No abstract

Mulberry (Morus alba L.) is an important cash crop and medicinal plant that has been cultivated for more than 5,000 years in China. The area of mulberry production in Guangxi Province is 45% of total production in China, with 1.3 million ha planted. In recent years, a mulberry root rot occurred in Heng County covering all the mulberry planting farms. Observations of 200 diseased plants were made. The xylem of infected roots first turned brown, and then became black followed by cortex rot. The xylem and cortex of infected roots were easily separated. The xylem of the stem of symptomatic plants was also brown and the bark was slightly darker than normal. Leaves of diseased plants turned yellow and wilted, but the wilted leaves remained on the affected branches for about 3 weeks. All affected branches and stem dried after a month. The affected area was 12,000 ha with incidences varying from 13 to 52%. About 8% of young mulberry trees died in severely infested orchards. The disease caused more than $3 million in losses within a year in Heng County alone. The causal fungus was isolated from xylem tissues of symptomatic roots of 62 mulberry plants with an isolation rate of 90%. Pathogenicity test was made by inoculating 5-month-old healthy mulberry plants with PDA plugs (5 × 5 mm) grown 5 days with viable mycelia of the fungus. Nine healthy plants were wounded on the roots with a sterile knife, and mycelial plugs of three Lasiodiplodia theobromae (Pat.) Griffon & Maubl isolates were placed on the wounds, covered with sterile moist cotton, and wrapped with Parafilm. Nine control plants were treated with PDA plugs. The test was repeated three times. All treated plants were kept in a greenhouse at ~28°C and 40% RH. After 3 days, the root xylem of inoculated plants turned brown and gradually became dark, similar to symptoms observed in the field. After 8 days, inoculated seedlings gradually wilted, and all the treated plants died after 11 days with leaves undetached. The fungus was re-isolated from all nine diseased plants and no symptoms were observed on the roots of control plants. The causal agent, of which conidia were dark brown, one-septate, thick walled, and ellipsoid with 4 or 6 vertical lines of dashes, 12.50 to 13.75 × 13.75 to 25.63 μm (n = 100), was identified as L. theobromae based on morphological characters described by Punithalingam (3) and sequences of the ITS region of rDNA using primers ITS1 and ITS4 and EF1-α using primers EF728F and EF986R. The ITS sequence (HG917932) was similar to the ITS sequences of AY640255 (CBS164.96) and AY236952 (CMW9074) in GenBank with identities of 98.8 and 99.8%, respectively. The EF1-α sequence HG917934 was similar to that of AY640258 (CBS164.96) and AY236901 (CMW9074) with identities of 99.7 and 99.7%, respectively. L. theobromae is a cosmopolitan fungus causing both field and storage diseases on more than 280 plant species including crops, fruits, and cash fruit trees (1,2,5). Mulberry root rot caused by L. theobromae has been reported in India (4) and ours is the first report in China. This finding clarifies the pathogen of mulberry root rot previously thought as Fusarium sp. in China, which is critical to develop management strategies to control this disease. References: (1) N. M. Celiker and T. J. Michailides. New Dis. Rep. 25:12, 2012. (2) I. H. Fischer et al. Australia Plant Dis. Notes 3:116, 2008. (3) E. Punithalingam. Botryodiplodia theobromae. CMI Descriptions of Pathogenic Fungi and Bacteria No. 519. CAB International, Wallingford, UK, 1976. (4) N. V. Radhakrishnan et al. Indian Phytopathol. 48:490, 1995. (5) B. C. Sutton. The Coelomycetes. Commonwealth Mycology Institute, Kew, Surrey, England, 1980.

