3.4.37
GENETIC DETERMINISM AND INHERITANCE OF RESISTANCES AGAINST BLACK LEAF STREAK ( MYCOSPHAERELLA FIJIENSIS) IN BANANAS

C ABADIE1, F CARREEL2, K TOMEKPE1, J CARLIER3, E FOURE3, JV ESCALANT1 and F BAKRY3

1Centre Regional Bananiers et Plantains, BP 832, Douala, Cameroon; 2CIRAD-FLHOR, Stabon of Neufchateau, 97 130 Capesterre Belle-Eau, Guadeloupe; 3CIRAD-FLHOR, BP 5035, 34032 Montpellier, France

Background and objectives
Banana and plantain production is the fourth most important global food crop. Black leaf streak (BLS), caused by the fungus Mycosphaerella fijiensis, is one of the most important diseases of banana and plantain in the world. Symptoms of foliar necrotic lesions destroy large areas of leaf tissue which result in reduced yields and premature ripening of fruits. Although chemical control of BLS is effective, genetic improvement for resistance to BLS appears as the most durable control and the most appropriate to plantain cultivated by smallholders. This study aims to understand the genetic determinism of two forms of resistance against BLS reported as high resistance (HR, hypersensitive response) and partial resistance (PR) [1], and to map the quantitative trait loci (QTL) coding for resistance to BLS. The results of these studies are expected to be used by breeders to assist banana selecton.

Materials and methods
Crosses were conducted between three diploid and fertile Musa acuminata clones displaying susceptible reaction (M. acuminata banksii type Madang), partial resistance (M. acuminata zebdna type Zebdna) and high resistance (M. acuminata burmannica type Calcutta 4) against BLS. The F1 and F2 progenies derived from the three crosses HR x S (CAM), PR x S (ZEMA) and PR x HR (CAZEB) were established in Cameroon. These populations were set up in two different locations to get through the environment effect on resistance phenotypes. The disease incidence and molecular characterization were undertaken on these populations. Two disease evaluation methods were used : in vitro using natural inoculated leaves grown in indoor survival conditions, and field evaluations for resistance using the youngest leaf spotted (YLS) method and symptoms evaluation time. The population is also being characterised with mapped molecular markers [2].

Results and preliminary conclusions
All the 35 F1 individuals derived from the CAM cross expressed a HR type of resistance. This suggests that the HR form of resistance expressed by Calcutta 4 is dominant over the susceptible form expressed by Madang. Although the HR parent is generally considered by the breeders as an homozygous clone, its heterozygosity rate was estimated as 26% with molecular markers. The homogeneous behavior of all the CAM F1 confirms that the HR parent Calcutta 4 is homozygous for the genes involved in BLS resistance. The selfing of one F1 CAM plant produced 154 F2 individuals which were characterized. The F2 population segregated and individuals with S, PR and HR phenotypes were observed with the most important proportion for the PR phenotype. DNAs of all F2 individuals were quantified and characterized with molecular markers such as sequence tagged microsatellite site.

Concerning the ZEMA and CAZEB populations, observations of the F1 individuals are in progress. Preliminary results suggest that the PR phenotype is dominant over the HR and S phenotypes. The F2 population will allow the genetic determinants of PR resistance to be characterized. All these results and the correlations between evaluations of BLS resistance and molecular characterization are discussed.

References
1. Foure E, 1993. Breeding bananas and plantains for resistance to diseases and pests, pp. 159-170.
2. Faure S et al., 1993. Theoretical Applied Genetics 87, 517-526.