Suppression of Pythium root rot in bulbous Iris in relation to biomass andactivity of the soil microflora

Disease suppression against Pythium root rot can be influenced by managemen t practices applied in ornamental bulb culture. Different levels of suppres sion were established in sandy soil after several soil treatments. Percenta ge of infection in Iris caused by Pythium macrosporum was lowest in untreat ed soil and progressively increased in sterilized soil amended with 1% comp ost, fumigated soil (methylisothiocyanate) and flooded soil (fumigation and flooding corresponding to the same level of disease severity) and was high est in sterilized soil. The relationship between the level of disease incid ence, Pythium growth rate through soil, and various microbial parameters wa s investigated. Soil microbial biomass and, after amendment of glucose as a non-selective carbon source, dehydrogenase activity, glucose uptake and re spiration were determined. By using C-14-labelled glucose, measurement of u ptake and assimilation of amended carbon could be distuingished from soil o rganic matter decomposition. All microbial parameters were negatively assoc iated with Pythium growth rates, indicating that high microbial biomass and activity induced suppression of Pythium growth through soil. However, with neither of the quantitative microbial parameters could chances in disease incidence be fully explained. It is hypothesized that competition for carbo n may not be the main mechanism in disease suppression. (C) 2001 Elsevier S cience Ltd. All rights reserved.