3.8.6
MICROBIAL POPULATIONS IN PYTHIUM-SUPPRESSIVE ROCKWOOL GROWN WITH CUCUMBER

J POSTMA and JD VAN ELSAS

DLO Research Institute for Plant Protection (IPO-DLO), PO Box 9060, 6700 GW Wageningen, The Netherlands

Background and objectives
The policy of the the Dutch government in greenhouse horticulture is focused on the development of closed culture systems. Nutrient solutions have to be circulated to minimize pollution. However, root pathogens, and particularly those which produce zoospores, have the potential to spread rapidly in the aqueous environment. Pythium aphanidermatum has been responsible for considerable crop losses in cucumber and control of this pathogen is difficult. Pythium spp., however, are poor competitors relative to other root-colonizing microorganisms. The objectives of this study are (i) to evaluate the capacity of the indigenous microflora to suppress root rot of cucumber plants grown on rockwool, and (ii) to find the key organisms involved in suppressiveness.

Results and conclusion
Rockwool previously grown with cucumbers was found to be suppressive to P. aphanidermatum. In several experiments with nursery plants in an ebb and flow system [1], used rockwool showed 53 to 100 percent less diseased plants than sterilized rockwool after inoculation with P. aphanidermatum. A 26 to 88 percent decrease in disease was present in used compared to new rockwool. The suppressiveness was regained after sterilized rockwool was recolonized by the original microflora due to contact with non-sterilized rockwool, or by addition of nutrient solution obtained from non-sterilized rockwool. However, sterilized rockwool left up to four months at room temperature without any contact with non-sterilized rockwool did not become suppressive.

In order to stimulate or introduce a suppressive microflora, we studied the relation between suppressiveness and the occurrence of certain microbial groups by plate counts and by denaturing gradient gel electrophoresis (DGGE) [2]. This molecular technique allows genetic fingerprinting of populations. Results with plate counts showed that the bacterial numbers in sterilized rockwool increased rapidly, whereas the build up of fungal populations was much slower. Within 2 days total numbers of bacteria were similar in sterilized and non-sterilized rockwool (7.5-7.9 log cfu/g moist rockwool). Furthermore, correlations between suppressiveness and numbers of actinomycetes and Trichoderma spp. were found. Community profiling with DGGE showed distinct shifts in the bacterial populations in rockwool. As soon as a correlation between suppressiveness and specific DGGE patterns is found, interesting bands will be analysed in more detail.

Suppressiveness in rockwool is a new phenomenon which will lead to new opportunities to control diseases in greenhouse horticulture; i.e. in stead of a complete sterilization of substrates, solutions, etc., research will develop methods to optimize the beneficial microflora suppressing Pythium.

References
1. Postma J, Willemsen-de Klein MJEIM, Hoogiand EE, 1996. IOBC wprs Bulletin 19(6), 42-46.
2. Rosado AS, Seldin L, van Eisas JD, 1998. Applied and Environmental Microbiology 59, 695-700.