Downy mildew outbreak on Chrysanthemum boreale caused by Paraperonospora minor

Chrysanthemum boreale is a wild flower, cultivated as an ornamental and for its medicinal properties throughout East Asia. Planting of wild flowers along roads and in gardens has recently become popular in Korea and new businesses have developed to provide C. boreale seedlings. In July 2004 a downy mildew infection of C. boreale was first noticed in Pyeongchang county and since then in flower beds and gardens of other regions in Korea. The disease was most severe during the monsoon season, but was also commonly found in autumn, sometimes completely destroying flower beds. Infected leaves turned pale green to yellowish with whitish fungal-like growth developing on the lower surfaces. As the disease progressed, the leaves wilted and curled (Figs. 1, 2).

Microscopic examinations of two representative samples (KUS-F 22711 and KUS-F 22970) were made to identify the pathogen. The conidiophores were hyaline, straight, 310-620 × 6-13 µm, tree-like and mostly dichotomously to monopodially branched in 4-5 orders (Fig. 3). Ultimate branchlets were straight to slightly curved, 10-17 µm long and had truncate or obtuse tips (Fig. 4). Conidia were almost subhyaline, broadly ellipsoidal to ellipsoidal, and measured 25-31(-35) × 20-23 µm (length/width ratio = 1.17-1.50) (Figs. 5 & 6). On the basis of symptoms and morphology the pathogen was identified as Paraperonospora minor (Constantinescu, 1989).

The amplification and sequencing of the 28S rDNA was performed with procedures described by Riethmüller et al. (2002), and the sequence deposited in GenBank (Acc. No. EU287693). Comparison of the sequences available in the GenBank database revealed that the isolate shared 98 % similarity with P. leptosperma. Since this was the first sequence submitted for P. minor, no comparable data was available.

Paraperonospora minor has been previously recorded in Eurasia, as a causal agent of downy mildew on Achillea clusiana, Artemisia sieversiana, A. tilesii, A. vulgaris, and Leucanthemum nipponicum (=Chrysanthemum nipponicum, but not C. boreale (Farr et al., 2007). To our knowledge, this is the first report of downy mildew disease on C. boreale in the world.