First Report of black spot of Shine Muscat fruit caused by Cladosporium allicinum in China

Shine muscat is a Vitis vinifera hybrid (Akitsu-21 × Hakunan), that has emerged as a popular table grape cultivar in China. In recent years, shine muscat has been widely cultivated with 66,667 ha being cultivated in 2021. In November 2021, symptoms of fruit spot were observed on shine muscat during the storage at 0~3℃ and 85%~90% RH, while stored at National Agricultural Product Preservation Engineering Technology Research Center, in Tianjin (N 116°20', E 39°09'), China. The incidence of this disease was about 35%. Affected grape berries initially had small brown spots. The spots on the fruit expanded to an ellipse or circular sunken area with a black center. The central peel of the diseased spots were ruptured and collapsed. The diseased fruits eventually fell off the vine. To isolate the pathogen, grape peels with typical symptoms were cut into small pieces, sterilized with 75% ethanol for 45-sec, rinsed with sterilized distilled water three times, and then transferred onto potato dextrose agar (PDA) medium.The plates were incubated at 25°C in the dark. After 10 days, 26 single spore isolates with similar morphology were obtained from 30 symptomatic grape berries. Fungal colonies were grayish brown, with abundant conidia on the obverse-side on PDA. Conidiophores were cylindrical, straight with unbranched, solitary or clustered, elongation at the tip and ranged in size from 3.2-6.8 × 35.6-150.9 µm (n=50). Conidia were grew in chains, ovoid, aseptate, and 2.2-6.0 × 8.3-16.8 µm (n=50). The morphological characteristics were consistent with Cladosporium allicinum (Bensch et al. 2012). Molecular data were also used to support the microscopic identification by extracting genomic DNA from 26 isolates using a Plant Genomic DNA kit (Tiangen, China). Amplicons were generated for the internal transcribed spacer (ITS), translation elongation factor 1-alpha(tef1-α), and actin (act) using the following primers ITS1/ITS4, EF1-728F/ EF1-986R and ACT-512F/ ACT-783R, respectively (Bensch et al. 2012). Blast analysis showed that three amplified fragments of 26 isolates were highly similar to C. allicinum, with 98.96~100% sequence identity with Cladosporium allicinum accessions in GenBank (ITS, OK661041; tef1-α, MF473332; act, LN834537). Three amplified fragments of representative isolate YG03 were deposited in GenBank with accession nos. OP799670 for ITS, OP888001 for tef1-α and OP887999 for act, respectively. Neighbor-joining trees based on concatenated sequences of three genes were constructed using MEGA5.2. The results showed that the strain YG03 from shine muscat was closely related to C. allicinum. Pathogenicity tests of 26 isolates were performed on healthy shine muscat berries using pin pricks and a humidor. In each wound, 5 μL of conidial suspension (1×106 conidia/mL) and sterile distilled water were inoculated on 30 berries, and maintained in a dark incubator at 25°C, 90% relative humidity. Each treatment was repeated twice. After 10 days, the wounded berries inoculated with the spore suspension showed dark brown spots, similar to the original diseased fruits, while no symptoms were observed on the control treament. Pathogen re-isolated from inoculated fruits were identical to the original strains on colony and microscopic morphology, and identified to Cladosporium allicinum based on act gene by molecular method, thereby fulfilling Koch's postulates. C.allicinum has been reported causing leaf spot on 11 host plants around the world (Bensch et al. 2012, 2015; Quaedvlieg et al. 2014; Jurisoo et al. 2019). To our knowledge, this is the first report of C. allicinum causing black spot on fruit of Vitis vinifera worldwide. The identification of this disease could establish a foundation for developing management strategies to reduce losses in storage period.

