Downy mildew caused by Peronospora farinosa f. sp. betae newly reported on Swiss chard in Korea

Swiss chard (Beta vulgaris var. cicla) is a popular culinary vegetable crop grown worldwide for its edible leaves. In November 2007, Swiss chard showing typical downy mildew symptoms was initially collected in polytunnels in Yongin and Namyangju, Korea. In Yongin area, many growers abandoned the crop before harvest due to significant damage by the disease. Infected leaves turned pale green to yellowish with greyish to dark brown fungal growth developing mainly on lower surfaces, but spreading to the upper surface in wet conditions. As the disease progressed, the leaves began to curl and wilt (Figs. 1-1 & 1-2). Microscopic examination of two representative samples (KUS-F23152 and KUS-F23163) was done to identify the pathogen. Conidiophores (250-600 x 7-10 μm) were hyaline, straight to substraight, mostly monopodially branched in 4-6(-7) orders (Fig. 2-3). Ultimate branchlets were straight to substraight, 5-13 μm long, 1.5-3 μm wide at the base, with conical to obtuse tips (Fig. 2-4). Conidia were olivaceous brown, broadly ellipsoidal to ellipsoidal, and measured 21.5-30.0 x 17.5-22.5 μm (length/width ratio = 1.17-1.35)(Figs. 2-5 & 2-6).

Amplification and sequencing of the ITS rDNA region were performed and data deposited in GenBank (Accession No. EU591713). Comparison of sequences available in GenBank revealed that the ITS sequence is identical to Peronospora farinosa found on sugar beet, B. vulgaris var. saccharifera (DQ643843 and DQ643844), and shows two base pair substitutions with an isolate on spinach, Spinacia oleracea (DQ643901). Based on the present results of morphological characteristics and molecular approaches and previous work of host specificity (Byford, 1967), the pathogen was identified as P. farinosa f. sp. betae.

Downy mildew of Swiss chard has been previously recorded from USA (as Peronospora schachtii), Mexico (as Peronspora sp.) and China (as P. farinosa) (Farr and Rossman, 2009), but not from Korea. Though downy mildew infection on sugar beet has been recorded in the northern part of Korea before (Nakata, 1914), there has been no sugar beet cropping in Korea since the 1920s. The recent outbreak of the disease seems to be closely related to high humidity and poor ventilation occurring in the polytunnels. This is the first Korean report of downy mildew on Swiss chard, an emerging threat to sheltered production of this important crop.