Hexinia polydichotoma (Ostenf) H.L. Yang (synonym Chondrilla polydichotoma Ostenf.) is an indigenous sand-binding plant that is widely distributed only in the desert regions of Northwest China. During the summer of 2007, severe outbreaks of a previously unknown powdery mildew were observed in the Taklimakan Desert in Xinjiang, China. Almost 95% of the plants surveyed were affected in this area. The upper surfaces of the stem were covered with white mycelia and the corresponding abaxial surfaces of infected leaves were chlorotic. Affected young, green stems also showed extended chlorosis. As the disease progressed, the infected stems turned yellow and necrotic. Heavy infection resulted in death of the plants. The primary conidia of the fungus were lanceolate with apical pointed, rarely cylindrical or subcylindrical with attenuated apex. They measured 53 to 73 × 15 to 21 μm and had a surface with a net of irregular rides and warts. Subcylindrical or subclavate secondary conidia with rounded ends measuring 50 to 77 × 13 to 20 μm were observed. The ascomata are subgregarious to scattered, globose, and 165 to 200 μm in diameter that are immersed in the dense mycelial tomentum. Numerous and well-developed appendages on the lower half of the ascomata are irregularly branched and can be as long as up to the ascomata diameter. The appendages measure 79 to 106 × 5 to 10 μm and are aseptate, thin walled, and smooth. Asci are numerous (usually more than 20 per ascoma), stalked, clavate-ovoid to nearly cylindrical, and contain two spores (rarely one or three). Ascospores are ellipsoid, hyaline, and measure 25 to 35 × 14 to 20 μm. On the basis of these characteristics, the fungus was identified as Leveillula lactucae-serriolae (2). A voucher specimen was deposited in the Herbarium of Martin Luther University, Halle, Germany (Accession No. HAL 2439F). To confirm the identification, the internal transcribed spacer (ITS) rDNA was amplified and sequenced, and deposited in GenBank (Accession No. HQ821500). Comparison with sequences available in the GenBank database revealed that the ITS sequence shares 99% similarity with that of L. lactucae-serriolae on Lactuca serriola from Iran (Accession No. AB044375.1) (1). Thus, the pathogen was identified as L. lactucae-serriolae based on the host plant species, anamorph morphology, and ITS sequence. Pathogenicity was confirmed through inoculation by gently pressing a diseased stem onto the stem of healthy H. polydichotoma plants. Five inoculated plants were kept under a plastic humid chamber, whereas the same number of noninoculated plants served as the control. The plants were placed under natural conditions (25 to 28°C) with 80 to 90% humidity. At 15 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 report of L. lactucae-serriolae in China and the first record of L. lactucae-serriolae on H. polydichotoma in the world (http://nt.ars-grin.gov/fungaldatabases/index.cfm). Because the plant is becoming widely cultivated in the Taklimakan Desert for use in sand-binding, the powdery mildew poses a serious threat to desertification control.
References: (1) S. A. Khodaparast et al. Mycol Res. 105:909. 2001. (2) S. A. Khodaparast et al. Mycoscience 43:459, 2002.
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