Stemphylium lycopersici Causing Leaf Spot of Watermelon (Citrullus lanatus) in China

Watermelon (Citrullus lanatus) is an important cucurbit crop in China. During September 2020, an unknown leaf spot disease was observed on watermelon in two greenhouses (640m2 per greenhouse) of Sangzi town, Jizhou district, in Tianjin, China (117°10'E, 39°55'N), where approximately 10% of plants were infected. Disease symptoms began as small, circular, brown spots on leaves. As these spots increased in size, they developed confluent, irregular lesions surrounded by dark brown edges. Severely affected plants had many wilted leaves followed by defoliation. Ten symptomatic leaves were collected for pathogen isolation. Diseased tissues (3×3 mm) were cut from the margins of lesions and surface disinfected with 1% NaClO for 1 min, rinsed three times with sterile distilled water and then placed on potato dextrose agar (PDA) at 25±2°C with a 12-h photoperiod for 7 to 10 days. Seven morphologically similar isolates were obtained from the ten infected leaves and purified by single-spore culturing for further study. The initial growth of the isolates on PDA appeared grayish white in obverse and bright yellow pigmentation in reverse. Colony color gradually deepened to grayish brown in obverse and brownish red in reverse. Conidia (n=50) were solitary, light brown, oblong to long elliptic, pointed or obtusely rounded at the top, constricted at the transverse septum, with verrucous processes on the surface, 36.3 to 64.2×16.6 to 25.1 μm, and the L/W ratio of conidia was 1.5-2.5. All characteristics were consistent with the description of Stemphylium lycopersici (Ellis 1971; Woudenberg et al. 2017). Total genomic DNA was extracted from a representative isolate (XG2-2) using a Fungal DNA Kit (GBCBIO, Guangzhou, China). The internal transcribed spacer (ITS) and translation elongation factor 1-α (EF1-α) genes (Sun et al. 2015) were amplified and sequenced with the primer pairs ITS1/ITS4 (5'-TCCGTAGGTGAACCTGCGG-3'/5'-TCCTCCGCTTATTGATATGC-3') and EF-1α-F/EF-1α-R(5'-TCACTTGATCTACAAGTGCGGTGG-3'/5'-CGATCTTGTAGACATCCTGGAGG-3'), respectively. The two sequences of strain XG2-2 (GenBank Accession No. MW362344 and MW664941) showed 100% and 99% identity to S. lycopersici strain 01 and strain KuNBY1 (GenBank Accession No. KR911814 and AB828256) respectively. The phylogenetic analysis using MEGA7 based on the sequences of ITS and EF1-α regions showed that the isolate XG2-2 was clustered with S. lycopersici isolates (strain 01 and strain KuNBY1). For the pathogenicity test, a spore suspension (1×106 spores/ml) in sterile distilled water from a 7-day-old culture of the fungus grown on PDA and counted with a hemacytometer was sprayed on leaves and stems of five healthy watermelon plants, grown for 2 months in the greenhouse at 25 to 30 °C, with 85% relative humidity. Conditions remained the same for inoculation experiments. Negative controls were healthy plants inoculated with sterile distilled water. The experiment was repeated twice. Six days after inoculation, typical leaf spot symptoms were observed on inoculated leaves, whereas control leaves remained symptomless. To satisfy Koch's postulates, the causal fungus was re-isolated from the lesions of inoculated plants, with morphological and cultural characteristics identical with the original isolate. Stemphylium lycopersici is a common fungus with a relatively extensive host range (Kee et al. 2018). In recent years, new host plants infected by S. lycopersici have been reported in Asia including Physali (Yange et al. 2020), common bean (Li et al. 2019), and potato (Kee et al. 2018). To our knowledge, this is a new host record for S. lycopersici causing leaf spot on watermelon in China. Sangzi watermelon is a special local product in the Jizhou district of Tianjin. At present the cultivated area in 1000 ha including 667 ha in controlled conditions and 333 ha of field-grown plants with a total annual output of 45000 Mg. In this survey, we found the disease caused by S. lycopersici on watermelon only in these two greenhouses, but cannot rule out the possibility of large-scale spread in the future. Therefore, integrated management strategies for this fungus need to be developed to reduce economic losses in commercial cultivation.

