Utilisation of rhizosphere microflora in the biocontrol of root rot and growth enhancement of lettuce under hydroponic systems

Abstract

About 150 rhizobacteria and 49 rhizosphere fungi were isolated from the rhizosphere of grasses and sedges at Nylsvley Nature Reserve, Limpopo Province, South Africa. The rhizobacterial isolated were mostly Gram negative (72%) and rod shaped (73%). The dominant fungal genera were Trichoderma, Aspergillus, Pencillium, Neosartorya and Fusarium. The antagonistic activity of the above isolates were determined in a dual culture assay against a range of plant pathogens namely Colletotrichum gloeosporioides, Pythium irregulare, Penicillium digitatum, Fusarium solani and Geotrichum candidum. All rhizosphere fungal isolates showed positive antagonism against G. candidum (100%) and F. solani (100%). The rhizobacterial isolates showed positive antagonism against G. candidum (71%) and C. gloeosporioides (76%). The growth promotion and biocontrol activity of the rhizosphere isolates that showed broad-spectrum antagonistic activity against the fore mentioned pathogens were further evaluated on Canadian peat substrate under greenhouse condition. Although most of the rhizosphere isolates resulted in improved fresh leaf weight in comparison with the non-inoculated control in final growth promotion experiments, no statistical difference could be found in increasing leaf weight by one of the tested isolates. Some isolates and BactolifeTM prevented root infection by Pythium. However, only isolate 68B showed significant prevention of root infection compared to the Pythium inoculated control. The selected rhizobacteria, fungal and commercial biocontrol products that showed the most effective growth promotion and biocontrol activities were further evaluated in a re-circulating hydroponic system. Overall, isolate BSB (Bacillus subtilis) consistently enhanced the fresh leaf and root weight by 29.82 and 24.31% compared to the untreated control. Treatments with rhizobacteria isolate 91B and 43B significantly increased fresh leaf weight and suppressed Pythium root infection of lettuce. Isolate 91B and 121B significantly decreased the incidence of Pythium after the 1st and 2nd week of inoculation respectively. The combination of rhizobacteria BSB and 43B showed a synergistic effect as reflected in increased fresh leaf weight and total biomass per plant whilst suppressing root rot caused by Pythium group-F. Copyright 2003, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Begashaw, L 2003, Utilisation of rhizosphere microflora in the biocontrol of root rot and growth enhancement of lettuce under hydroponic systems, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-05092005-103113 / > E642/ag</gm>