Effect of the indigenous microflora on the development of root and crown rot caused by Pythium aphanidermatum in cucumber grown on rockwool

The capacity of the microflora in rockwool to suppress Pythium aphanidermatum, the causative agent of root and crown rot in cucumber, was assessed. Disease development of cucumber (Cucumis sativus) grown on rockwool was evaluated in an "ebb-and-flood" system with a recirculating nutrient solution after inoculation with P. aphanidermatum. In five independent experiments from 1995 to 1998, 11 batches of used rockwool were tested. All batches without P. aphanidermatum problems in the preceding cucumber crop had significantly lower numbers of diseased plants in nonautoclaved than in autoclaved used rockwool; the disease incidence was reduced by 52 to 100%. Suppressiveness also was present in rockwool previously used to grow other vegetable crops. Rockwool originating from a cucumber crop that was severely attacked by Pythium resulted in a high disease incidence. Previously unused (new) rockwool had higher or similar percentages of diseased plants than did nonsterilized used rockwool. Disease suppression in used rockwool could also be measured in a smaller test system. In both systems, autoclaved rockwool became suppressive to Pythium after recolonization with the indigenous microflora. Population sizes of total culturable aerobic bacteria as well as of fluorescent pseudomonads did not correlate with disease suppressiveness, as numbers of bacteria and pseudomonads were similar or lower in nonautoclaved (suppressive) than in autoclaved (nonsuppressive) rockwool. Differences in the structure of the bacterial populations could be visualized by using eubacterial polymerase chain reaction (PCR) followed by denaturing gradient gel electrophoresis (DGGE). Interestingly, the nonautoclaved and the recolonized used rockwool, which were both suppressive, showed different dominating bacterial groups as compared with the autoclaved rockwool. PCR-DGGE patterns obtained at different sampling times showed that the composition of the bacterial populations changed during plant growth. Fungal populations were present in the treatments that yielded suppressive rockwool, i.e., the nonautoclaved and the recolonized rockwool, but they were absent or present in low numbers in the autoclaved rockwool, which permitted a high disease incidence. Suppressiveness of rockwool to Pythium root and crown rot is a hitherto undescribed phenomenon, and knowledge of the mechanism and microorganisms involved will stimulate the development of microbially balanced soilless growing systems.