Location: Tropical Crops and Germplasm ResearchTitle: First report of Lasiodiplodia theobromae causing inflorescence blight and fruit rot of longan (Dimocarpus longan L.) in Puerto Rico Author
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2013
Publication Date: 2/3/2014
Citation: Serrato-Diaz, L.M., Rivera-Vargas, L.I., Goenaga, R.J., French-Monar, R.D. 2014. First report of Lasiodiplodia theobromae causing inflorescence blight and fruit rot of longan (Dimocarpus longan L.) in Puerto Rico. Plant Disease. 98(2):279. Interpretive Summary: Fungi are a very 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. Longan is a tropical plant that produces delicious edible fruits. In this research, a fungus that causes fruit rot and inflorescence blight was discovered for the first time in Puerto Rico in longan trees. Knowledge of the identity of this plant pathogen is the first step to assess its impact on production of trees and to develop control measures, if necessary.
Technical Abstract: Longan is a tropical fruit tree in the Sapindaceae family. During a disease survey from 2008 to 2010, fruit rot and inflorescence blight (rotting of the rachis, rachilla and flowers) were observed at the USDA-ARS Research Farm in Isabela, Puerto Rico. Tissue sections (1 mm2) of diseased inflorescences and fruits were surface disinfested with 70% ethanol, rinsed with sterile water and transferred to acidified potato dextrose agar (APDA). Three isolates of Lasiodiplodia theobromae (Pat.) Griffon & Maubl. (Lt) from symptomatic tissue were identified morphologically using a taxonomic key for the Botryosphaeriaceae. In APDA, colonies of Lt had initial greenish-gray aerial mycelia that turned dark brown with age. Pycnidia were dark brown to black. Immature conidia were sub-ovoid to ellipsoid, apex rounded, truncate at the base, thick-walled, hyaline and one-celled, becoming dark brown, two-celled with irregular longitudinal striations when mature. Conidia (n=50) averaged 26.88 µm long by 12.98 µm wide. For molecular identification, DNA analysis of the ITS1-5.8S-ITS2 region, and fragments of both ß-tubulin and elongation factor (EF1-a) genes were sequenced and compared using BLASTN with sequences available in the GenBank. Accession numbers of gene sequences of Lt submitted to GenBank were: KC964546, KC964547 and KC964548 for ITS region; KC964549, KC964550 and KC964551 for ß-tubulin; and KC964552, KC964553 and KC964554 for EF1a. For all genes used, sequences were 99 to 100% identical to reference isolate CBS164.96 of Lt reported in GenBank. Pathogenicity tests were conducted on six random healthy non-detached inflorescences of longan and six healthy detached fruits per isolate. Both inflorescences and fruit were inoculated with 5-mm mycelial disks from 8-day-old pure cultures grown in APDA. Inflorescences were kept in a humid chamber using plastic bags for five days under field conditions while fruits were kept in a humid chamber using plastic boxes for five days under laboratory conditions of 25°C and 12 hours of fluorescent light. Untreated controls were inoculated with APDA disks only. The experiment was repeated once. Five days after inoculation, isolates of Lt caused inflorescence blight, fruit and aril (flesh) rot. Inflorescences turned brown and flower mummification was observed on the inflorescences. The exocarp (peel) and endocarp (aril) turned dark brown and mycelial growth and pycnidia of Lt were observed on fruits. Untreated controls did not show any symptoms and no fungi were reisolated from tissue. In diseased inflorescences and fruits, Lt was reisolated from diseased tissue, thus fulfilling Koch's postulates. Lasiodiplodia theobromae has been reported causing dieback, stem end rot and fruit rot on a wide range of plants host. In longan, Lt has been reported as causing fruit rot in Thailand. To our knowledge, this is the first time that Lt has been reported as causing inflorescence blight in longan and the first report of Lt causing fruit rot in Puerto Rico.