Phytophthora theobromicola sp. nov. a new species causing black pod disease on cacao in brazil

Authors: DECLOQUEMENT, JENNIFER - University Of Brasilia; RAMOS-SOBRINHO, ROBERTO - Consultant; GALVAO ELIAS, SAMUEL - University Of Brasilia; SANTOS BRITTO, DAHYANA - Mars, Inc; Puig, Alina; REIS, AILTON - Centro Nacional De Pesquisa E Conservação De Primatas Brasileiros; BATISTA PINHO, DANILO - University Of Brasilia; MARELLI, JEAN-PHILIPPE - Mars Chocolate North America; FERNANDES DA SILVA, RILDO - University Of Brasilia; MARTINS NEWMAN LUZ, EDNA DORA - Centro De Pesquisas Rene' Rachou; HONORATO-JUNIOR, JAIME - Centro De Pesquisas Rene' Rachou

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2021
Publication Date: 3/15/2021
Citation: Decloquement, J., Ramos-Sobrinho, R., Galvao Elias, S., Santos Britto, D., Puig, A.S., Reis, A., Batista Pinho, D., Marelli, J., Fernandes Da Silva, R., Martins Newman Luz, E., Honorato-Junior, J. 2021. Phytophthora theobromicola sp. nov. a new species causing black pod disease on cacao in brazil. Frontiers in Microbiology. 12: 486. https://doi.org/10.3389/fmicb.2021.537399.
DOI: https://doi.org/10.3389/fmicb.2021.537399

Interpretive Summary: Black pod disease, caused by Phytophthora spp., is among the main limiting factors of cacao production in Brazil, the world’s sixth largest producer. In Brazil, only P. palmivora, P. capsici, P. citrophthora, and P. heveae have been described infecting cacao, however, recent research in Bahia, Brazil revealed the presence of a previously undescribed species of Phytophthora. In addition to being more prevalent than P. palmivora, the most common black pod pathogen, the new species is also more aggressive. As with most disease situations, whether involving animals or plants, understanding the genetics and biology of pathogens is critical to developing effective disease management strategies. Here, we present for the first time, a comprehensive evaluation of the biology, and pathogenicity of P. theobromicola sp. nov, and its genetic relationship to other members of the Phytophthora genus. These results will be used by scientists and breeders to develop cacao genotypes resistant to infection by Phytophthora pathogens. The newly discovered, more aggressive species of Phytophthora infecting cacao, will be incorporated into disease resistance screening to improve the effectiveness of breeding programs. Ultimately this research benefits producers and consumers by stabilizing cacao yields and commodity supplies.

Technical Abstract: Black pod disease, caused by Phytophthora species, is among the main limiting factors of cacao (Theobroma cacao L.) production. High incidence levels of black pod disease have been reported in Brazil, being induced by P. capsici, P. citrophthora, P. heveae, and P. palmivora. To assess the diversity of Phytophthora species affecting cacao in Brazil, 40 new isolates were obtained from cacao pods exhibiting symptoms of black pod disease collected in different smallholder farms in 2017. Further, ten cacao-infecting isolates morphologically identified as P. citrophthora and P. palmivora were molecularly characterized. The genomic regions beta-tubulin, elongation factor 1 alpha, heat shock protein 90, and internal transcribed spacer, and the mitochondrially encoded cytochrome c oxidase I and II genes were PCR-amplified and Sanger-sequenced from the cacao-infecting Phytophthora isolates. The morphological characterization and evaluation of the mycelial growth rates for the Phytophthora isolates were performed in vitro. Based on the molecular analysis and morphological comparisons, 19 isolates were identified as P. palmivora (clade 4). Interestingly, 31 isolates grouped together in the phylogenetic tree and were placed apart from previously known species in Phytophthora clade 2. Therefore, these isolates are considered as a new species herein referred to as Phytophthora theobromicola sp. nov., which produced papillate, semipapillate and persistent sporangia on simple sporangiophores. The Phytophthora palmivora isolates were identified as A1 mating type by pairing each isolate with known A1 and A2 tester strains of P. capsici, but no oogonia/antheridia were observed when P. theobromicola was paired with the different tester strains. The Phytophthora theobromicola and P. citrophthora isolates showed higher mycelial growth rates, when compared to P. palmivora, on different media at 10, 15, and 20°C, but similar values were observed when grown on clarified CA media at 25 and 30°C. The pathogenicity tests carried out on pods of four cacao clones (CCN51, PS1319, Cepec2004, and CP49) showed significant variability among the isolates of both Phytophthora species, with P. theobromicola inducing higher rates of necrotic lesion expansion, when compared to P. palmivora. Here, two Phytophthora species were found associated with black pod disease in the state of Bahia, Brazil, and the previously undescribed P. theobromicola seems to be prevalent in field conditions. This is the first report of Phytophthora theobromicola on T. cacao. Also, these findings are crucial to improve the disease control strategies, and for the development of cacao materials genetically resistant to Phytophthora.