Fact sheet on Fusarium wilt of banana: varietal susceptibility, symptoms, pathogenesis, control

Bentley, S.

International Network for the Improvement of Banana and Plantain

Jones, D.R.

Moore, N.Y.

Pegg, K.G.

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Fusarium wilt or Panama disease of banana is widely regarded asone of the most destructive plant diseases in recorded history. Itis caused by the soil-inhabiting fungus Fusarium oxysporumSchlecht f. sp. cubense (E. F. Smith) Snyd. & Hans (Foc). Thedisease was first recognized in Australia in 1874. Fusarium wilthas now been reported from all banana growing regions of theworld except Papua New Guinea, the South Pacific Islands andsome of the countries bordering the Mediterranean.Fusarium wilt is a serious problem on many banana cultivarsgrown by smallholders for local consumption. These include La-tundan (Philippines), Maçã (Brazil), Pisang Rastali (Malaysia),Rasthali (India) which belong to the AAB ‘Silk’ subgroup; LadyFinger (Australia), Prata (Brazil), Virupakshi (India) which belongto the AAB ‘Pome’ subgroup and Chuoi Tay (Vietnam), Kayinja(East Africa), Kluai Namwa (Thailand) which belong to the ABB‘Pisang Awak’ subgroup. Other locally important cultivars, such asthose in the East African Highland subgroup (AAA ‘Muti-ka/Lujugira’) and Pisang Mas subgroup (AA ‘Sucrier’) have alsobeen reported as susceptible in some environments. If thedisease were to spread to the South Pa-cific, cultivars in the popular Pacific cook-ing banana subgroup (AAB ‘Maia Maoli/Popoulou’) would also bevulnerable as representatives have shown susceptibility in fieldtests. In addition, widely grown clones in the ABB ‘Bluggoe’ andAAA ‘Gros Michel’ subgroups are also susceptible.Gros Michel was the basis of the early export banana trade in theLatin America/Caribbean region and it was the progressive de-cline of plantations of this cultivar due to Fusarium wilt in the 1940-50s that led to the adoption of cultivars in the AAA ‘Cavendish’subgroup as the main export banana types. Cavendish cultivarshave remained the mainstay of the world’s export industries todate. These types are also highly favoured for local consumptionin countries such as Australia, China, Vietnam, India, Pakistan,Egypt and South Africa. Unfortunately, plantations of Cavendishcultivars in subtropical countries such as Taiwan, Spain (CanaryIslands), Australia and South Africa are being increasingly attack-ed by Fusarium wilt. It is thought that plants in these areas arepredisposed to systemic infection by certain strains of Foc by coldstress during winter. However, recent losses of Cavendish grow-ing in export plantations in Malaysia, Sumatra and Java make itclear that other strains are quite capable ofsystemically infecting cultivars such asMusa Disease Fact Sheet N¡ 5FUSARIUM WILT OF BANANAN. Y. Moore, S. Bentley, K. G. Pegg, and D. R. Jones (June, 1995)International Network for the Improvement of Banana and Plantain, Parc Scientifique Agropolis, 34397 Montpellier, Cedex 5, FRANCE - Tel: (33) 67 61 13 02, Telex: 490376F, Fax: (33) 67 61 03 34A Lad y Fing er (AAB ‘P ome’) plantation in A ustralia almost completel y destr oyed  by Fusarium wilt.Valery, Grande Naine and Williams under tropical conditions. Al-though Cavendish growing in the Latin America/Caribbean regionhave not been reported as succumbing in large numbers, theworld trade is again threatened by Fusarium wilt.The classical and conspicuous external symptom of Fusarium wilt ofbanana initially appears as yellowing of the leaf margins of olderleaves (this symptom may initially be confused with potassium defi-ciency, especially under dry or cold conditions). Yellowing progres-ses from the oldest to the youngest leaves. Leaves gradually col-lapse at the petiole or, more commonly, towards the base of themidrib and hang down to form a “skirt” of dead leaves around thepseudostem. In some cultivars, leaves of affected plants remainpredominantly green until the petioles buckle and leaves collapse.The youngest leaves are the last to show symptoms and often standunusually erect giving the plant a “spiky” appearance. Growth doesnot cease in an infected plant and leaves which emerge are usuallypaler in appearance than those of the healthy plant. The lamina ofemerging leaves may be markedly reduced and exhibit wrinklingand distortion. Longitudinal splits may also develop in thepseudostem. A susceptible banana plant infected with Foc rarelyrecovers. However, poor growth of the clump may continue for sometime and many infected suckers may be produced before the clumpfinally dies. No disease symptoms have been observed in fruit.Infection occurs when the pathogen penetrates the roots of thebanana plant. The fungus then invades the xylem vessels and, ifnot blocked by vascular occluding responses of the host, ad-vances into the corm. Internal symptoms are characterized byvascular