Outbreak of powdery mildew caused by Erysiphe heraclei on Pleurospermum kamtschaticum in Korea

Pleurospermum kamtschaticum, family Apiaceae, is a perennial herb used as a salad ingredient and also as a traditional medicine in Asia (Cho et al., 2004). It has been widely planted in private gardens for personal consumption at altitudes of over 600 m above sea level, in Korea. Commercial cultivation of this herb is of recent development, providing fresh leaves to urban consumers. In September 2008, severe outbreaks of powdery mildew were noticed on plants growing in Pyeongchang, Korea. The disease was recorded in more than ten farms in Pyeongchang and Youngwol during 2009. White superficial mycelia and conidia were present on both sides of the leaves as well as on young shoots, causing leaf distortions and reduced growth (Fig. 1A). Chasmothecia started to form from July and were fully mature by September (Fig. 1B).

Conidiophores were cylindrical, 60-130 µm and composed of 3-4 cells. Conidia produced singly, were cylindric oval to ellipsoid, 28-42 x 12.5-16.5 µm, without distinct fibrosin bodies.Chasmothecia were dark brown,spherical, and scattered subgregariously,90-115 µm in diameter. Each chasmothecium contained 4-8 asci. Appendages were 6-20 per chasmothecium, and 0-3-septate. Asci were shortly stalked, 3-4(-5)-spored, 60-75 x 38-50 µm. Ascospores were ellipsoid-ovoid, 24-30 x 13.5-18.5 µm (Fig. 2 A, B). Based on these characteristics, this fungus was identified asErysiphe heraclei (Braun, 1987). To confirm the identification, the ITS rDNA was amplified and sequenced, and the sequence was deposited in GenBank (Accession No. GU173850). Comparison with sequences available in the GenBank database revealed that the ITS sequence shares 100% (560/560 bp) similarity with that of E. heraclei on Torilis leptophylla (AB104514) and 99% (559/560 bp) with that of E. heraclei on Daucus carota (EU371725). Therefore, the sequence analysis confirmed the identification of the pathogen as E. heraclei. Powdery mildew on P. kamtschaticum has been known only from Japan and recorded as Oidium sp. (Nomura, 1997). Though Erysiphe heraclei was recorded on P. uralense in USSR (Farr & Rossman, 2009), there have been no previous records of E. heraclei infections on P. kamtschaticum. As this plant is becoming widely cultivated in polythene tunnels for use in salads in Korea, powdery mildew infections pose a serious threat to the safe production of this leaf vegetable.