First Report of Colletotrichum capsici Causing Anthracnose on Alocasia macrorrhizos in China

Alocasia macrorrhizos (Linnaeus) G. Don is a perennial herb in the Araceae family. It is native to South Asia and the Asia-Pacific and has long been cultivated as it is an economically important medicinal and ornamental plant. During July 2012 and 2013, severe outbreaks of leaf spot and stem rot disease on this plant occurred in a greenhouse of Shunyi district, in Beijing, China (117°05'E, 40°13'N). The disease incidence was greater than 30%. The leaf spots first appeared as yellow dots. As lesions expanded, the symptoms were circular to subcircular, light brown lesions with darker brown edges, Around the lesions the leaf tissue was chlorotic causing the formation of a yellow halo (Suppl. Fig1). Initial symptoms on the stems were brown, round or fusiform spots . As the disease progressed, lesions enlarged and merged together. When humidity was high, black acervuli with grey brown cirrhus of conidia were rapidly produced in lesions. Infected plants eventually withered or collapsed from the stem rot (Suppl. Fig2). Infected tissues were surface-sterilized in 1% NaOCl for 1 min, washed three times with distilled water, and placed on potato dextrose agar (PDA). Colonies on PDA, growing at 25°C in darkness, showed grayish brown and grey brown conidial masses produced from acervuli with black seta (Suppl. Fig3). Acervuli (n=30) were dark brown to black and approximately round, 121 to 210 μm in diameter, averaging 166.5 μm (Suppl. Fig4). Setae (n=30) scattered in acervuli, black, septate, 94.4 to 128.4×3.4 to 4.7 μm, base inflated, and narrower toward the top (Suppl. Fig5). Conidiophores (n=50) were phialidic, hyaline, unicellular. Conidia (n=50) were hyaline, monospora, falcate, base obtuse, apices acute, and 20.5 to 24.7 ×2.8 to 3.4 μm (Suppl. Fig6). Six monoconidial isolates were made, and the morphological characteristics of the fungus were similar to those of Colletotrichum capsici (Syd.) Butler & Bisby (Mordue, 1971). In the greenhouse (25 to 30 °C, relative humidity 98%), pathogenicity tests were conducted by spraying a 106 spores /mL suspension on leaves and stems of 10 healthy potted A. macrorrhizos plants (3-year-old). A control was included that consisted of ten plants sprayed with sterile distilled water. All treated plants were covered with a plastic bag and removed 48 h later. After 12 days, all inoculated leaves and stems appeared with typical Anthracnose symptoms, whereas control plants remained healthy. The fungus was reisolated from diseased tissues, fulfilling Koch´s postulates. The ITS region of a representative isolate was amplified and sequenced using the primers ITS1/ITS4 (White et al. 1990).The obtained ITS sequence (GenBank Accession No. KJ018793.1) showed 100% similarity to Colletotrichum capsici (Accession No. HQ271469.1 and DQ454016.1). Colletotrichum capsici is synonymous to Colletotrichum truncatum. Colletotrichum capsici is a major phytopathogen with a broad host range which causes anthracnose disease. The first report of C. capsici as a pathogen of Alocasia macrorrhizos was reported in India in 1979 (Mathur, 1979). To our knowledge, this is the first record of C. capsici causing anthracnose on A. macrorrhizos in China.

First Report of Corynspora cassiicola Causing Leaf Spot on Jasminum sambac in China

