was most active in stimulating sporulation, whereas summer soil was inactive. When peptone was added to summer soil, the stimulatory activity of the extract was restored. A wide variety of materials added to distilled water failed to stimulate sporulation. Soil extract lost its stimulatory properties after sterilisation by autoclaving, steaming, Seitz or sintered glass filtration, dialysis, ultrasonic disintegration, ultracentrifugation, or treatment with propylene oxide or alcohol. Root exudates and micro-organisms other than bacteria had no effect on sporulation. Fifty-nine bacterial cultures classified in 15 genera stimulated sporulation on one or more occasions when transferred from nutrient broth cultures but not from nutrient agar slopes. Of 16 morphologically distinct bacteria isolated from wet autumn soil, two Pseudomonas species stimulated abundant sporulation when transferred from agar plates to sterile soil extract. The activity of these isolates was lost after six weeks in culture, but was partially restored by growing them in nutrient broth. Extracts from sterilised soil inoculated with attenuated cultures of these Pseudomonas species were stimulatory, but subcultures from these extracts were only stimulatory if grown in broth. The difference in ability to stimulate sporulation between cultures from broth and from agar was not due to bacterial numbers, flagellation, or fimbriation, but was associated with the additional nutrient transferred with the bacterial inoculum from a broth culture. Streptomycin was the only one of several antibiotics tested which inhibited sporulation. Irrigation of P. cinnamomi mycelium with continuously-produced sterile filtrate from soil extracts produced sporangia in 48 hours.