Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2012
Publication Date: 4/19/2012
Citation: Rossman, A.Y., Melgar, J.C., Walker, D.M., Gonzales, A., Ramirez, T., Rivera, J.M. 2012. First report of Dolabra nepheliae associated with corky bark disease of Rambutan and Pulasan in Honduras. Plant Disease. 96:765. Interpretive Summary: Fungi are a large and diverse group of organisms that cause serious diseases of crop and forest plants. Accurate knowledge of fungi is critical for controlling the diseases they cause. Rambutan and pulasan are tropical plants that produce delicious edible fruits. A little known fungus was discovered to cause a stem canker disease known as corky bark of rambutan and pulasan in Honduras. The fungus causing this disease was identified using both microscopic structures and molecular sequence data. This research will help plant pathologists to accurately identify the cause of this disease of specialty crops. In addition, knowledge of this fungus will be useful to plant regulatory officials working to control the spread of this disease and prevent its introduction into the United States.
Technical Abstract: Rambutan (Nephelium lappaceum L., Sapindaceae) was first introduced into Honduras in 1927 but commercial production was not started until after 1980 when cultivation of pulasan (N. mutabile Blume) was initiated. In the last decade these crops have become popular as exotic fruit for export to North America. Recently a disease was observed that produced dark brown to black, fissured cankers varying from 1 to 3 cm long (average 1.5) and 1 to 4 cm wide (average 2.0). The infected bark tissue is swollen with the thickest middle region ranging from to 3 to 8 mm (average 3.6) as compared to 1 mm of healthy bark tissue. Symptoms were seen initially when the trees were about three years old. As the trees mature, the cankers increase in size and weaken the branches often resulting in breakage due to the additional weight of fruit before harvesting. This disease has been spreading throughout commercial plantations and is the cause of substantial plant damage and fruit loss in Honduras and neighboring Guatemala and Costa Rica. In August, 2010, Branch samples of rambutan and pulasan were collected from six to eight year old trees with typical cankers from the Humid Tropical Demonstrative Agroforestry Center. The causal agent was identified as Dolabra nepheliae. Fungi from both rambutan and pulasan were isolated from single ascospores plated on cornmeal agar plus 0.5% dextrose and antibiotics. On potato dextrose agar the ascospores produced slow growing colonies, 5 mm per week, and numerous black pycnidia. In culture isolates from both hosts produced pycnidia with elongated, slightly to strongly curved or S-shaped, hyaline conidia, 22.8–46.4 × 2.8–3.7 µm. The pathogen produces black, stipitate, elongate ascomata within the fissures of the cankers. This fungus was first reported as a causal agent of stem canker or corky bark disease on rambutan and pulasan from Malaysia and later reported on rambutan and litchi in Hawaii and Puerto Rico. The specimens from Honduras represent the first report of D. nepheliae on pulasan and rambutan in that country; they have been deposited at the U. S. National Fungus Collections (BPI 882442 on N. lappaceum, BPI 882443 on N. mutabile). Cultures were deposited in CBS 131490 on N. lappaceum and CBS 131491 on N. mutabile with sequences of the ITS region including ITS1, 5.8S, and ITS2 intergenic spacers deposited in GenBank (JQ004281 on N. lappaceum, JQ004280 on N. mutabile). Based on ITS sequence data the isolates from Honduras were confirmed as the same species, D. nepheliae, from Puerto Rico. Efforts are being initiated to develop management strategies to control the disease on these three exotic fruit crops.