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First Report of Powdery Mildew of Snow Lotus (Saussurea involucrata) Caused by Leveillula taurica in China

Snow lotus (Saussurea involucrata Kar. et Kir.) is a rare and beneficial traditional Chinese medicinal herb, growing in the mountains at heights of 4,000 to 4,300 m in the Tianshan and A'er Tai areas in China (2). Because of its very slow growth and exhaustive plant collection, the wild population has been listed as a protected plant by the Chinese government. In recent years, it has succeeded in artificial cultivation. Since 2010, severe powdery mildew infections were repeatedly observed on leaves of the plant of artificial cultivation in Urumqi, Xinjiang Autonomous Region of China. White superficial mycelia were present epiphyllously on both sides of the leaves as well as on young stems, forming a thin, irregular covering. Infections often cause leaf yellowing, reduced growth, and premature defoliation. On the basis of microscopic examination, the morphology of the fungus can be described as follows: Conidiophores emerged through leaf stomata, singly or branched, and form dimorphic conidia. Primary conidia are lanceolate with distinct apical points and 44 to 63 × 12 to 22 μm. Secondary conidia are ellipsoid to cylindrical and 41 to 59 × 12 to 20 μm. The germ tube has indistinct appressoria, is longer than conidia length, and arises from the subterminal region of conidia. These morphological features are typical of the anamorphic stage description of Leveillula taurica (Lév.) Arnaud (1). No ascomata were observed. Representative voucher specimens were deposited in the fungal herbarium of Xinjiang Agriculture University (HMACC 40731). To verify the identity of the fungus, the internal transcribed spacer (ITS) rDNA was amplified and sequenced, and the sequences were deposited as GenBank Accession No. KC292212. Comparison with sequences in the GenBank database revealed that the ITS sequence showed 99% homology with the sequence of L. taurica on Helianthus sp. (AB044378) and Gundelia tournefortii (AB667874). Thus, the pathogen was identified as L. taurica on the basis of the morphological characters and the ITS sequence. Pathogenicity was confirmed through inoculation by gently pressing diseased leaves onto leaves of healthy potted plants. Three inoculated plants were kept under a plastic humid chamber, whereas the same number of non-inoculated plants served as the control. The plants were placed under natural conditions (25 to 28°C) with 80 to 90% humidity. At 5 days after inoculation, typical symptoms of powdery mildew developed on the inoculated plants. No symptoms were seen on the control plants. To our knowledge, this is the first record of L. taurica on S. involucrata in the world. Because the plant is becoming widely cultivated in the Urumqi for use as a Chinese medicinal herb, the occurrence of powdery mildew poses a potential threat to the health of snow lotus.

References: (1) U. Braun. A Monograph of the Erysiphales (Powdery Mildews). Nova Hedwigia Beiheft 89:1, 1987. (2) L. G. Fu. China Plant Red Data Book - Rare and Endangered Plants, vol 1. Chinese Science Press, Beijing, 1992.