MOLECULAR CHARACTERIZATION AND DIVERSITY ASSESSMENT OF COCOA GERMPLASM IN THE AMERICAS
Title: A NEW MORPHO-TYPE OF PHYTOPHTHORA PALMIVORA ON CACAO IN CENTRAL AMERICA.
| Johnson, Elizabeth |
| Aime, Mary |
| Crozier, Jayne - CABI BIOSCIENCE |
| Flood, Julie - CABIE BIOSCIENCE |
| Schnell Ii, Raymond |
Submitted to: Proceedings of the International Cocoa Producer's Conference
Publication Type: Proceedings
Publication Acceptance Date: August 19, 2006
Publication Date: March 20, 2007
Citation: Johnson, E.S., Aime, M.C., Crozier, J., Flood, J., Schnell Ii, R.J. A new morpho-type of phytophthora palmivora on cacao in central america. Proceedings of the International Cocoa Producer's Conference.pp. 31-39.
Several species of Phytophthora have been implicated as the cause of black pod disease, one of the most serious constraints to cocoa production worldwide. These include (i.e., P. arecae, P. capsici, P. citrophthora, P. megakarya, P. megasperma, P. nicotianae var. parasitica and P. palmivora). Only P. palmivora has a global distribution and can cause up to 100% of crop losses in the Caribbean and Central America during periods of particularly high rainfall or in wet years. In the Americas, black pod disease is also caused, to a lesser extent, by P. capsici and P.citrophthora has been isolated from diseased cooca in Brazil. The primary infective agents are zoospores for all known isolates of P. palmivora, which are released singly and require free water for their liberation and subsequent infection of the host. Two Phytophthora isolates (Cr10 and Ni5) purified from infected cacao pods from the Atlantic coasts of Costa Rica and Nicaragua, respectively, were observed to release the majority of their zoospores ‘en masse’. These zoosporic balls germinate to form the infective agent causing the characteristic necrotic circular lesion on the pod surface. Isolates are papillate and perform as well as known P. palmivora isolates in aggressiveness tests though releasing thousand-fold fewer single zoospores. Sporangia characteristics for Cr10 (L=47.2 µm, B=29.9 µm, L/B ratio=1.6:1, pedicel length=2.6 µm) and Ni5 (L=49.5 µm, B=29.5 µm, L/B ratio=1.7:1, pedicel length=2.3 µm) are well within the range reported for P. palmivora. Both Cr10 and Ni5 were also determined to be of Mating Type A2 typical for the P. palmivora cacao pathogen in Central America.
Previous studies have shown sequence variation of five genes to be good indicators of species-level diversity in Phytophthora: these include three nuclear-encoded genes, the ITS (internal transcribed spacer regions 1 and 2 and intervening 5.8S rDNA gene), translation elongation factor EF-1', and '-tubulin; and two mitochondrial-encoded genes, cytochrome c oxidase subunit I, and the NADH dehydrogenase subunit. Sequence data for these five genes were generated to compare Ni5 with six geographically diverse isolates of P. palmivora from Colombia (Co4), Costa Rica (Cr15), Ecuador (Ec1), Panama (Pa1), Trinidad (TRD- P47) and Tobago (TBO-P1) Sequence homology for all seven isolates were confirmed by BLAST analyses against the GenBank databases (http://www.ncbi.nlm.nih.gov/) and found to share 100% identity to each other and with other known strains of P. palmivora. We report a new morpho-type of P. palmivora releasing zoospores ‘en masse’. Implications to cacao cultivar development will be discussed.