Stem Canker of Giant Dogwood (Cornus controversa) Caused by Fusarium lateritium in Korea

Abstract

In May 2012, a stem canker was observed on a ~20-year-old giant dogwood (Cornus controversa) in Suwon, Gyeonggi Province, South Korea, which consisted of necrotic lesions on stem bark with orange sporodochial fruiting bodies. A single fungal colony was obtained from hyphal tips that were grown out of affected tissues plated on potato dextrose agar (PDA) acidified with 0.1% lactic acid after surface sterilization with 1.0% NaOCl for 30 s and 70% ethanol for 30 s, and incubated at 25°C for 7 days in the dark. The fungal isolate was grown on PDA and carnation leaf agar (CLA) to examine its mycological characteristics. The fungal colonies grown on PDA at 25°C for 7 days had diameters of 31 to 36 mm, with the colony surface sparsely cottony or with little or no aerial mycelium, very pale brown to pink, becoming progressively lighter toward the center; the colony reverse was pinkish-white to reddish-yellow, producing very few hyaline microconidia that were ellipsoidal, mostly 1-celled, and 15.4 to 22.8 × 4.1 to 4.8 μm. It produced hyaline macroconidia that were slightly curved, frequently 3 septate, a hooked or beaked apical cell and a foot-shaped or notched basal cell, 28.0 to 35.5 × 4.0 to 5.5 μm, borne on pink sporodochia. On CLA, the colony surface was lighter toward the center with no or sparse aerial mycelium, growing to 33 to 43 mm diameter at 25°C for 7 days. Microconidia were ellipsoidal, mostly 1-celled, and 9.2 to 17.5 × 2 to 2.5 μm on CLA. Macroconidia were produced on pink sporodochia near or on carnation leaf pieces, falcate to almost straight or slightly curved, frequently 5 to 7 septate, with a hooked or beaked apical cell and a foot-shaped or notched basal cell, and 45.5 to 59 × 5.5 to 6.5 μm. Chlamydospores were rare or absent. Based on these morphological characters, the isolate was identified as Fusarium lateritium (1,2). Sequences of the internal transcribed spacer (ITS) rDNA region of the fungus (GenBank Accession No. KC453998) amplified using primers ITS1/ITS4 had 100% sequence identity to F. lateritium (JN198452). The DNA sequences of translation elongation factor-1α (EF-1α) amplified using primers EF1/EF2 (KC453997) also had 100% sequence identity to F. lateritium (AY707172 and AY707156). The culture was deposited in the Korean Collection for Type Cultures (KCTC 46029). Pathogenicity tests were conducted using 1-year-old giant dogwood seedlings grown for 3 weeks before inoculation in a 1:1:1 mixture of peat moss, perlite, and sand in 10” × 10” × 12” plastic pots. The stems of three seedlings were inoculated with the mycelial plugs from the edge of the fungal culture on PDA grown at 25°C for 7 days, which were placed on three barkless cuts per stem and sealed with Parafilm that was removed 3 weeks later. Canker symptoms on the inoculated seedlings developed after 30 days of incubation at 25 to 32°C and relative humidity of 50 to 60% in a glasshouse, from which the same fungus was isolated. Non-inoculated control seedlings showed no canker development. To our knowledge, this is the first report of stem canker on giant dogwood caused by F. lateritium in Korea and also the family Cornaceae as new host for the fungus. References: (1) D. M. Geiser et al. Mycologia 97:191, 2005. (2) J. F. Leslie and B. A. Summerall. The Fusarium Laboratory Manual. Blackwell Publishing. Ames, Iowa, 2006.