Management of Fusarium wilt of banana by means of biological and chemical control and induced resistance

Abstract

Management of Fusarium wilt of banana, one of the most important diseases of agricultural crops, is complicated and involves the consideration of factors such as the biology, epidemiology and population structure of the pathogen, and genetic resources and production practices of the crop. The development of an integrated disease management programme, therefore, is of great importance in countries where the Fusarium wilt pathogen, Fusarium oxysporum f.sp. cubense Foc, has been introduced into banaria fields, and where resistant cultivars are not acceptable to local markets. To achieve this, it is important to investigate new management strategies and to review methods that have been less successful in the past. These management practices need to be practical and affordable. Since certain cultural practices have proven to be effective, management practices that could compliment them should be considered. This thesis has attempted to investigate such practices in order to develop an integrated disease management programme for Fusarium wilt of banana. One of the most important findings of this study, was that the surface sterilant previously used to prevent the introduction of the Fusarium wilt into uninfected areas in South Africa, are not effective. The sterilants Sporekill and Prazin proved to be highly effective, and are now recommended to replace the sterilants previously used. Several fungicides reduced mycelial growth of Foc in vitro, with the OMI fungicides and Benomyl found to be the most effective. The same fungicides reduced the disease severity of Fusarium wilt in the greenhouse significantly, especially when they were applied as root dip treatments. None of the fungicides found effective against Foc have been evaluated in the field against Foc before. The next step, therefore, would be to evaluate root dip treatments combined with drench treatment in the field. Although it is expected that these fungicides might have a negative effect on the microbial populations in the soil, this has yet to be investigated. Fungicides may even weaken or stress the pathogen, making it more vulnerable for the action of an effective biocontrol agent or agents. Chemical activators are probably one of the most attractive strategies to combat Fusarium wilt of banana, since it stimulate the plants' own defence system. Banana plantlets were found to be quite sensitive to the amount and method whereby chemical activators were applied. The activator benzo-(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester induced resistance against Foc on the susceptible Williams cultivar in the greenhouse, but not in the field. In field studies, environmental conditions were much more variable than in the greenhouse, which made it difficult to evaluate the effectiveness of chemical activators. Sodium nitroprusside and a product containing the harpin protein showed promising results on the Williams and DRSI cultivars, respectively. These activators need to be considered as part of an integrated disease management programme. Since they are not directly applied to the soil, they will not have a negative effect on the microbial populations in the soil. Several Fusarium isolates had been collected from banana fields with disease suppressive soils in Kiepersol, South Africa. Most of these isolates were F. oxysporum, and with the exception of one isolate, proved to be non-pathogenic to banana plants. A PCR-based restriction fragment length polymorphism (RFLP) analysis of the intergenic spacer region of the ribosomal RNA operon grouped the non-pathogenic F. oxysporum isolates into 12 distinct genotypes. A great diversity could be seen among the non-pathogenic isolates compared to the pathogenic Foc isolates. The known-biological control agent F047 grouped with three of the South African isolates, while the one pathogenic isolate grouped with the pathogenic Foc from diseased Cavendish bananas in South Africa By using PCR-RFLPs, we were able to rapidly characterize the structure of non-pathogenic isolates of F. oxysporum in disease suppressive soils in Kiepersol. This could assist us in our search for potential biological control agents for Fusarium wilt of banana. Representative isolates from the 12 genotype groups were selected for evaluation of Fusarium wilt suppressive properties in banana. These non-pathogenic F. oxysporum isolates appeared to be good biological control candidates and was compared to known biological control agents and commercial biological control products. Fourteen of the non-pathogenic isolates, the combination of two Trichoderma strains form suppressive soils in South Africa, and two Pseudomonas fluorescens isolates were found to significantly reduce Fusarium wilt development in the greenhouse. The commercial products Patostop, B-rus and a mixture of arbuscular mycorrhizae were also found to suppress the disease severity of Foc significantly. The well-know biological control agent F047 proved to be not effective. Results concluded that two of the non-pathogenic F. oxysporum isolates and the two P. fluorescens isolates, one of which was the well-known WCS 417, were the most effective of all the agents evaluated. Since combinations of biocontrol agents may provide even more consistent and effective control than a single agent, future research will include the combination of biocontrol agents found effective in this study. It would also be of great value to determine the mode of action of these