First Report of Alternaria Leaf Spot on Eleutherococcus senticosus Caused by Alternaria tenuissima in China.

Abstract

Eleutherococcus senticosus (Ruper et Maxim) Maxim, a very important potential medicinal plant used for the treatments of neurasthenia, anti-aging, and kidney deficiency, is a perennial herb belonging to Araliaceae and mainly distributed in northeast China. With the development of its cultivation, many diseases start to occur and a previously unknown leaf spot was observed on this plant in July 2007 in Linjiang City, Jilin Province, China. This disease incidence reached 100% in some planting grounds and it has resulted in serious loss of acanthopanax production. This disease generally happens during July and August in Jilin Province, China. At the initial stage of the infection, some small, light brown spots appeared on the leaves that gradually become round or irregular, dark brown, concentrically zonate with a dark brown margin, frequently surrounded by light yellow haloes and conspicuous black brown concentric rings in the advanced stage of the infection. The necrotic areas often coalesce and result in the appearance of larger spots with a diameter of 13.0 to 15.0 mm. Severely affected plants were defoliated. On leaf spots, conidia, generally in short chains, were straight, multicellular, obclavate or obpyriform, olivaceous brown or dark brown, with three to eight transverse and rarely zero to four longitudinal or zero to three oblique septa, and measured 8.3 to 27.5 × 17.3 to 55.0 μm. Conidiophores arose singly or in groups, straight or flexuous, cylindrical, expand in base cell, branch occasionally, septate, pale to olivaceous brown, 25.0 to 75.0 μm long, 3.0 to 6.0 μm wide; beak or false beak cylindrical, septate, colorless or light brown, and measured 2.0 to 5.1 × 10.4 to 47.7 μm. The morphological descriptions and measurements of the fungi are similar to Alternaria tenuissima (2). Six single cultures from the infected leaves were isolated on potato dextrose agar. Pathogenicity tests were carried out on the potted, healthy, 1-year-old plants (n = 10). These plants were divided into two groups, one group was sprayed with a conidial suspension of 105 conidia per ml and the other was sprayed with sterilized water as control plants. All plants were covered with polyethylene bags for 3 days. Symptoms of the disease appeared 5 days after inoculation. Symptoms on the inoculated leaves were similar to those that naturally occurred on the plants. The fungal pathogen was consistently reisolated from the inoculated plants but not from the control plants. The internal transcribed spacer (ITS) region of rDNA was amplified from DNA extracted from single-spore isolate cwz-2 of the pathogen using the ITS1/ITS4 and sequenced (GenBank Accession No. HQ402558). The ITS sequence had 99% identity with that of A. tenuissima strain XSD-83 (GenBank Accession No. EU326185). Therefore, the pathogen was identified as A. tenuissima on the basis of its morphological characteristics and ITS sequence. A. tenuissima was reported to occur on many plants such as blueberry in China (1). However, to our knowledge, this is the first report of A. tenuissima occurring on E. senticosus in China.