EFFICIENT SCREENING PROCEDURE FOR BLACK SIGATOKA DISEASE OF BANANA

Current methods for screening for reaction of Mycosphaerella fijiensis are mostly field based and subject to variability caused by weather conditions. The objective of this study was to develop an efficient screening procedure for black sigatoka disease in order to provide a reliable controlled environment screening method. A fragmented mycelia-based infection system was developed using weighed mycelia suspensions, in 0.18% water agar. Consistent and repeatable black sigatoka infection was achieved in cv. Gross Michel as a reference susceptible cultivar. The infection system developed was used to validate screenhouse reaction of banana genotypes to M. fijiensis. Three East African highland genotypes (AAA-EA), one plantain genotype (ABB), two dessert (AAB and AAA) and one hybrid genotype (AAAA) were tested. Inoculum of M. fijiensis was adjusted to 15 mg ml-1 fragmented mycelium and applied to the abaxial surfaces of the first two open leaves. Plants were evaluated for 6 weeks post inoculation (PI). Significant differences in the response of the different genotypes, to M. fijiensis infection were observed at 3, 4, 5 and 6 weeks PI. All East African highland and dessert genotypes were susceptible. M9 hybrid and Psang awak (ABB) expressed resistance, characterised by high disease incubation period and slow rate of symptom development, with Psanag awak being the most resistant. Artificial inoculation of in vitro plants with weighed fragmented mycelial suspension, was practicable for determining resistance to M. fijiensis. This is the first report demonstrating that use of weighed fragmented mycelial inoculum spread over the entire leaf surface, can be used to quantitatively assess reaction of banana genotypes to M. fijiensis under screenhouse conditions.Les m\ue9thodes actuelles de d\ue9pistage de Mycosphaerella fijiensis sont essentiellement bas\ue9es sur des \ue9valuations dans le champ et sont souvent sujets \ue0 des variabilit\ue9s climatiques. L\u2019objectif de cette \ue9tude \ue9tait de d\ue9velopper une proc\ue9dure efficace de d\ue9pistage de la maladie foliaire du bananier Sigatoka dans le but de proposer une m\ue9thode de d\ue9pistage non influenc\ue9e par l\u2019environnement. Un syst\ue8me d\u2019infection a \ue9t\ue9 d\ue9velopp\ue9 en se servant de myc\ue9lium en suspension dans 0,18% de solution agar. Des infections consistantes et r\ue9p\ue9t\ue9es ont \ue9t\ue9 r\ue9alis\ue9es in cv. La vari\ue9t\ue9 Gross Michel a \ue9t\ue9 utilis\ue9e comme r\ue9f\ue9rence susceptible. Le syst\ue8me d\u2019infection d\ue9velopp\ue9 a \ue9t\ue9 utilis\ue9 pour valider la r\ue9action des accessions de bananiers en serre \ue0 M. fijiensis. Trois g\ue9notypes de bananiers adapt\ue9s aux hautes altitudes en Afrique de l\u2019Est (AAA-EA), un g\ue9notype de type plantain (ABB), deux d\ue9serts (AAB and AAA) et un hybride (AAAA) ont \ue9t\ue9 test\ue9s. Des inocula de M. fijiensis ont \ue9t\ue9 doses \ue0 15 mg ml-1 et appliqu\ue9s sur la face dorsale des deux premi\ue8res feuilles ouvertes. Les plants ont \ue9t\ue9 \ue9valu\ue9s pendant 6 semaines apr\ue8s inoculation (PI). Des diff\ue9rences significatives ont \ue9t\ue9 observ\ue9es dans les r\ue9ponses des g\ue9notypes \ue0 l\u2019infection \ue0 diff\ue9rents stages apr\ue8s inoculation (3, 4,5 et 6 semaines) de M. fijiensis. Tous les g\ue9notypes de haute altitude et les d\ue9serts ont \ue9t\ue9 r\ue9v\ue9l\ue9s susceptibles. L\u2019hybride M9 et Psang awak (ABB) se sont montr\ue9s r\ue9sistants, ils sont caract\ue9ris\ue9s par une longue p\ue9riode d\u2019incubation et un taux bas de d\ue9veloppement de sympt\uf4mes, Psanag awak s\u2019est r\ue9v\ue9l\ue9 le plus r\ue9sistant. L\u2019inoculation artificielle in vitro des plants avec des suspensions de fragments myc\ue9liens, a \ue9t\ue9 pratique dans la d\ue9termination de la r\ue9sistance \ue0 M. fijiensis. Ceci est le premier r\ue9sultat d\u2019\ue9tude scientifique ayant d\ue9montr\ue9 que cette technique peut \ueatre utilis\ue9e pour l\u2019\ue9valuation de la r\ue9action des g\ue9notypes de bananiers \ue0 l\u2019infection de M. fijiensis dans des conditions d\u2019exp\ue9riences en serre.