First report of leaf spot of Paederia scandens caused by Pseudocercospora paederiae in Korea

Abstract

Paederia scandens (P. foetida), a fast-growing, perennial, deciduous climbing plant in the Rubiaceae, is a native of eastern and southern Asia. The plant is widely distributed in grassland on the seashore of the subtropical island Cheju, as well as in the foothills of the middle and southern districts of Korea. Paederia scandens has traditionally been used as a medicinal herb in Asia. In Florida, where it is known as skunk vine weed, it creates dense canopies leading to injury, structural alteration, or death of native vegetation (Walker et al., 2001). In France and Germany it has been cultivated as an ornamental and used to produce a sweet juice. In July 2000 a brown leaf spot was observed on P. scandens for the first time in Keryong Mountains in Chungnam district, South Korea. Further surveys for Paederia leaf spot in the southern districts, Chonnam, Kyeongnam, and Cheju island revealed that the disease was widespread under moist and warm conditions. Typical disease symptoms were generally observed in late July and August due to a long rainy period. In addition, the long period of rainfall during late June and July 2000 favoured more development and spread of the disease than in 2001. The initial symptoms generally observed in early July consisted of small, brown leaf spots that expanded and coalesced, resulting in discoloration, blight, necrosis and defoliation. Leaf spots were amphigenous, circular, irregular, distinct, orbicular, blackish or bright brown, sometimes reddish brown, often bordered with a raised, dark linear margin on both leaf surfaces, and with grey to brown centres, commonly 1–5 mm (up to 20 mm) in diameter. The leaf spots were more severe on leaves near the ground and progressed to the upper leaves. Small blackish brown leaf spots were also observed until late October. Conidiomata were densely fasciculate to synnematous and the scars of the conidiogenous cells, which are rather inconspicuous, unthickened and not darkened, were the only definitive diagnostic signs observed. The morphological characteristics of conidiomata and scars matched previously reported descriptions of Pseudocercospora paederiae (Cercospora paederiae) (Hsieh & Goh, 1990; Guo & Liu, 1991), although the morphology resembled Phaeoisariopsis, whose conidiomata were generally synnematous. Conidiophores were chiefly hypophyllous, but sometimes epiphyllous with globular or subglobular stromata, about 10–30 in dense synnemata, 119–210 μm long and 3–5 μm wide, arising from stromata, multiseptate, generally straight but often curved at the apices. They were caespitose, slightly wider at the apex than base, and sometimes forming rather loose synnemata at the apex. Synnemata were black-brown, often branched, 102–153 μm long and 14–41 μm wide. Conidia were hyaline to brown, single, obclavate, generally slightly curved, with a rounded apex and an obconic truncate base, mostly 2–7 (up to 10) septate, 31–120 μm long, 2–6 μm wide at the apex and scar, respectively. Pathogenicity of the fungus was determined on 4-weekold leaves of P. scandens. For inoculation, a conidial suspension (approximately 10 spores mL) containing fungal mass such as synnematous conidiophores and mycelia was prepared from cultures grown on Paederia leaf extract-potato dextrose agar for 3 weeks. Slightly wounded (done by softly scratching the leaf surface with a needle) and nonwounded leaves of two plants were inoculated using a hand sprayer, and placed in a moist chamber at 25°C for 5 days, then moved into a growth chamber at about 25°C, or moved into a greenhouse. Symptoms similar to those originally observed began to appear on the leaves 2–3 weeks after inoculation. Non-inoculated control leaves exposed to the same environmental conditions remained healthy. Pseudocercospora paederiae was consistently reisolated from inoculated plants. This is the first report of P. paederiae on P. scandens in Korea.