First confirmed report of tomato powdery mildew caused by Oidium neolycopersici in Korea

In February 2009, severe powdery mildew infections were found on tomato plants (Solanum lycopersicum) grown in organic farming polytunnels in Iksan, located in the southern part of Korea. Further surveys in several localities of Korea including Suwon showed that this powdery mildew disease is common with low severity due to successful chemical control. Symptoms first appeared as white circular patches on the upper leaf surface and expanded to the lower leaf surface, petioles and stems, which were finally covered by white mycelial mats (Fig. 1A, B).

Microscopic examination showed cylindrical conidiophores, 60-120 x 7.5-9 μm, consisting of two to three straight cells, with a terminal single conidium (Fig. 2A). Conidia were ellipsoidal, 26-42 x 16-20 μm, contained no distinct fibrosin bodies, and produced a sub-terminal germ tube (Fig. 2B). The wrinkling pattern of the outer walls of older conidia was angular/reticulated (Fig. 2C). Appressoria on hyphae were single and multi-lobed (Fig. 2D). These features are characteristic of Oidium neolycopersici (Kiss et al., 2001). No chasmothecia were found in our observations. DNA analyses were performed to confirm the identity of the pathogen. The ITS rDNA region was amplified with primers ITS5 and P3as described by Takamatsu et al. (2009). The ITS sequencesobtained (GenBank Accession Nos. GQ169502, GQ169503) have one base pair insertion compared with 18 ITS sequences of O. neolycopersicifrom tomato plantsstudied by Jankovics et al. (2008), which confirms the identity of the fungus.

Cho et al. (1997) recorded tomato powdery mildew in Korea as Erysiphe cichoracearum. However, the accompanying description and figure suggest that the fungus was not E. cichoracearum but an anamorph belonging to Oidium subgen. Pseudoidium, and thus possibly O. neolycopersici. We examined 12 samples of powdery mildew collected in both polytunnel and field tomatoes from 2004 to 2009 in Korea and found that they all can be identified as O. neolycopersici. Therefore, the present work is the first well-founded report of O. neolycopersici in Korea, but it is likely that the pathogen has been present on tomatoes in previous years. The presence of O. neolycopersici in Korea represents a serious threat to tomato production because vegetable production in polytunnels and greenhouses is rapidly increasing in this country.