L. M. Serrato-Diaz, Department of Plant Pathology and Microbiology, Texas AgriLife Research Extension Service, Texas A&M System, Amarillo;
L. I. Rivera-Vargas, Department of Crop and Agroenvironmental Sciences, University of Puerto Rico-Mayagüez Campus;
R. Goenaga, USDA-ARS, Tropical Agriculture Research Station, Mayaguez, Puerto Rico;
G. J. M. Verkley, Fungal Biodiversity Centre, Centraalbureau voor Schimmelcultures, Utrecht, the Netherlands; and
R. D. French-Monar, Department of Plant Pathology and Microbiology, Texas AgriLife Research Extension Service, Texas A&M System, Amarillo
Rambutan (Nephelium lappaceum L.) is a tropical fruit tree that has increased in importance for fruit growers in Puerto Rico. In 2008 and 2009, fruit rot and lesions on leaves and inflorescences were observed. A total of 276 diseased samples were collected from commercial orchards, orchards at the University of Puerto Rico, and the USDA-ARS in Mayaguez. Plant tissue was disinfested and plated on acidified potato dextrose agar (APDA). Besides other typical fungi associated with these tissue samples (2,3), 130 unknown isolates were identified as a Lasmenia sp. at the Fungal Biodiversity Centre (CBS), the Netherlands and the University of Puerto Rico using taxonomic keys (1,4). Sequencing of the rDNA with primers ITS 1 and ITS 4 and Lr5 and LR0R corresponding to the (internal transcribed spacer) ITS1-5.8S-ITS2 region and the partial region of the large ribosomal subunit (LSU), respectively, was completed. Five isolates (CBS 124122 to 124126) were deposited at the CBS. In APDA, colonies of a Lasmenia sp. were cream-colored with dark brown concentric rings and immersed, hyaline, branched, and septate mycelium. Acervuli were produced on APDA and plant tissue that was sampled from field and clean tissue that was inoculated with a Lasmenia sp. Conidia were 10 to 12 × 4 to 5 μm, light brown, thick walled, obclavate, aseptate, and the apex was obtuse with a scar at the base. Conidiophores were hyaline, septate, cylindrical, and sparingly branched. The conidiogenous cells were hyaline, cylindrical, and holoblastic. Pathogenicity tests were done on 12 healthy, superficially sterilized fruits under laboratory conditions, on four random leaves in each of six 6-month-old rambutan seedlings under greenhouse conditions, and on four flowers in six random inflorescences for each of six mature trees from an orchard. Tests were repeated. Either wounded or unwounded tissues were inoculated with a conidial suspension (2 to 4.5 × 106 conidia/ml) and 5-mm mycelial disks from each fungal isolate grown in APDA. After 5 days, a Lasmenia sp. produced necrotic spots on leaves, rachis necrosis and flower abortion, fruit rot, and water-soaked lesions on the fruit surface that spread to cause an aril (flesh) rot. Acervuli were produced on fruit spintems (hair-like appendages). Koch's postulates were fulfilled by reisolation of inoculated fungi from diseased tissue. A complete sequence for the ITS region for four isolates of a Lasmenia sp. was submitted to NCBI GenBank (Accession Nos. GU797405, GU797406, GU797407, and JF838336). Complete sequences of the LSU region for all five isolates were submitted to GenBank (Accession Nos. JF838337, JF838338, JF838339, JF838340, and JF838341). For both types of sequences, the identity was 100% between isolates. Although there is no DNA sequence data for the genus Lasmenia, a BLASTN search indicates a closer affinity to the Cryphonectriaceae (Diaporthales) (1). A Lasmenia sp. has been reported from Hawaii as causing fruit rot in rambutan (2). To our knowledge, this is the first report of a Lasmenia sp. causing rachis necrosis and flower abortion worldwide, and the first report of fruit rot and necrotic spots on leaves of rambutan in Puerto Rico.
References: (1) M. N. Kamat et al. Rev. Mycol. 38:19, 1973. (2) K. A. Nishijima and P. A. Follett. Plant Dis. 86:71, 2002. (3) L. M. Serrato et al. Phytopathology (Abstr.) 100(suppl):S176, 2010. (4) B. C. Sutton. The Coelomycetes: Fungi Imperfecti with Pycnidia Acervuli and Stromata. CMI. Kew, Surrey, England, 1980.