ARS | Publication request: FIRST REPORT OF PHYTOPHTHORA PALMIVORA, CAUSAL AGENT OF BLACK POD, ON CACAO (THEOBROMA CACAO L.) IN PUERTO RICO

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 21, 2006
Publication Date: August 1, 2007
Citation: Irish, B.M., Goenaga, R., Park, S., Kang, S. 2007. First Report of Phytophthora palmivora, causal agent of black pod, on cacao (Theobroma cacao L.) in Puerto Rico. Plant Disease. 91:1051.

Interpretive Summary: Black pod is the most economically important and widespread disease of cacao. It is caused by multiple Phytophthora species, although P. palmivora is most significant. Symptoms resembling those of black pod in the national clonal germplasm collection of cacao at the USDA-ARS Tropical Agriculture Research Station (TARS) in Mayaguez, Puerto Rico have been observed for a number of years. So in an effort to better characterize the disease, in May 2005 several isolates were collected. The presence sporangia, which fit the previously reported description of P. palmivora, were confirmed microscopically. Pathogenicity tests were conducted for nine isolates and DNA was extracted. The internally transcribed spacer sequences (ITS) were amplified and compared. The ITS sequences for all isolates were identical and exhibited strong similarity (>99% identity) to that of previously described isolates of P. palmivora. Its source in this collection is uncertain, but it may have originated from previously reported P. palmivora populations in Puerto Rico on their respective hosts. Control of this disease is critical for proper management and maintenance of accessions in the collection.

Technical Abstract: Black pod (Phytophthora pod rot) is the most economically important and widespread disease of cacao. It is caused by multiple Phytophthora species, although P. palmivora (E. J. Butler) E. J. Butler is most significant. Infection of all plant parts occurs, but is particularly important on pods. Total losses due to these pathogens exceed $400 million worldwide (1). Symptoms resembling those of black pod in the national clonal germplasm collection of cacao at the USDA-ARS Tropical Agriculture Research Station (TARS) in Mayaguez, Puerto Rico have been observed for a number of years. So in an effort to better characterize the disease, in May 2005 small surface sterilized sections from pod lesions were placed on water agar and incubated for 4 days. The presence of papillate, caducous sporangia, which fit the previously reported description of P. palmivora, was confirmed microscopically (4). Single hyphal tips were selected for nine isolates and sub-cultured in PD broth for mycelium production. Each hyphal tipped isolate was tested in a standard detached pod pathogenicity test (2). DNA was isolated from mycelium with the FastDNA® kit (Q-Biogen, Irvine, CA1) for eight isolates. The internally transcribed spacer (ITS) regions of the ribosomal RNA gene cluster were then amplified purified and sequenced. The ITS sequences of all isolates (GenBank accession numbers DQ987915 and DQ987922) were identical and exhibited strong similarity (>99% identity) to that of previously described isolates of P. palmivora (e.g., GenBank accession numbers AF467093 and AF467089). P. palmivora has been reported on many plant hosts worldwide and in Puerto Rico on citrus, coconut, black pepper, and Arracacia xanthorrhiza (2,3); however this is the first report of P. palmivora on cacao in Puerto Rico. Its source in this collection is uncertain, but it may have originated from previously reported P. palmivora populations in Puerto Rico on their respective hosts. The USDA-ARS TARS is the official cacao germplasm repository for the National Plant Germplasm System and as such, efforts are underway to reduce the disease impact on the cacao germplasm collection by implementing a detailed integrated pest management plan. The plan also includes the introduction and evaluation of germplasm accessions for resistance to this important plant pathogen.