Abstract:
The productivity of cocoa (Theobroma cacao L.), an important economic tree in Nigeria, is limited by Phytophthora Pod Rot (PPR). Efficient management of PPR requires identification and characterisation of the Causal Pathogen (CP). The use of Host Plant Resistant Varieties (HPRV) was successful in the management of fungal diseases of some crops with minimal environmental effect. However, there is dearth of information on the management of PPR with HPRV. Therefore, morphological and molecular characterisation of CP and management of PPR with HPRV were investigated in southwestern Nigeria.
Five States: Ekiti, Ogun, Ondo, Osun and Oyo which are the major cocoa growing areas in southwestern Nigeria were purposively selected. Nine cocoa plantations were randomly selected and five PPR-infected pod samples collected per plantation in each of the states. Fungal isolates were obtained from cocoa pods and pure cultures assessed for pathogenicity following standard procedures. Morphological characteristics of 45 isolates (nine/state) were determined using Mycelial Growth (MG), Colony Pattern (ClP), Sporangial Shape (SS), Sporangial Pedicel Length-SPL to Sporangial Breadth-SB (SPL:SB) were used for CP identification. Deoxyribonucleic acid was extracted from 10-day old V8 cultures of the 45 isolates, followed by Polymerase Chain Reaction (PCR) amplification of the Internal Transcribed Spacer (ITS) gene following standard procedures. Amplicons were sequenced to show similarities among PPR isolates and compared with CP genomes within the Phytophthora data base. In the laboratory, three-month old pods and two-month old seedlings’ Leaf Discs-LD (1.5 cm) of each of the six cocoa varieties (TC-1, TC-3, TC-4, TC-5, TC-6, TC-7) with different levels of resistance and N38 (susceptible control) were inoculated with pure cultures of CP 2x105 spore/mL at 10µL/pod and 10µL/LD in a completely randomised design replicated thrice to screen for resistance. Seven days after inoculation, Infection Growth (IG) on pods were measured, while a rating scale of 0 (highly resistant) to 5 (highly susceptible) was used to assess IG on LD. Data were analysed using descriptive statistics and ANOVA at α0.05.
A total of 135 fungal isolates were identified in all the states and 45 were pathogenic. The MG of all isolates was cottony with slightly papillate ClP. The SPL ranged from 29.9±7.9 to 38.8±8.7, while SB ranged from 24.0±5.4 to 30.8±5.4. The SPL:SB was 1.2:1.4 and the SS was ellipsoid in all isolates. The PCR amplification and sequencing of the CP characterised cacao PPR as Phytophthora megakarya. The pathogen yielded an estimated 550-bp ITS product with ≥99.9% isolate similarities. The gene accessions of Phytophthora megakarya isolates showed evolutionary relationship to strains in Cameroon and Ghana. The IG on pods was least on TC-4 (±9.3mm) and was similar to TC-6 (±23.3mm), TC-3 (±27.8mm), but was significantly lower than TC-5 (±52.5mm), TC-1 (±64.0mm), TC-7 (±68.1mm) and N38 (±73.1mm), Leaf discs of TC-4 had the least IG of (1.2) and were similar to TC-3 (1.5), TC-6 (1.8), TC-5 (2.6) and TC-1 (2.8) but significantly lower than TC-7 (3.4) and N38 (4.2).
Phytophthora megakarya predominantly caused Phytophthora pod rot on cocoa in the study site. The variety TC-4 was resistant to Phytophthora pod rot of cocoa.