isolates, so that isolates with different modes of action could be combined to enhance the suppression effect. Biological control can be a very useful component of an integrated disease management programme, since the effective agent or agents can easily be established on tissue culture banana plantlets before they are planted in the field. AFRIKAANS : Een van seker die mees belangrikste grondgedraagte siektes in lanbou, is Fusarium verwelksiekt van piesangs. In Suid-Afrika, is die siekte verantwoordelik vir emstige verliese in die piesang produksie. Aangesien daar geen weerstandbiedende kultivars beskikbaar is wat deur die mark aanvaar word nie, is dit van kardinale belang dat 'n geintegreerde siekte beheer program vir Suid-Afrika ontwikkel word. Voordat so 'n program saamgestel kan word, is dit belangrik dat verskeie faktore aangaande die patogeen en piesang poduksiepraktyke in ag geneem moet word. Beheermaatreëls moet prakties en bekostigbaar wees, en moet die reeds bestaande praktyke kan bevoordeel. Studies wat in hierdie tesis aangebied word, oorweeg beheermaatreëls wat gekombineer kan word met die huidige praktyke, nadat vorige praktyke ook in ag geneem is. Daar word gesoek na nuwe meer doeltreffende en ekonomiese metodes om siektes te beheer. Metodes wat doeltreffend aangewend kan word om die voorkoms van die siekte te vermirider. In vitro en in vivo studies het getoon dat die DMI swamdoders en Benomil die groei van die patogeen en die ontwikkeling van Fusarium verwelksiekte die meeste onderdruk. Die beste resultate is in die glashuis gevind nadat die wortels van plante in die middels geweek is. Positiewe resultate is ook verkry met die grondtoediening van Benomil 'n week nadat plante geplant is in Foc geïnfekteerde grond. Die chemiese beheer van Fusarium verwelsiekte kan verder ondersoek word deur die effek van die grondtoedienings en wortelbehandelings in die veld te ondersoek. Daar word egter verwag dat die swamdoders moontlik 'n negatiewe uitwerking op die mikrobiese aktiwiteit in die grond kan veroorsaak. Die gebruik van effektiewe ontsmettingmiddels is uiters belangrik vir die voorkomende beheer van Fusarium verwelkdiekte op piesangs. Die ontsmettingmiddel, koper oxichloried, wat tot onlangs in Suid Afrika gebruik was, is ondoeltreffend gevind vir ontsmettingsdoeleindes. Prazin en Sporekill, twee omgewingsvriendelike middels, is baie effektief gevind en word dus aanbeveel vir die ontsetting van voertuie, skoene en veld toerusing. Chemiese plant aktiveerders stimuleer plante om hulleself te beskerm deur middel van weerstandsmeganisms. Piesang plante het sensitiwiteit getoon toonoor die konsentrasie en die toedieningsmetode van hierdie chemiese aktiveerders. In die glashuisproewe het die aktiveerder benzo-(1,2,3)thiadiazole-7-carbothioic suur S-metiel ester weerstand gestimuleer in die Williams kultivar. As gevolg van veranderende toestande in die veld was dit moeiliker om die chemiese aktiveerders se werking te evalueer. Nogtans het die middels natrium nitroprussied en 'n produk wat die protein harpin bevat die voorkoms van siekte op die Williams en DRS 1 plante verlaag. Chemiese aktiveerders behoort sterk oorweeg te word as deel van 'n geintegreerde beheer program, aangesien chemiese aktiveerders nie direk tot die grond aangewend word nie, en geen negatiewe uitwerking op die natuurlike mikrobiese populasies in die grond uitoefen nie. Verskeie Fusarium isolate is geisoleer vanuit siekte onderdukkende gronde in die Kiepersol area van Suid-Afrika. Die meeste van die isolate is geidentifiseer as F. oxsysporum. 'n PKR-gebaseerde restriksie fragment lengte polimorfisme (RFLP) ontleding van die "intergenic spacer region" van die ribosomale DNS operon het die niepatogeniese F. oxysporum isolate in 12 verskillende genotypes opgedeel. 'n Groot diversiteit was sigbaar onder die nie-patogeniese isolate in vergelyking met die patogeniese foc isolate. Die bekende beheer agent, Fo47 het gegroepeer saam met drie van die Suid Afrikaanse nie-patogene. Hierdie tegniek het ons in staat gestel om die nie-patogeniese populasie van onderdrukkende gronde in Kiepersol vinnig te karakteriseer en potentiele biologiese agente te identifiseer. Verteenwoordigende isolate van die 12 genotipiese groepe wat geidnetifiseer is, is geselekteer vir verdere evaluasie. Dit is gevind dat die isolate goeie kandidate vir moontlike bio-beheer agente maak. Die onderdrukkingsvermoe van die nie-patogene is vergelyk met die van bekende bio-beheer agente en komersiele produkte wat beskikbaar is. Veertien van die nie-patogene, die kombinasie van twee Trichoderma spp., en twee Pseudomonas fluorescens isolate het die siekte ontwikkeling van Fusarium verwelking merkwaardig onderdruk in die glashuis. Die komersiele produkte Patostop®, B-rus en die kombinasie van twee mycorrhizae isolate is ook gevind om die voorkoms van siekte te verlaag. Die wel-bekende biobeheer agent Fo47 is oneffektief gevind teen Fusarium verwelksiekte van piesangs. Resultate van die studie het bewys dat twee van die nie-patogeniese F.Oxysporum isolate en twee P. Fluorescens isolate, waarvan een die welbekende WCS 417 is, uiters effektiewe beheer agente teen Foc is. Toekomstige studies sal fokus op die kombinasie van die bio-beheer agente wat die meeste potensiaal getoon het in die studie, asook hulle meganismes van werking. Biologiese beheer is van groot waarde vir 'n geïntegreerde beheer program. Dit kan maklik met bestaande beheer maatreëls gekombineer word en potensiële biologiese beheer agente kan vooraf op weefselkultuur plante in die kwekery gevestig word.