discoloration beginning with yellowing of the vasculartissues in the roots and corm which progress to form the conti-nuous yellow, red or brown discolored vascular stands in thepseudostem and sometimes the bunch stalk.As the plant dies, the fungus grows out of the xylem into the surround-ing tissues, forming many chlamydospores which are returned to thesoil when the plant decays. Foc is also able to colonize and persist inthe roots of alternative hosts, including close relatives of the bananaand several species of weeds and grasses, even though these plantsremain symptomless under field conditions. The fungus can survivein soil for up to 30 years as chlamydospores in infested plant debrisor in the roots of alternative hosts. Spread of the pathogen locally,nationally and internationally is most commonly by infected rhizomesor suckers and attached soil. Infected planting material may notexhibit symptoms. Foc can also be effectively spread in soil attachedto implements or vehicles. From an isolated point of introduction in adisease-free plantation, Foc will spread slowly from plant to plant. If,however, spores are carried in surface run-off water or contaminatean irrigation reservoir, the disease can spread very rapidly,decimating a plantation within months if conditions are favorable.Several factors influence the development of this disease. Thebanana cultivar is of primary importance and other factors such asdrainage, environmental conditions and soil type also influencedisease development. Suppressive soils, in which microbial pop-ulations suppress the pathogen population, were first described inSection thr ough banana pseudostem sho wing discolored v ascular tissuecaused b y Foc.Pisang A wak (ABB) with f oliar symptoms of Fusarium wilt.the 1930s in Central America. Such soils have also been recordedin the Canary Islands, Australia and South Africa.Research is currently underway to determine the pathogenic andgenetic variation within Foc and the geographic distribution of vari-ants. Before resistant cultivars can be deployed to combat thisdisease, the pathogenic and genetic diversity within and amongpopulations of Foc needs to be assessed at local, national andinternational levels. Classical genetic techniques cannot be used tostudy the genetic diversity of this pathogen since no sexual stagehas been found for Foc. Several analytical techniques have beenapplied to differentiate isolates of Foc. These include vegetativecompatibility, volatile production, pectic enzyme and DNA analyses.Four races of Foc are currently recognized. The term race is usedless formally for this pathosystem since the genetic basis for sus-ceptibility and resistance has not been characterized. Thecurrently described races of Foc refer to strains of the pathogenwhich have been observed to be pathogenic to particular hostcultivars in the field. Race 1 is pathogenic to cultivars in the AAB‘Silk’ and ‘Pome’ subgroups and on AAA ‘Gros Michel’. Race 2 ispathogenic to ABB ‘Bluggoe’ and other closely related cookingbananas. Race 3 has been recorded in Honduras, Costa Rica andAustralia on Heliconia species and has little to no effect on bana-na. Race 4 attacks AAA ‘Cavendish’ and all cultivars attacked byraces 1 and 2. The current set of host differentials does not ade-quately assess the virulence which exists among populations ofthe currently designated races. It is not always possible to char-acterize populations of Foc using pathogenicity tests due to plant-environment interactions.Vegetative compatibility is a technique based on asexualmechanisms which occur naturally in Foc that can be used in thelaboratory to identify genetically isolated groups known as veg-etative compatibility groups (VCGs). Strains of Foc which belong tothe same VCG are genetically identical or very similar with respectto other characteristics and are, therefore, more closely related toeach other than vegetatively incompatible strains. Vegetative com-patibility analysis can be used to assess the diversity of strains ofFoc within a given region and is much faster and more reliabletechnique than pathogenicity testing. Studies have shown a strongcorrelation between VCG and pathogenicity in Australia, but, sincewith Foc more than one race can occur within a VCG and there canbe more than one VCG in each race, it is often difficult to use VCGfor pathotype discriminations. Currently, twenty-one VCGs havebeen described for Foc and a pattern of distribution of these VCGsthroughout the world is emerging. Recent studies show that fifteenof the twenty-one described VCGs have been recorded in popula-tions of Foc in Asia, where Musa is indigenous.As only asexual reproduction is involved in the population biology ofthis pathogen, genetic diversity can be measured by genotypediversity. This can be assessed by methods such as arbitrarilyprimed PCR (e.g. RAPD and DAF techniques) to generate DNAfingerprints. Using