Jasminum sambac (Linnaeus) Aiton belonging to Oleaceae family, has been cultivated in China over 1500 years. It is an important tea crop in South China, and popular ornamental plant in North China. In June 2015, Corynespora leaf spot on Jasminum sambac was first observed in Caozhuang in Tianjin(E: 117°20', N: 39°13'). In a series of investigations, the disease incidence of Corynespora leaf spot on Jasminum sambac in Tianjin was found to be as high as 20%. The disease was also found in flower planting greenhouses in Beijing and Hebei region. The initial symptoms appeared as small circular spot with gray white in the center and dark brown in the edge. The lesions expanded rapidly to 5-10 mm diameter, surrounded by a yellow halo. The affected leaves became covered with the circular spots and defoliated eventually. Fifty-six leaf samples were collected from 20 greenhouses, then directly examined and disease tissues set aside for isolation. Conidiophores were either solitary or in clusters, straight or curved and un-branched, with 3-10 septate, pale to dark brown in color. Those were measured 80.6 to 234.8 μm long and 4.5 to 8.0 μm wide in size and were formed with 0-8 successive cylindrical proliferation (n = 50). Conidia borne from conidiophore, were single or in chains, obclavate to cylindrical, smooth. The average size of 50 conidia was 43.6 to 130.7 μm long×6.4 to 13.5 μm wide, with 4 to 8 pseudosepta (n = 50). The infected leaves were cut into small pieces, sterilized with 75% ethanol, rinsed in sterilized distilled water and then plated on potato dextrose agar (PDA). The isolated plates were incubated at 25℃ for 5 days. Thrity-five pure culture isolates were obtained from diseased leaves. The colonies were pale brown on the front and dark brown on the underneath, with thickly aerial mycelia. Morphological characteristics of the pathogen from the infected leaves and purified strains matched descriptions of Corynesora cassicola (Ellis and Holliday 1971). To confirm the pathogenicity, conidial suspensions (106 conidia mL-1 ) of 35 isolated strains were sprayed on the healthy plants at the seedling stage of Jasminum sambac respectively, 5 plants per strains. 10 healthy plants were sprayed only with sterilized water as control. Conidial suspensions were prepared by washing conidia from the pure C. cassiicola cultures. Conidial suspensions were adjusted to a concentration of 1 × 106 conidia/mL with the hemocytometer. All plants were incubated in artificial climate-room at 90% relative humidity, 25±2℃. After 7 days, small circular spots, similar to the field samples were observed on all plants inoculated with the pathogen. By comparison, control plants displayed no symptoms. The same pathogens were re-isolated from the symptomatic leaves fulfilling Koch's postulates. For molecular identification, rDNA-ITS and two housekeeping genes (TUB2 and EF-1α) gene were amplified and sequenced (Zhu et al. 2020). The sequences of three high pathogenicity isolats showed 99% homology to C.cassiicola (MK571399) with 464 bp for rDNA-ITS, 100% homology to C.cassiicola (MN512639) with 469 bp for TUB2 gene, 98.61% homology to C.cassiicola (MH263735) with 287 bp for EF-1α gene. And all 9 sequences were submitted to GenBank (GenBank accession No. MT334575-77 for rDNA-ITS, MT886387-89 for TUB2, and MT886390-92 for EF-1α).The molecular identification and morphological characteristics showed that the isolated strains from Jasminum sambac were identified as C.cassiicola. C. cassiicola has an extensive host range, covering 397 host plants (Farr and Rossman 2020). Recently, C.cassiicola has been reported on Jasminum mesnyi of Oleaceaein China (Zhang et al. 2018). To our knowledge, this is the first report of leaf spot caused by C.cassiicola on Jasminum sambac in China and also worlwide.

Quantification of Viable Cells of Pseudomonas syringae pv. tomato in Tomato Seed Using Propidium Monoazide and a Real-Time PCR Assay

Pseudomonas syringae pv. tomato is a seedborne pathogen that causes bacterial speck disease in tomato. P. syringae pv. tomato is typically detected in tomato seed using quantitative real-time PCR (qPCR) but the inability of qPCR to distinguish between viable and nonviable cells might lead to an overestimation of viable P. syringae pv. tomato cells. In the present study, a strategy involving a propidium monoazide (PMA) pretreatment followed by a qPCR (PMA-qPCR) assay was developed for quantifying viable P. syringae pv. tomato cells in contaminated tomato seed. PMA could selectively bind to the chromosomal DNA of dead bacterial cells and, therefore, block DNA amplification of qPCR. The primer pair Pst3F/Pst3R was designed based on gene hrpZ to specifically amplify and quantify P. syringae pv. tomato by qPCR. The PMA pretreatment protocol was optimized for selectively detecting viable P. syringae pv. tomato cells, and the optimal PMA concentration and light exposure time were 10 μmol liter^-1 and 10 min, respectively. In the sensitivity test, the detection limit of PMA-qPCR for detecting viable cells in bacterial suspension and artificially contaminated tomato seed was 10² CFU ml^-1 and 11.86 CFU g^-1, respectively. For naturally contaminated tomato seed, viable P. syringae pv. tomato cells were quantified in 6 of the 19 samples, with infestation levels of approximately 10² to 10⁴ CFU g^-1. The results indicated that the PMA-qPCR assay is a suitable tool for quantifying viable P. syringae pv. tomato cells in tomato seed, which could be useful for avoiding the potential risks of primary inoculum sources from contaminated seed.