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Study of inheritance and identification of QTLs linked to Ceratocystis wilt resistance in cacao

Ferreira Santos R.M., Lopes U.V., Vieira Midlej Silva S.D., Micheli F., Clément D., Peres Gramacho K.. 2012. In : COPAL. 17th Conférence Internationale sur la Recherche Cacaoyère, Yaounde, Cameroun, 15 - 20 Octobre 2012. s.l. : s.n., 4 p.. Conférence Internationale sur la Recherche Cacaoyère. 17, 2012-10-15/2012-10-20, Yaoundé (Cameroun).

Ceratocystis wilt (CW) in cacao, caused by Ceratocystis cacaofunesta, is a drastic disease that results in plant death. The pathogen was recently identified in the major cacao-producing region of Brazil-Bahia. There are still many unanswered questions regarding the sources and mode of inheritance to CW. Phenotypic evaluation of CW resistance has been based on qualitative measures of the disease, therefore hindering the determination of the type of inheritance involved in this pathosystem. The identification of genetic markers tightly-linked to disease resistance loci are valuable tools for the development of resistant cultivars using marker-assisted selection (MAS). Branches of 143 six year old progenies of the F2 Sca 6 x ICS 1 population were wounded by making a 3 mm deep cut with a sterile scalpel, and inoculated with a 20 ul drop of a spore suspension of 3 x 104 CFU/ml. Fifteen days after the inoculation (DAI) stems were collected and transported to the laboratory for evaluation. Stems were sliced open vertically above and below the point of inoculation and the length of the xylem discoloration (LXD) recorded. Associations between molecular markers and resistance to CW were evaluated by a simple interval mapping (SIM) and composite interval mapping (CIM) using the software MapQTL v. 5.0. Mean parental disease scores were 12.14 ± 3.5 cm for Sca 6 and 29.57 ± 6.7 cm for ICS 1. The mean disease score for the entire population was 14.22 ± 3.5 cm. Individual progenies varied from 4.28 to 33.75 cm. The length of the LXD followed a continuous distribution. The inoculation method used allowed to quantitatively phenotype the population. QTL analysis using the SIM and CIM revealed two genomic regions located in linkage groups 3 (LXD-LG3) and 9 (LXD-LG9) associated with the expression of the CW resistance with a LODmax of 2.57 and 3.1, respectively. The QTLs explained individually from 6.9 to 8.6% of the phenotypic variation. The identification of two QTLs involved in resistance to CW offers the possibility to improve the durability of resistance in cocoa by a possible accumulation of many different resistance genes located in different chromosome regions using marker-aided selection. The marker alleles used for the introgression survey on MAS can be also used for characterization of unrelated germplasm and finding new sources of resistance..

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