PCR-based identification of cacao black pod causal agents and identification of biological factors possibly contributing to Phytophthora megakarya’s field dominance in West Africa
Plant Pathology, 65 (7) : p. 1095-1108
Among the Phytophthora species that cause black pod of cacao, P. megakarya is the most virulent, posing a serious threat to cacao production in Africa. Correct identification of the species causing the black pod and understanding the virulence factors involved are important for developing sustainable disease management strategies. A simple PCR-based species identification method was developed based on the species-specific sequences in the ITS regions of the rRNA gene. A phylogenetic tree generated for 119 Phytophthora isolates based on the 60S ribosomal protein L10 gene and rDNA sequence verified the PCR-based identification assay and showed high interspecific variation among the species causing black pod. P. megakarya isolates were uniformly virulent in a zoospore inoculated pod husk assay using susceptible cacao pods, while the P. palmivora isolates showed greater divergence in virulence. The virulence of P. megakarya was associated with earlier sporangia production along with an accelerated induction of necrosis. While zoospore germ-tubes of both species penetrated pods through stomata, only P. megakarya produced significant numbers of appressorium. A hypersensitive-like response was observed when attached SCA-6 pods were inoculated with P. palmivora. SCA-6 pods became vulnerable to P. palmivora when wounded prior to zoospore inoculation. P. megakarya was more aggressive on attached SCA-6 pods causing expanding necrotic lesions with or without wounding. P. megakarya is predominant in the Volta region of Ghana and it remains to be seen if it can displace P. palmivora from cacao plantations of Ghana as it has done in Nigeria and Cameroon.