First Report ofCorynesporaLeaf Spot Caused byCorynespora cassiicolaonDuranta repensin China

Abstract

Duranta repens Linn. is a shrub in the family Verbenaceae, widely used as an ornamental plant in tropical and subtropical areas of the world. In April 2017, a new leaf spot disease was observed on D. repens in several public parks of Zhanjiang City in Guangdong Province. The symptoms were usually found on mature leaves, represented by dark brown spots with grayish centers and surrounded by a yellow halo. The disease occurred on about 40% of leaves in some plantings, which caused the infected leaves to abscise. Abundant conidiophores and conidia formed on the lesions. Conidiophores were straight to slightly flexuous, unbranched, cylindrical, light brown to brown, 52.6 to 347.4 μm long and 5.1 to 9.8 µm wide. Conidia were solitary or in chains, obclavate to cylindrical, straight to slightly curved, pale olivaceous to brown, 58.8 to 258.6 μm long and 5.5 to 10.7 μm wide, with 4 to 13 pseudosepta. The morphological characteristics of the conidiophores and conidia were consistent with the descriptions for Corynespora cassiicola (Ellis 1971). To isolate the causal pathogen, symptomatic leaf tissues were surface sterilized in 1% sodium hypochlorite for 1 min, rinsed in sterile water, and transferred to potato dextrose agar (PDA). Three single-spore cultures of the isolates were isolated from different locations and designated SMT1-1, SMT1-2, and SMT1-3. DNA was extracted from the mycelium of the three isolates, and the internal transcribed spacer (ITS) region of rDNA was amplified using the primer pairs ITS1 and ITS4 (White et al. 1990). Amplicons were 520 bp (GenBank accession nos. MG190032, MG190033, and MG190034) and shared more than 99% similarity with the published sequences C. cassiicola (KX638445 and KY764323) (Lee et al. 2017; Yeh and Kirschner 2017). To conduct a pathogenicity test, 50 ml of conidial suspension (10⁶ conidia/ml) were sprayed over 1-year-old D. repens plants in each pot with a high-pressure sprayer. Four separate pots were used for each isolate and the same number of plants were inoculated with sterilized water as a control. All pots were covered with translucent plastic bags to maintain high relative humidity, and placed into a growth chamber at 25°C under a 12-h fluorescent light/dark regimen. One week after inoculation, dark brown lesions, similar to those observed in nature, occurred on all inoculated plants. No symptoms were observed on control plants. The pathogen was reisolated from the inoculated plants, and confirmed to be C. cassiicola based on morphology as described above, confirming Koch’s postulates. To our knowledge, this is the first report of leaf spot caused by C. cassiicola on D. repens in China and worldwide.