Effet de deux isolats de Phytophthora megakarya sur quelques clones parentaux et hybrides de cacaoyer (Theobroma cacao L.) du groupe forastero

Abstract

The black pod disease caused by Phytophthora megakarya is the most common and most important disease in Cameroon's cocoa production. She is responsible for the decrease of production yield to about 80%. To overcome this problem the study was performed to determine the behavior of two isolates of P. megakarya together with a co-infection (two isolates) on clones and hybrids of T. cacao Forastero group. Semi-aoutes leaves of four clones (SCA 12; UPA 134; T79 / 501; T79 / 467) reconstituted by grafting T. cocoa Forastero group and two hybrid families were inoculated with a zoosporale solution of two isolates of P. megakarya (NPV and NGF) calibrated. 106 zoospores / ml. Then the index of severity was measured three, five and seven days after inoculation. The results showed that clones T 79/467 and UPA 134 had the best percentage of success (85%) after performing grafting side slit. Isolates of the three that were studied; only the strain NPV + NGF (coinfection) showed a high degree of virulence (75%) on the leaves of Theobroma cacao L. On the other hand, the NGF strain was more virulent on hybrid genotypes with 55.55 % contrary to coinfection (11.11%). However, SCA 12 clone proved the more tolerant compared to strains used. Similarly, hybrid F19.04, F23.03, F19.03 and F23.01 have been very tolerant. The calculation of heritability (h²) in the strict sense of the nature area of necrosis crosses gave for F19 and F23 families the values 0.613 and 0.722 respectively. All these results reflect a better performance of clones T 79/467 UPA and 134 for vegetative multiplication. In general the SCA 12 'clone was more tolerant P. megakarya.