1. Department of Mathematics and Computer Science; The University of Douala; Douala Cameroon
2. UMI 209 IRD/UPMC UMMISCO; University of Yaounde I; Cameroon
3. EPITAG; LIRIMA; France
4. CIRAD; UPR Bioagresseurs; 30501 Turrialba Costa Rica
5. Bioagresseurs; Univ Montpellier, CIRAD; Montpellier France
6. IRAD; BP 2123 Yaoundé Cameroon
7. CIRAD, Umr AMAP; Pretoria South Africa
8. AMAP; Univ Montpellier, CIRAD, CNRS, INRA, IRD; Montpellier France
9. University of Pretoria; Department of Mathematics and Applied Mathematics; Pretoria South Africa