First report of Diaporthe tulliensis causing necrotic spots and leaf blight on rambutan in Puerto Rico

Authors: SERRATO DIAZ, LUZ; AVILES-NORIEGA, ASHLEY - UNIVERSITY OF PUERTO RICO; RIVERA-VARGAS, LYDIA - UNIVERSITY OF PUERTO RICO; Goenaga, Ricardo

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2022
Publication Date: 7/31/2023
Citation: Serrato Diaz, L.M., Aviles-Noriega, A., Rivera-Vargas, L.I., Goenaga, R.J. 2023. First report of Diaporthe tulliensis causing necrotic spots and leaf blight on rambutan in Puerto Rico. Plant Disease. https://doi.org/10.1094/PDIS-09-22-2058-PDN.
DOI: https://doi.org/10.1094/PDIS-09-22-2058-PDN

Interpretive Summary: Rambutan is a tropical exotic fruit belonging to the Sapindaceae family. Several pathogens have been identified in rambutan causing different diseases on fruits, inflorescences, and branches but few on leaves. From 2015 to 2021, a disease survey was conducted at greenhouses and at experimental rambutan field orchards of the USDA-ARS Tropical Agriculture Research Station located at Isabela, Puerto Rico. Necrotic spots and leaf blight were commonly observed with an 80% of incidence. Diseased leaves had a necrosis starting from the apex and spreading through the lamina. Leaves were disinfected and plated on common medium to grow fungi. Two isolates of Diaporthe tulliensis, were identified by morphology and DNA amplifications of four nuclear genes. Pathogenicity tests were conducted on rambutan seedlings of variety R-167. Three healthy non-detached leaves were inoculated per isolate with 5mm mycelial disks from pure D. tulliensis cultures. Rambutan seedlings were kept in a humid chamber using plastic bags for 8 days under greenhouse conditions. Two of six seedlings were used as control. Eight and 14 days after inoculation, isolates caused necrotic spots and leaf blight on leaves. Diseased leaves turned from light brown to dark brown starting from the apex and spreading through the lamina with necrosis tissues ranging from 5 – 10 mm. Untreated controls showed no symptoms, and no fungi were re-isolated from tissue. D. tulliensis was re-isolated from diseased leaves, fulfilling Koch’s postulates. D. tulliensis has been reported as a pathogen in Boston Ivy, Bodhi trees, Jasmin, coffee, kiwifruit and cacao. To our knowledge, this is the first report of D. tulliensis causing necrotic spots and leaf blight on rambutan. Recognizing that leaf blight of rambutan in Puerto Rico is caused by D. tulliensis and not a potassium deficiency as commonly reported in the literature, is important to establish adequate control measures against this disease.

Technical Abstract: Rambutan (Nepehelium lappaceum) is a tropical exotic fruit belonging to the Sapindaceae family. Several pathogens have been identified in rambutan causing different diseases on fruits, inflorescences, and branches (Serrato-Diaz et al., 2015, 2017, 2020) but few on leaves. From 2015 to 2021, a disease survey was conducted at greenhouses and at experimental rambutan field orchards of the USDA-ARS Tropical Agriculture Research Station located at Isabela, Puerto Rico, on varieties: Benjai, Gula Batu, Jitlee, R-134, R-156Y, R-162, R-167 and Rongren. Necrotic spots and leaf blight were commonly observed with an 80% disease incidence. Diseased leaves showed necrosis starting from the apex and spreading through the lamina. Diseased leaves were disinfected and plated on potato dextrose agar (PDA) and oatmeal agar (OA). Two representative isolates of Diaporthe tulliensis, A3 and A4, were obtained, purified, and identified morphologically and by PCR amplifications of the Internal Transcribed Spacer (ITS1-5.8S-ITS2) region of the nuclear ribosomal DNA, portions of the Beta tubulin (BT) and the translation elongation factor 1 alpha (TEF1-a) genes. On PDA and OA colonies of isolates A3 and A4 were initially white, flat with sparse mycelia that turned yellowish-white to grey with age. Pycnidia were black with cream to pale yellow conidial droplets that exuded from ostioles. Hyaline, unicellular alpha conidia were oval to cylindrical, rounded at the apex and obconically truncate at base. Alpha conidia (n = 50) for isolates of D. tulliensis were from 4.9 to 5.9 µm long by 2.2 – 2.5 µm wide. DNA sequences of the ITS region and partial sequences of TEF1-a and BT genes were compared by BLASTN with D. tulliensis sequences deposited in GenBank. ITS, BT and EF1-a sequences of isolates A3 and A4 (OP219651 and OP161553 for ITS region; OP222137 and OP168832 for TEF1-a; OP222136 and OP168831 for BT, respectively) were grouped to the holotype BRIP 62248a (Bootstrap BS=100) of Diaporthe tulliensis R.G. Shivas, Vawdrey & Y.P. Tan. Pathogenicity tests were conducted on six rambutan seedlings of R-167 variety. Three healthy non-detached leaves were inoculated per isolate with 5mm mycelial disks from pure cultures grown on PDA. Rambutan seedlings were kept in a humid chamber using plastic bags for 8 days under greenhouse conditions. Two of six trees were used as controls and inoculated with PDA disks only. Eight and 14 days after inoculation (DAI), D. tulliensis isolates caused necrotic spots and leaf blight, on leaves. Diseased leaves turned from light to dark brown starting from the apex and spreading through the lamina with necrotic lesions ranging in size from 5 – 10 mm. Untreated controls showed no symptoms, and no fungi were re-isolated from tissue. D. tulliensis was re-isolated from diseased leaves, fulfilling Koch’s postulates. D. tulliensis has been reported in Taiwan causing Diaporthe leaf spot in Boston Ivy and Bodhi trees, as well as Jasmin stem canker. It has been reported as causing leaf blight of coffee, and kiwifruit stem canker in China, and most recently causing cacao pod rot in Puerto Rico. To our knowledge, this is the first report of D. tulliensis causing necrotic spots and leaf blight on rambutan.