Powdery mildew caused by Erysiphe cruciferarum is found for the first time on Chinese cabbage in Korea

Chinese cabbage (Brassica rapa ssp. pekinensis; syn. B. pekinensis) is the second most important vegetable in Korea after red pepper. In November 2006 seedlings in a glasshouse in Suwon, Korea, showed typical powdery mildew symptoms. Symptoms first appeared as circular to irregular white colonies, which subsequently developed into abundant growth on both leaf surfaces (Fig. 1). Severe infections often caused leaf distortion, withering and premature senescence.

Superficial mycelium and other structures were detached from the leaves by using a razor blade for morphological characterisation. Appressoria on the mycelium were lobed. Conidiophores were unbranched, 3-4 celled and cylindrical, 84-120 × 7-9 µm (Fig. 2). The foot-cells of the conidiophores were 24-34 µm long and generally equal to or shorter than the upper cells. Singly produced conidia were oblong to cylindrical or oval, 32-52 × 16-20 µm, with angular/rectangular wrinkling of outer wall, and no distinct fibrosin bodies (Fig. 3). No chasmothecia were found. These structures are typical of the powdery mildew Oidium subgenus Pseudoidium, anamorph of the genus Erysiphe. The measurements coincide with those of E. cruciferarum as described by Braun (1987).

The ITS region of the fungal DNA was amplified with ITS1/P3 and ITS1/PM6 primers according to Takamatsu & Kano (2001) and the sequence (GenBank EF592611) compared to Erysiphe cruciferarum ex Arabidopsis thaliana (AF031283). The DNA analysis confirmed the powdery mildew fungus from Chinese cabbage to be E. cruciferarum.

Powdery mildew of Chinese cabbage has been recorded from Europe (Finland and Germany), North America, China and Korea (Farr et al., no date). However, the economic losses due to powdery mildew infections have so far not been assessed in any countries. In Korea, an opportunistic infection on seed capsules in a breeding experiment under glass was noted by Cho et al. (1997) but no infections were observed on the leaves. Despite annual disease surveys on Chinese cabbage in glasshouse and in the field, powdery mildew has not been found previously in Korea (WD Cho, personal communication).

This is the first report from Korea of powdery mildew on Chinese cabbage caused by E. cruciferarum. Since the 1990s, production of Chinese cabbage seedlings, produced mostly as plugs under glass for later transplantation into the field, has become a big business in Korea, since an increasing number of growers demand timely supply of seedlings of adequate quality and quantity. Powdery mildew poses a potential treat to safe production of Chinese cabbage seedlings.