Phytophthora palmivora causing disease on theobroma cacao in hawaii

Authors: Puig, Alina; Quintanilla, Wilber - Wil; Matsumoto Brower, Tracie; Keith, Lisa; Gutierrez, Osman; MARELLI, JEAN-PHILLIPE - Mars, Inc

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/22/2021
Publication Date: 4/27/2021
Citation: Puig, A.S., Quintanilla, W.E., Matsumoto Brower, T.K., Keith, L.M., Gutierrez, O.A., Marelli, J. 2021. Phytophthora palmivora causing disease on theobroma cacao in hawaii. Phytopathology. 11(5). Article 396. https://doi.org/10.3390/agriculture11050396.
DOI: https://doi.org/10.3390/agriculture11050396

Interpretive Summary: Black pod disease, caused by several species of Phytophthora, has been reported on cacao in Hawaii but extent of the problem is unknown. In addition, the species present has not been confirmed using molecular tools. To address this, USDA-ARS scientists from Miami, FL and Hilo, HI as well as MARS Inc, collected symptomatic material from cacao farms on Hawaii Island and Oahu. The pathogen was detected on less than half of the sites visited, none of which were located on Oahu. All isolates were identified as Phytophthora palmivora based on sequences from two different genes. Although no genetic variation was observed, isolates differed among each other in terms of their temperature-growth responses and virulence. They were also tested in the laboratory to determine their sensitivity to commonly-used fungicides, and it was found that they were more sensitive to mefenoxam, chlorothalonil, and fosetyl-Al, than P. palmivora strains from parts of the globe where cacao production is more well-established. Cacao production is increasing in Hawaii and farmers have rated diseases as the most import issue facing the industry.

Technical Abstract: Commercial production of cacao in Hawaii has doubled in the past 10 years, and farmers are receiving premium prices for their beans from the expanding local confectionery industry. Black pod, caused by Phytophthora spp., is the only major cacao disease that has been reported in Hawaii but distribution and molecular identification are lacking. To determine the species of Phytophthora affecting Theobroma cacao, a sampling trip was conducted on Hawaii Island and Oahu. Ten isolates of Phytophthora palmivora were obtained from diseased cacao on Hawaii Island, but none from Oahu, despite the presence of symptomatic pods. No other Phytophthora species were found. Laboratory studies showed that all isolates produced lesions on unwounded cacao pods, but they differed in terms of their temperature–growth responses. Fungicide sensitives for a subset of isolates (n = 4) were determined using media amended with a range of fungicide concentrations. The Hawaiian isolates of P. palmivora were more sensitive to mefenoxam, chlorothalonil, and fosetyl-Al, than isolates from Ghana (n = 2) and Mexico (n = 1). This study identifies P. palmivora as a causal agent of black pod in Hawaii based on molecular data and provides valuable preliminary information on fungicide resistance and temperature response that can be used to improve disease management.