DNA fingerprinting, it is possible to determine thegenetic relationships between different VCGs as well as isolatesFoc isolates f ound in A ustralia, Indonesia, Mala ysia, Philippines, T aiwan, SouthAfrica and the Canar y Islands are capab le of attac king Ca vendish (AAA) c lones.Silk (AAB), a popular cultiv ar in man y Asian countries, is especiall y susceptib leto Fusarium wilt.within each VCG. Molecular and genetic characterization which hasbeen carried out to date places isolates of Foc into two distinctgroups suggesting a biphyletic origin for this pathogen. Thesestudies suggest that Foc and Musa coevolved in Asia to generategenetically diverse strains of the pathogen. However, additionalstudies are required to confirm the coevolution hypothesis.Chemical control, flood fallowing, crop rotation and the use oforganic amendments have not been effective in managing Fusa-rium wilt. It is now generally accepted that the only effectivemeans of control is by host resistance. Natural sources ofresistance exist in wild species and cultivars and in syntheticdiploids developed by breeding programs. There are now fourmajor conventional banana breeding programs and these arelocated in Honduras (FHIA), Brazil (EMBRAPA-CNPMF), Nigeria(IITA) and Guadeloupe (CIRAD-FLHOR). These programs haveconcentrated on using resistance in Pisang Jari Buaya, PisangLilin and Musa acuminata ssp. burmannicoides (Calcutta 4).Although it has not yet been possible to breed a Cavendishreplacement because of fertility constraints, useful hybrids can bebred to replace AAB dessert and ABB cooking banana types.FHIA-01 (Goldfinger), a dessert banana from the FHIA breedingprogram with an acidic or “apple” flavor, has been identified ashaving resistance to race 1 and race 4 populations of Foc inAustralia. Biotechnology, mutation breeding and somaclonal varia-tions are also being used to produce resistant genotypes. Hybridsfrom breeding programs are to be tested in the field at selectedsites around the world in the International Musa Testing Program(IMTP) organized by INIBAP with funds from UNDP.Quarantine and exclusion procedures are effective in controllingthe disease by restricting the movement of corms, suckers andsoil that could be carrying Foc from infested to clean areas. Theuse of micropropagated planting material should be encouragedas this, if managed correctly, should be free from Foc. It isimperative that Cavendish attacking Foc populations recentlyrecognized in Southeast Asia do not spread to other geographiclocations, especially Latin America.A method needs to be developed by which host-pathogen res-ponses can be quickly and reliably ascertained in a growth cabinetor glasshouse environment. Pathogenic variability among gene-tically characterized isolates could then be determined. Epidemio-logical studies are also required to provide information on how differ-ent host genotypes and pathogen genotypes interact in the field indifferent geographical locations. A PCR-based detection system toidentify pathotypes of Foc would be useful for screening plantingmaterial and identifying isolates in infested soil and diseased plants.Continuing analysis of populations of Foc, particularly from Asiawhere maximum variability of host and pathogen are to be expec-ted, will enhance our understanding of the genetic diversity of Foc.Your help is needed . INIBAP requests that specimens of vasculartissue from diseased plants be collected as is indicated in thediagram above. They should then be sent by airmail to Dr NatalieMoore (Plant Protection Unit, Department of Primary Industries, 80Meiers Road, Indooroopilly, Q 4068, Australia) for analysis.Collaborators will be informed of results which will help nationalplant protection programs identify existing and potential problems.The Department of Primary Industries (Dr N. Moore and Mr K.Pegg) and the Cooperative Research Centre for Tropical PlantPathology (Dr S. Bentley) are two of INIBAP’s key partners inQueensland, Australia. INIBAP has supported research inQueensland on the genetic analysis of Foc isolates from Asia inassociation with ACIAR.Cut out sections of tissue from low in pseudostem where many discoloredstrands are present. Dissect out individual discolored vascular strands andplace between sterile blotting paper to air dry.When dry, place the blotting paper andstrands in a paper envelope, seal andlabel with:• sample number• cultivar name (including local names)• location• collector’s name• dateIn addition tovascular strands,small cubes (2-3cm 2) ofdried, discolored, corm tissuemay be included in a separate enve-lope. This material would act as an addi-tional source of diseased vascular tissue shouldisolations from vascular strands prove unsuccessful.

Fusarium wilt of banana

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Fusarium wilt of banana

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