1Bulb Research Centre, PO Box 85, 2160 AB Lisse, The Netherlands; 2DLO-Institute for Agrobiology and Soil Fertility, PO Box 14, 6700 AA Wageningen, The Netherlands

Background and objectives
Pythium is a soil-borne fungus which causes root rot in several ornamental bulb crops on sandy soil. It is a potentially fast-growing fungus which is relatively susceptible to competition by other organisms. A light infestation in field soil causes a light root rot infection, whereas the same infestation in sterilized soil leads to severe disease development. Cultural practices, such as flooding and soil fumigation, are generally applied in ornamental bulb culture to control some diseases and weeds. However, these treatments do not control Pythium (flooding) or only to a limited extent (fumigation) and are harmful to many other soil organisms. Therefore, flooding and fumigation may result in an imbalance between Pythium and other competing microorganisms. The impact of these cultural practices on Pythium root rot has been studied in bioassays with Iris . Infestation of flooded or fumigated field soil results in enhanced root rot compared with infestation of untreated soil. In the absence of other microorganisms (heat-sterilized soil) flooding and fumigation have no effect on disease development, indicating the crucial role of the microflora in natural disease suppression. In this context, the introduction of microorganisms by adding composted organic household waste has also been investigated. Addition of 1% compost to heat-sterilized soil 1 ;week prior to infestation with Pythium results in reduced disease development compared with treatments without compost or with sterilized compost.

In order to quantify the role of the soil microflora in suppression of Pythium several microbial parameters were measured in treated and untreated soil and related to the disease development in iris.

Materials and methods
Field soil was treated, or not treated, by flooding (8 ;weeks), fumigation (methylisothiocyanate) or heat sterilization and 1% compost was added to sterilized soil. Subsequently, biomass, glucose uptake (indicating capacity for food competition) and respiration (CO2-production) of the soil microflora were determined using a chloroform fumigation extraction method [1]. Dehydrogenase activity was determined [2] as indicator of microbial activity in soil. The growth rate of Pythium through soil was determined by periodically collecting soil samples at regular distances from the point of inoculation and plating on a selective medium to assess for presence of the fungus. All measurements, except biomass, were performed after addition of supplementary substrate (glucose or oat meal) to the soil in order to simulate a substrate-rich environment as can be expected in the proximity of roots caused by the presence of root exudates.

Results and conclusions
Dehydrogenase activity and respiration increased in sterilized, flooded, fumigated and untreated soil, correlating with increasing disease suppression observed in the bioassay. Pythium growth rates decreased with increasing disease suppression in these soils. Flooding and fumigation, however, had no significant effect on the microbial biomass or on the glucose uptake compared with untreated soil. The addition of compost to sterilized soil resulted in extremely high scores for microbial biomass and activity, and an extremely low growth rate of Pythium in relation to the merely moderate disease suppression observed in the bioassay and compared with the other soil treatments. Apparently, these quantitative microbial parameters do not consistently reflect the potential disease suppression in soil against Pythium. Possibly, the presence of specific species is also important for disease suppression.

1. van Ginkel JH, Merckx R, van Veen JA, 1994. Soil Biology and Biochemistry 26, 417-19.
2. Smith SN, Pugh GJF, 1979. Enzyme Microbial Technology 1, 279-81.