Molecular Identification of Phytophthora palmivora in the Cocoa Tree Orchard of Côte d’Ivoire and Assessment of the Quantitative Component of Pathogenicity

Abstract

Black pod rot is one of the most widespread diseases in the species Theobroma cacao L. In Côte d'Ivoire, it causes significant yield losses, estimated at about 10 to 60% depending on the ecological zones. This study was conducted in order to identify Phytophthora spp isolates in the cocoa orchard by the analysis of their molecular features and assess their pathogenicity on cocoa tree leaf disks having different sensitivity. Thirty-six isolates of Phytophthora spp., from 06 producing regions (Divo, Soubré, Daloa, Duékoué, Abengourou and Aboisso) were taken from immature pods naturally affected by black pod rot. Methods of molecular analysis based on PCR-RFLP techniques, sequencing the ITS1 region of ribosomal DNA and artificial inoculation tests on cocoa tree leaf disks were used to identify Phytophthora isolates and assess their aggressiveness. The results obtained enabled to identify Phytophthora palmivora and showed a difference of aggressiveness between isolates of this species with average grades of foliar sensitivity ranging from 0.90 to 3.93.

Comparative efficacy of essential oils of three aromatic plants as alternatives in the control of Phytophthora spp., agent of cocoa tree (Theobroma cacao L.) black pod disease in Côte d'Ivoire

“…However, the minimum concentration of these three essential oils that completely inhibited mycelial growth of the P. megakarya strain was 0.5 mL/L. This difference in susceptibility of the two Phytophthora strains could be explained by the fact that the Phytophthora megakarya strain is slower growing in vitro compared to the Phytophthora palmivora strain (Mpika et al, 2009;Coulibaly et al, 2018;Ouattara et al, 2020). Thus, with the Phytophthora megakarya strain, the oils have time to act on the mycelial walls before the expiration of the strain's latency time, which varies from 48 to 72 hours at 26 °C in darkness.…”

“…Thus, with the Phytophthora megakarya strain, the oils have time to act on the mycelial walls before the expiration of the strain's latency time, which varies from 48 to 72 hours at 26 °C in darkness. This is not the case for Phytophthora palmivora strain which has rather a short latency time (24 to 48 hours 26 °C in darkness); (Mpika et al, 2009;Coulibaly et al, 2018;Ouattara et al, 2020). Similar results were obtained by Soro et al (2020), who observed in vitro a total inhibition of Phytophthora palmivora strain with a concentration (CI100) of 200 ppm with Ocimum gratissimum species essential oil.…”

“…Despite this remarkable performance, cocoa farming remains subject to numerous production constraints (Assiri et al, 2012;Ploetz, 2016). Among these constraints, black pod disease is currently one of the main sources of productivity losses in Ivorian cocoa farms (Coulibaly et al, 2018). It has been on the rise in recent years in Côte d'Ivoire, mainly in the South-East, Centre-West and West zones with estimated losses of 35 to 50% (Coulibaly, 2014;Coulibaly et al, 2018).…”