Microbiological differences between limed and unlimed soils and their relationship 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 carrot 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 measured by the hydrolysis of fluorescein diacetate and arginine ammonification. The application of lime resulted in a significant (P < 0.01) increase in the total numbers of colony forming units (cfu) of aerobic bacteria, fluorescent pseudomonads, Gram negative bacteria, actinomycetes and a significant (P < 0.01) decrease in the cfu of filamentous fungi 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 using polyvalent Streptomyces phages and the dry heat technique to reduce the dominance of streptomycetes on isolation plates. The non streptomycete actinomycetes isolated included species of Actinoplanes, Micromonospora, Streptoverticillium, Nocardia, Rhodococcus, Microbispora, Actinomadura, Dactylosporangium and Streptosporangium. The numbers of actinomycetes antagonistic to Pythium coloratum, a causal agent of cavity spot disease of carrots increased in soil amended with lime. Application of lime also reduced the isolation frequency of P coloratum from asymptomatic carrot roots grown in soil artificially infested with the pathogen, 3, 4 and 5 weeks after sowing.