Naked oats (Avena nuda L.) is an independent species of Avena, which can be used as both food and forage for rich nutritional value. In August 2019, leaf spot was observed at a naked oats planting base in Zhangbei County, Zhangjiakou City, Hebei Province. The incidence of disease was 40% to 50%. The symptoms of the lesions were chlorosis and gradually developing light brown spots with light yellow halos. The spots were irregular, enlarged and even coalesced to form large areas of necrosis on leaves. To identify the pathogen, twenty symptomatic leaves were collected, and one disease spot was isolated from each samples. Small square leaf pieces (3 to 5 mm) were excised from the junction of diseased and healthy tissues with a sterile scalpel and were sterilized with 75% alcohol for 30s, 0.1% mercuric chloride solution for 1 min, and then rinsed three times with sterile water, then transferred cultured on potato dextrose agar (PDA) at 25°C for 7 days. Four fungal isolates were obtained and purified by single-spore isolation method. All fungi have the same morphology and no other fungi were isolated. Colonies of the isolates had round margins, and thick fluffy aerial mycelia with brown coloration after 7 days on PDA. Conidiophores were brown, straight or flexuous, septate, single or in clusters. Conidia were obclavate or oval, dark brown, and size ranging from 4.61 to 15.68 × 6.61 to 35.49μm (n=100), with longitudinal and transverse septa varying from 1 to 3 and 1 to 7, respectively. The transverse median septum of the central section was especially thick. On the basis of morphological characteristics, the isolates were identified as Alternaria spp. (Simmons 2007). To further assess the identity of the species, the genomic DNA of pathogenic isolate (YM3) was extracted by CTAB protocol. The ribosomal DNA internal transcribed spacer (ITS) region, the glyceraldehyde-3-phosphate dehydrogenase (GAPDH), the RNA polymerase II second largest subunit (RPB2), and the plasma membrane ATPase genes were amplified and sequenced with primers ITS1/4, gpd1/2, RPB2-6F/7cR and ATPDF1/ATPDR1 respectively (Nishikawa and Nakashima 2015; Woudenberg et al. 2015). Sequences of ITS, GAPDH, RPB2 and ATPase (MN646900, MT233043, MT233042, MN640794) of the isolate was 99.82%, 99.68%, 100% and 99.51% similar to the fungus A. alternata (MK461082.1, MK451978, KP124770.1, MK804115). A neighbor-joining phylogenetic tree was constructed by combining all sequenced loci in MEGA7. The isolate YM3 clustered in the A. alternata clade with 100% bootstrap support. Therefore, the pathogen was identified as A. alternata based on the morphological characteristics and molecular identification. A pathogenicity test of the A. alternata isolates was performed by placing mycelial disks (5 mm) with conidia on the surface of the first unfolding leaves of naked oats. Each leaf was inoculated with three disks. The pathogenicity test was repeated four times, and 10 leaves were inoculated in each repetition, while sterile PDA was used as the control. All treated plants were placed in a moist chamber (25°C, 16-h light and 8-h dark period). Leaf spot symptoms developed on the inoculated plants about 10 days post inoculation while all control plants remained healthy. The similar isolates were re-isolated from the inoculated and infected leaves and identified as A. alternata by DNA sequencing, fulfilling Koch’s postulates. It has been reported that A. alternata can cause leaf spots on A. Sativa(Chen et al. 2020). However, to our knowledge, this is the first report of A. alternata causing leaf spots on A. nuda in China. It can be concluded that A. alternata can cause leaf spot disease of oats (A. Sativa and A. nuda). The spots disease is worthy of our attention for its harm to the production of oats.
