Alternaria tenuissima Causing Leaf Spot Disease on Amygdalus triloba in Xinjiang of China

Amygdalus triloba (Rosaceae; previously Prunus triloba) is a deciduous, flowering shrub that is widely used in the greening and beautification of lawns, parks and courtyards in China. In late May 2019, a leaf spot disease of A. triloba was observed on approximately 35% of plants in the Xinjiang Alaer city (40˚33'20''N, 81˚17'19''E). The disease symptoms began as small, suborbicular, brown spots on the leaves. As the disease progressed, the spots enlarged and coalesced into large necrotic areas and resulted in premature defoliation. Leaf sections (5 x 5 mm) from infected leaves were surface - sterilized with 75% ethanol for 30 s and 0.1% HgCl2 for 1 min, rinsed three times in sterile distilled water and then incubated on potato dextrose agar (PDA). Fifteen fungal isolates showing similar morphological characteristics were obtained by single-spore isolation. On the PDA plates, all fungal colonies had a dark olive color with loose, cottony mycelium. On the potato carrot agar, the fungus formed unbranched spore chains, but occasionally formed one or two lateral branches. Conidiophores were short, hazel-colored, septae, arising singly, and measuring 15.1 to 61.8 × 1.8 to 4.2 µm (35.2 ± 1.4 × 2.3 ± 0.1 µm, n = 50). Mature conidia were ellipsoidal to ovoid with a short conical beak at the tip, light brown with zero to three longitudinal septa and one to five transverse septa, and measuring 19.3 to 30.8 × 7.2 to 12.5 µm (21.8 ± 0.3 × 9.5 ± 0.2 µm, n = 50). Based on the cultural and morphological traits, the pathogen was preliminary identified as Alternaria tenuissima (Simmons 2007). Genomic DNA was extracted from the representative isolate YALAR-1, and the internal transcribed spacer (ITS) region, the partial coding sequence of endopolygalacturonase (endoPG), the glyceradehyde -3- phosphate dehydrogenase (GAPDA), the partial region of the histone 3 (H3) genes were amplified using primers ITS1/ITS4 (White et al. 1990), PG2b/PG3a (Andrew et al. 2009), GDF1/GDR1 (Berbee et al. 1999) and H3-1a/H3-1b (Glass and Donaldson 1995), respectively. The amplicons were sequenced and deposited in GenBank [MT459807 (ITS), MT459808 (endoPG), MT459805 (GAPDA), MT459806 (H3)]. MegaBLAST analyses revealed that our ITS, endoPG, GAPDA, and H3 sequences were 99-100% identical to those of A. tenuissima isolates in GenBank [AF347032 (ITS), KP124026 (endoPG), AY278809 (GAPDA), KF997086 (H3)], confirming the identity of the pathogen as A. tenuissima. Pathogenicity tests were performed by in