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First Report of a Fruit Spot of Elaeagnus angustifolia Caused by Alternaria tenuissima in Xinjiang, China

Chen, X. F., Li, H., Teng, L. P., Dan, H. X., Xiong, R. C.
Plant disease 2018 v.102 no.11 pp. 2377
Alternaria tenuissima, Elaeagnus angustifolia, agar, branches, carrots, cities, color, conidia, conidiophores, culture media, disease incidence, edaphic factors, ethanol, fruits, fungi, genes, histones, internal transcribed spacers, leaf spot, leaves, ornamental plants, pathogenicity, pathogens, polygalacturonase, potatoes, sequence analysis, trees, windbreaks, China
Elaeagnus angustifolia (Russian olive) is a small tree with silvery twigs and foliage, flowers that are yellowish white, and ellipsoid or subglobose fruit (Sun and Lin 2010). It is not only an important windbreak or landscape tree but also favored for the high nutritional and medicinal values of its fruits. This tree can tolerate a wide range of moisture and edaphic conditions and is widely distributed in Xinjiang. In August 2017, diseased fruits with 5 to 40% disease incidence were observed in windbreaks of four different counties and cities of Xinjiang, China. Early fruit symptoms appeared as elliptical or irregular-shaped black spots that ranged in size from 0.8 to 2 mm. Later, tissue under the spots turned brown to dark brown. Pathogen isolation attempts were made from diseased fruits collected from four different locations in Xinjiang. Fruits were rinsed with sterile water, sprayed with 75% ethanol, and skins removed with a sterile scalpel. Infected tissue (about 1 mm³) was excised and placed on potato dextrose agar (PDA). Twelve fungal isolates showing similar morphological characteristics were obtained from as many diseased fruits. On a PDA plate, all fungal cultures developed white colonies that later turned gray-green or brown-greenish. On potato carrot agar, all fungal isolates formed chains of 5 to 12 conidia that occasionally formed one or two lateral branches. Conidiophores were short, light brown in color, arising singly, and measured 14.2 to 62.7 (average, 35.6) µm long by 1.6 to 4.1 (average, 2.2) μm wide. Mature conidia were obclavate or pyriform, golden brown in color, 18.2 to 31.5 (average, 21.6) µm long and 7.3 to 11.4 (average, 9.6) µm wide with one to four transverse and zero to two longitudinal septa. Conidia of all fungal isolates either produced no beaks or very small beaks. Cultural and morphological traits were similar to those of Alternaria tenuissima described by Simmons (2007). Identity confirmation tests were performed by amplifying and sequencing the internal transcribed spacer (ITS) region, the partial coding sequence of endopolygalacturonase (endoPG), and histone 3 gene of all 12 fungal isolates using primers ITS1/ITS4 (White et al. 1990), PG2b/PG3a (Andrew et al. 2009), and H3-1a/H3-1b (Glass and Donaldson 1995). MegaBLAST analysis revealed that our ITS sequences (accession nos. MG744355 to MG744366) matched with 99 to 100% identity to those of A. tenuissima (AF347032.1). The endoPG sequences (accession nos. MH022723 to MH022734) matched with 99 to 100% identity to A. tenuissima (KP124026.1). The histone 3 sequences (accession nos. MG744367 to MG744378) were 99 to 100% identical with A. tenuissima (KF997086.1), confirming the identity of these fungal isolates as A. tenuissima. Spore suspensions (1 × 10⁶ spores/ml) of the 12 isolates were prepared for pathogenicity tests. Healthy E. angustifolia fruits (390) were collected, disinfected with 75% ethanol, allowed to dry, and then sprayed with the spore suspensions (10 fruits per fungal isolate). Ten fruits were mock-inoculated with sterile water to serve as controls, and tests were performed three times. Inoculated and control fruits were maintained in a moist chamber at 27°C. All fungal isolates produced symptoms within 9 days that were similar to those observed in the field, whereas control fruits remained healthy. The same fungal pathogen was reisolated from all inoculated fruits and identified as A. tenuissima based on morphological characters and sequence analysis. This fungus is a well-known pathogen that causes leaf spot diseases on a variety of crop plants in Asia (Wee et al. 2016). To our knowledge, this is the first report of A. tenuissima causing a black fruit spot disease on E. angustifolia in Xinjiang, China.