1.1.24
A LEAF DISK ASSAY TO ASSESS PARTIAL RESISTANCE OF BANANA GERMPLASM AND AGGRESSIVENESS OF MYCOSPHAERELLA FIJIENSIS, THE CAUSAL AGENT OF BLACK LEAF STREAK DISEASE

A EL HADRAMI, MF ZAPATER, F LAPEYRE, X MOURICHON and J CARLIER

Laboratoire PHYTROP, CIRAD-AMIS, BP-5035, Montpellier 34032, Cedex 1, France

Background and objectives
Black leaf steak disease (BLSD), caused by Mycosphaerella fijiensis (anamorph Paracercospora fijiensis) is the most destructive leaf disease of banana and plantains in the world [1]. Banana germplasm displays both resistant and susceptible interactions against this pathogen [2]. In the latter case, different levels of partial resistance between varieties are observed in the fields. As a high level of diversity is maintained in pathogen populations, breeding for partial resistance against M. fijiensis might be more durable than breeding for complete resistance. Thus it is first necessary to characterize partial resistance of parents and progeny, taking account of the pathogen's diversity. However, until now this characterization was carried out with difficulty and only in field trials to estimate all M. fijiensis life cycle parameters. A new, simplified pathogenicity test under controlled conditions has therefore been developed using inoculation of leaf disks.

Materials and methods
Diploid and triploid bananas derived from the wild species Musa acuminata (A genom) and Musa balbisiana (B genom) or from interspecific hybridization were cultivated in a greenhouse. The six bananas used in this work are cited below in an increasing order of partial resistance level evaluated in the field: Grande Naine (AAA, sub-group Cavendish), M. a. banksii Madang (AA), M. acuminata Pisang Berlin (AA), Saba (BBB), M. a. Zebrina (AA) and Fougamou (ABB, sub-group Pisang awak). Leaf disks of each accession are maintained on agar medium which contains benzimidazole. Conidial suspensions of 12 isolates from three banana-producing areas (Latin America, South-East Asia and Africa) were used for inoculation. Each inoculation was carried out on three leaf disks and the experiment was replicated four times.

Results and conclusions
Inoculation of leaf disks with M. fijiensis leads to similar infection type and symptom evolution to those obtained in field conditions. All the different stages of the M. fijiensis life cycle were reproduced under these conditions, including asexual and sexual reproductions. Since M. fijiensis is a heterothallic pathogen, sexual reproduction was obtained by co-inoculation of compatible isolates. This result is important since ascospores produced during sexual reproduction play a major epidemiological role.

Significant differences in infection efficiency (IE) and incubation time (IT50, defined as the time when 50% of the maximum lesion number became visible) among the six banana accessions and the 12 M. fijiensis isolates were detected. Bananas were classified with respect to their level of partial resistance in the same order as observed in the field. Thus the pathogenic test on leaf disks appears consistent with field observations. Estimation of these parameters allowed us also to detect significant groups of M. fijiensis with different levels of aggressiveness. The 12 isolates were ranked in a similar way among the six banana accessions. No specific isolate-accession interactions were detected with respect to IE and IT50 parameters. Other life-cycle parameters such as lesion extension speed, sporulation capacity, latencies and infectious periods for asexual and sexual reproduction cycles are being studied. The pathogenicity test developed in this work could be applied to large samples of accessions and isolates. Such studies will give more information on variation in aggressiveness and on the importance of each pathogen life-cycle parameter in partial resistance. Finally, this method could be used to detect quantitative trait loci (QTL) involved in interaction at pathogen life stages which play major epidemiological roles.

References
1. Mourichon X, Fullerton RA , 1990. Fruits 45, 213-218.
2. Fouré E, 1982. Fruits 37, 749-771.