Effet de deux isolats de phytophthora megakarya sur quelques clones parentaux et hybrides de cacaoyer (theobroma cacao l.) du groupe trinitario

Abstract

Phytophthora megarkarya is a Oomycete responsible about 80% of production losses cocoa in Cameroon. Thus, the study of the behavior two isolates of P. megakarya (strains NPV + NGF) at monoinfection and coinfection were performed on leaves of four clones generate by graft and two hybrid’s family (F20 et F25) of Theobroma cacao group Trinitario. The vegetal material is made up of the new leaves of four of Theobroma cacao group Trinitario generate by graft and two hybrid group. The fungi material is constitute by zoosporale solution of two strains of Phytophthora megarkarya at 106 zoospores/ml. after three, five and seven days inoculation, severity index are take. The results of this study have shown that the best success rate of grafting side slot was observed in the SNK clone 413 (85%) compared to the other clones. Inoculation of leaves by zoosporale suspension NPV different strains and co-infection NGF revealed a high virulence of the strain in NGF all clones studied compared to other strains. However, coinfection was more virulent in most hybrids on day 7 with the exception of F20.01 and F20.03 hybrids. At the seven day, the evaluation of the tolerance index showed values of 2.00 ± 0.20, 2.16 ± 0.36; 2.33 ± 0.26; 2.50 ± 0.30 respectively for SNK 413 clones; ICS 40, 16 and SNK 13 for three treatments. SNK 413 clone is the most tolerant clone. Moreover, the hybrid F25.03, F20.02 and F20.01 who presented a positive heterosis can be considered as performing genotypes manifesting all increased hybrid vigor. The heritability of reciprocal crosses necrosis surface character (h² = 0.780 to F20 and F25 to h² = 0.643) showed that the transmission of this character was nuclear but not cytoplasmic.