MICROBIOLOGICAL DIFFERENCES BETWEEN LIMED AND UNLIMED SOILS AND THEIRRELATIONSHIP WITH CAVITY SPOT DISEASE OF CARROTS (DAUCUS-CAROTA L) CAUSED BY PYTHIUM-COLORATUM IN WESTERN-AUSTRALIA

Application of lime (4000 kg ha(-1)) to a soil used for commercial car rot production (pH 6.9) significantly (p < 0.05) reduced the incidence of cavity spot disease of carrots compared to unlimed soil (pH 5.1). It significantly (p < 0.0 I) increased soil microbial activity as meas ured by the hydrolysis of fluorescein diacetate and arginine ammonific ation. The application of lime resulted in a significant (P < 0.01) in crease in the total numbers of colony forming units (cfu) of aerobic b acteria, fluorescent pseudomonads, Gram negative bacteria, actinomycet es and a significant (P < 0.01) decrease in the cfu of filamentous fun gi and yeasts compared to unlimed soil. Liming also increased the cfu of non-streptomycete actinomycetes rarely reported in similar studies. These non-streptomycete actinomycetes were estimated and isolated usi ng polyvalent Streptomyces phages and the dry heat technique to reduce the dominance of streptomycetes on isolation plates. The non streptom ycete actinomycetes isolated included species of Actinoplanes, Micromo nospora, Streptoverticillium, Nocardia, Rhodococcus, Microbispora, Act inomadura, Dactylosporangium and Streptosporangium. The numbers of act inomycetes antagonistic to Pythium coloratum, a causal agent of cavity spot disease of carrots increased in soil amended with lime. Applicat ion of lime also reduced the isolation frequency of P coloratum from a symptomatic carrot roots grown in soil artificially infested with the pathogen, 3, 4 and 5 weeks after sowing.