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 aphanidermat um, the causative agent of root and crown rot in cucumber, was assessed. Di sease development of cucumber (Cucumis sativus) grown on rockwool was evalu ated in an "ebb-and-flood" system with a recirculating nutrient solution af ter inoculation with P. aphanidermatum. In five independent experiments fro m 1995 to 1998, 11 batches of used rockwool were tested. All batches withou t P. aphanidermatum problems in the preceding cucumber crop had significant ly lower numbers of diseased plants in nonautoclaved than in autoclaved use d rockwool; the disease incidence was reduced by 52 to 100%. Suppressivenes s also was present in rockwool previously used to grow other vegetable crop s. Rockwool originating from a cucumber crop that was severely attacked by Pythium resulted in a high disease incidence. Previously unused (new) rockw ool had higher or similar percentages of diseased plants than did nonsteril ized used rockwool. Disease suppression in used rockwool could also be meas ured 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 fluore scent pseudomonads did not correlate with disease suppressiveness, as numbe rs of bacteria and pseudomonads were similar or lower in nonautoclaved (sup pressive) than in autoclaved (nonsuppressive) rockwool. Differences in the structure of the bacterial populations could be visualized by using eubacte rial polymerase chain reaction (PCR) followed by denaturing gradient gel el ectrophoresis (DGGE). Interestingly, the nonautoclaved and the recolonized used rockwool, which were both suppressive, showed different dominating bac terial groups as compared with the autoclaved rockwool. PCR-DGGE patterns o btained at different sampling times showed that the composition of the bact erial populations changed during plant growth. Fungal populations were pres ent in the treatments that yielded suppressive rockwool, i.e., the nonautoc laved and the recolonized rockwool, but they were absent or present in low numbers in the autoclaved rockwool, which permitted a high disease incidenc e. Suppressiveness of rockwool to Pythium root and crown rot is a hitherto undescribed phenomenon, and knowledge of the mechanism and microorganisms i nvolved will stimulate the development of microbially balanced soilless gro wing systems.