First Report of Powdery Mildew Caused by Podosphaera fusca on Herba eupatorii in China

Abstract

Herba eupatorii, one of the most important Chinese medicinal herbs, belongs to the Asteraceae family. In June 2012, a previously unknown disease, tentatively identified as powdery mildew, was observed on H. eupatorii growing in Shangqiu, in eastern Henan Province, China. Symptoms began as white mycelium partially covering upper leaf surfaces; as the disease progressed, it spread to cover entire leaf surfaces. The infected leaves became yellow and necrotic at advanced stages of infection. Specimens consisting of infected leaves were maintained at the Plant-Microbe Interaction Laboratory at Shangqiu Normal University. Microscopic observations of the morphology of the fungus revealed oval primary conidia measuring 18 to 27 × 15 to 22 μm. A long unbranched germ tube that germinated laterally from the ends of conidia was observed in some samples. Conidiophores were cylindrical, simple unbranched, and composed of a basal cell with a swollen base and three to six barrel-shaped conidia formed in chains, measuring 112 to 180 × 9 to 12 μm. Mycelial appressoria were nipple-shaped. Chasmothecia were not observed in the collected samples. To verify the identity of the fungus, the internal transcribed spacer (ITS) rDNA was amplified with ITS1 and ITS4 primers (3) and sequenced. The sequences were deposited as GenBank Accession No. JX546297. Comparison with sequences in the GenBank database revealed that the ITS sequence was 100% homologous with the sequence of Podosphaera fusca on Calendula officinalis (AB525914) (2) and Syneilesis palmata (AB040349) (1). The ITS sequence analysis verified that the causal agent was P. fusca, which is reported to be a cosmopolitan powdery mildew fungus, parasitic on numerous plant species in the Asteraceae family. Koch's postulates were completed by inoculating healthy H. eupatorii plants with a conidial suspension (prepared in distilled water) of 105 conidia/ml collected from infected plants. Five plants were sprayed until the suspension ran off the leaves, while five additional plants were sprayed with distilled water as a control. Plants were maintained in a climate cell under the following conditions: day, 24°C, 16 h; night, 20°C, 8 h; 85% humidity. After 10 days, inoculated plants developed symptoms similar to those observed in the field, whereas control plants remained healthy. Further examination showed that the inoculated plants were infected by P. fusca. To our knowledge, this is the first report of P. fusca affecting H. eupatorii in China. Because there are no fungicides labeled for use on this plant, the appearance of powdery mildew caused by P. fusca could result in substantial production loss of H. eupatorii. References: (1) T. Hirata et al. Can. J. Bot. 78:1521, 2000. (2) S. Takamatsu et al. Persoonia 24:38, 2010. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, CA, 1990.