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Title: 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

item ALI, SHAHIN - Non ARS Employee
item AMOAKO-ATTAH, ISHMAEL - Cocoa Research Institute Of Ghana
item BAILEY, REBECCA - Non ARS Employee
item Strem, Mary
item Schmidt, Martha
item YAW AKROFI, ANDREW - Cocoa Research Institute Of Ghana
item KOLAWOLE, OLUWASEUN - Cocoa Research Institute Of Ghana
item SURENDRA, SURUJDEO-MAHARA - University Of The West Indies
item GOSS, ERICA - University Of Florida
item DIDIER BEGOUDE, B - Institute Of Agricultural Research For Development (IRAD)
item MARTIJIN TEN, HOOPEN - Centro De Cooperation Internationale En Recherche Agronomique Pour Le Development (CIRAD)
item PHILLIPS, WILBERTH - Catie Tropical Agricultural Research
item TYLER, BRETT - Oregon State University
item Meinhardt, Lyndel
item Bailey, Bryan

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2015
Publication Date: 2/15/2016
Citation: Ali, S., Amoako-Attah, I., Bailey, R., Strem, M.D., Schmidt, M.E., Yaw Akrofi, A., Kolawole, O.O., Surendra, S.J., Goss, E., Didier Begoude, B.A., Martijin Ten, H., Phillips, W., Tyler, B., Meinhardt, L.W., Bailey, B.A. 2016. 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. doi: 10.1111/ppa.12496.

Interpretive Summary: Chocolate is produced from the seeds of the cacao tree. Black pod rot is a plant disease causing serious reductions in yield a major cacao producing areas in Africa. In Africa, black pod rot is caused by the two Phytophthora species, Phytophthora megakarya and Phytophthora palmivora. These two species are very similar in appearance but Phytophthora megakarya is by far the most destructive causing severe yield losses. In Africa, when trying to manage black pod it is important to know which species are present in specific regions and fields. For example, in Ghana a national spray program target management of Phytophthora megakarya but not Phytophthora palmivora. Since the two Phytophthora species look the same, we developed marker using simple polymerase chain reaction techniques which easily distinguish between the two species. In addition, we characterized differences in the manner in which the two species cause disease on cacao using newly developed laboratory techniques. The molecular markers and techniques developed, and the knowledge gained will aid scientists in breeding for resistance to black pod and government field managers in planning disease management approaches, helping assure a stable supply of cacao for the chocolate industry and consumers of chocolate products.

Technical Abstract: 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 rDNA sequence showed high interspecific variation among the species causing black pod. P. megakarya showed higher intraspecific variation compared to that of P. palmivora. 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 zoospore 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 appressoria. 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 growing regions in Ghana as it has done in Nigeria and Cameroon.