Naked oats (Avena nuda L.) is rich in protein, fat, vitamin, mineral elements and so on, and is one of the world's recognized cereal crops with the highest nutritional and healthcare value. In July 2019, leaf spot was detected on A. nuda in Zhangbei experimental station of Hebei Agricultural University. The incidence of disease is 10% to 20%. The symptoms were similar to anthracnose disease, the infected leaves had fusiform or nearly fusiform yellowish-brown spots, yellow halo around the spots. Numerous acervuli with black setae diagnostic of fungi in the genus Colletotrichum were present on necrotic lesions. To identify the pathogen, ten symptomatic leaves were collected, and only one disease spot was isolated from each leaf. Small square leaf pieces (3 to 5 mm) were excised from the junction of diseased and healthy tissues with a sterile scalpel and surface disinfested with 75% alcohol for 30s, 0.1% corrosive sublimate for 1 min, rinsed three times in sterile water. Plant tissues were then transferred on potato dextrose agar (PDA), and incubated at 25°C for 7 days. Two fungal isolates were obtained and purified by single-spore isolation method. All fungi have the same morphology and no other fungi were isolated. The aerial mycelium was gray black. The conidia were colorless and transparent, falcate, slightly curved, tapered toward the tips, and produced in acervuli with brown setae. The length and width of 100 conidia were measured and size ranged from 1.86 to 3.84 × 8.62 to 29.81 μm. These morphological characteristics were consistent with the description of Colletotrichum cereale (Crouch et al. 2006). To further assess the identity of the species, the genomic DNA of two fungal isolates (LYM19-4 and LYM19-10) was extracted by a CTAB protocol. The ribosomal DNA internal transcribed spacer (ITS) region as well as, the glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), and the beta-tubulin 2 (Tub2) partial genes were amplified and sequenced with primers ITS4/5, GDF/GDR, ACT-512F/ACT-783R, and T1/Bt2b, respectively (Carbone et al. 1999; Templeton et al. 1992; O'Donnell et al. 1997; Glass et al. 1995). The sequences of the ITS-rDNA region (MW040121, MW040122), the GAPDH sequences (MW052554, MW052555), the ACT sequences (MW052556, MW052551) and the Tub2 sequences (MW052552, MW052553) of the two single-spore isolates were more than 99% identical to C. cereale isolate CGMCC3.15110 (JX625159, KC843517, KC843534 and JX625186). Maximum likelihood tree based on concatenated sequences of the four genes were constructed using MEGA7. The results showed the strains isolated from A. nuda were closely related to C. cereale, as supported by high bootstrap values. A pathogenicity test of the C. cereale isolates was performed on first unfolding leaves of A. nuda. Koch's postulates were carried out with isolates by spraying a conidial suspension of 106 conidia/mL on leaves of healthy A. nuda. Four replicated pots were inoculated at a time, 10 leaves each pot, while sterile distilled water was used as the control. All treated plants were placed in a moist chamber (25°C, 16-h light and 8-h dark period). Anthracnose symptoms developed on the inoculated plants 7 days post inoculation while all control plants remained healthy. Microscopic examination showed the surface of infected leaves had the same acervuli, setae, and conidia as the original isolate. The pathogenicity test was repeated three times. C. cereale was previously reported as the causal agent of anthracnose on feather reed grass in US (Crouch et al. 2009). To our knowledge, this is the first report of C. cereale as the causal agent of A. nuda anthracnose in China.
Snow lotus (Saussurea involucrata (Kar. & Kir.) Sch. Bip.) is an economically important medicinal herb increasingly grown in China in recent years. During the summer and autumn of 2005, 2006, and 2007, a necrosis of unknown etiology was observed on leaves in commercial production areas in Xinjiang Province of China. Disease incidence was approximately 40 to 50% of the plants during the 2005 and 2007 growing seasons. Initial symptoms consisted of pronounced water-soaked, dark brown-to-black spots that were 1 to 2 mm in diameter on young, expanding leaves. Later, some leaf spots on older leaves enlarged and coalesced, causing leaf desiccation. Leaf samples were collected in 2005, 2006, and 2007 from the affected hosts. Bacterial streaming was evident from these samples, and 28 strains were isolated on nutrient agar or King's medium B (KMB). All strains were gram negative and fluoresced bluegreen under UV light after 48 h of growth at 28°C on KMB. On the basis of LOPAT tests, the strains were identified as Pseudomonas syringae (1). The identity of two strains was confirmed by sequencing the 16S rDNA gene, which revealed 98% similarity to P. syringae strains in NCBI (Accession Nos. FJ001817 and FJ001818 for XJSNL 111 and 107, respectively). Infiltration of tobacco leaves with bacterial suspensions resulted in typical hypersensitivity reactions within 24 h. Pathogenicity of the strains was confirmed by spray inoculating five snow lotus leaves of a six-leaf stage plant with 108 CFU ml–1 bacterial suspensions in sterile water and five plants sprayed with sterile distilled water served as controls. Inoculated and sterile water-sprayed controls were maintained in the growth chamber with 90% relative humidity for 15 days at 15 ± 2°C. Symptoms similar to the original symptoms were observed on inoculated plants after 2 weeks. No symptoms developed on controls. Bacteria reisolated from inoculated plants were identified as strains of P. syringae. Isolates were deposited at the Key Laboratory for Oasis Crop Disease Prevention and Cure, Shihezi University. Rust caused by Puccinia carthami and leaf spot disease caused by Alternaria carthami of snow lotus have been reported (2,3). To our knowledge, this is the first report of P. syringae as the cause of bacterial leaf spot on snow lotus in China. References: (1) A. Braun-Kiewnick and D. C. Sands. Pseudomonas. Page 84 in: Laboratory Guide for the Identification of Plant Pathogenic Bacteria. 3rd ed. N. W. Schaad et al., eds. The American Phytopathological Society, St. Paul, MN, 2001. (2) S. Zhao et al. Plant Dis. 91:772, 2007. (3) S. Zhao et al. Plant Dis. 92:318, 2008.
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