No abstract

Sisal (Agave sisalana Perrine) is an important hard fiber crop that is widely planted in Guangxi, Guangdong, Hainan, Yunnan, and Fujian provinces, China. In July 2019, a new leaf disease of sisal with a disease incident of about 36% was found in Guangxi (Fig.1a~d). The oval or circular black lesions were 2.3 cm to 15.9 cm in length and 1.6 cm to 5.5 cm in width on both sides of the diseased leaves. The central part of the lesions was slightly hollow. The lesions continuously enlarged and ultimately penetrated the leaves. Reddish brown and dark mucus was secreted from the lesions. The junction of lesions and healthy parts was reddish brown to yellow. The diseased leaf fiber and mesophyll tissues were reddish brown and necrotic. Fresh leaf yield was reduced about 30% by the disease, and fiber quality was significantly compromised every year in Guangxi. Six kinds of fungi distinguished by their morphology, size and color of the colonies were isolated from diseased leaf tissues of 60 sisal plants sampled from five different farms in Guangxi. Isolate JMHB1 was isolated at a rate of 95.67%. The isolate JMHB1 was initially white with dense and hairy aerial mycelium, gradually turning dark grey to olive green on PDA (Fig. 2). Conidia, arthrospores, and chlamydospores were observed on PDA in culture (Fig. 3). The conidia formed arthric chains, disarticulating, cylindrical-truncate, oblong-obtuse to doliiform, colorless and transparent, zero- to one-septate, and averaging 4.4 to 13.8 µm × 2.2 to 5.6 µm (n=100). Arthrospores were short columnar, pigmented and transparent, single or formed arthric chains, averaging 5.5 to 17.9 µm × 2.1 to 3.5 µm (n=100). Chlamydospores were dark brown, round or oval, averaging 4.5 to 9.6 µm × 4.5 to 8.6 µm (n=100). Pathogenicity testing was conducted by inoculating 3-year-old healthy sisal plants with PDA plugs (5 × 5 mm) on which the fungus had grown for 5 days. Nine healthy plants were wounded on the leaves with a sterile needle, and mycelial plugs were placed on the wounds, covered with sterile moist cotton, and wrapped with parafilm. Nine control plants were wounded and treated with PDA plugs as the negative control. The test was repeated three times. All treated plants were kept in a greenhouse at ~28 ℃ and 40% RH. After 5 days, only leaves inoculated with isolate JMHB1 showed lesions similar to symptoms observed in the field (Fig.1e~f). The fungus was re-isolated from all nine diseased plants, and no symptoms were observed on the leaves of control plants. Molecular identification of the fungus was made by PCR amplification of the internal transcribed spacer (ITS) region of rDNA, EF1-α gene and β-tubulin gene using primers ITS1/ITS4 (White et al. 1990), EFl-728F/EF1-986R (Carbone and Kohn 1999), TUB2Fd/TUB4Rd (Aveskamp et al. 2009) respectively. The ITS (MT705646), EF1-α (MT733516) and β-tubulin (MT773603) sequences of JMHB1 were similar to the ITS (AY819727), EF1-α (EU144063) and β-tubulin (KF531800) sequences of the epitype of Neoscytalidium dimidiatum (CBS 499.66) with 100%, 99.65% and 99.02% identity, respectively. Based on pathogenicity testing, morphological characteristics, and molecular identification, the pathogen of sisal causing black spot was identified as N. dimidiatum (Penz.) Crous & Slippers (Crous et al. 2006). To our knowledge, this is the first report of black spot caused by N. dimidiatum on sisal in China. Sisal is the main economic crop in arid and semi-arid areas that is widely planted in several provinces of southern China. The serious occurrence of the disease caused by N. dimidiatum has greatly affected the development of sisal industry and local economic income in China. Identification of the pathogen of the disease is of great significance to guide disease control, increase farmers' income and promote the development of sisal industry. References: Aveskamp, M. M., et al. 2009. Mycologia, 101: 363. https://doi.org/10.3852/08-199. Carbone, I., and Kohn, L. M. 1999. Mycologia, 91:553. https://doi.org/10.1080/00275514.1999. 12061051. Crous, P. W., et al. 2006. Stud. Mycol. 55:235. https://doi.org/10.3114/sim.55.1.235. White, T. J., et al. 1990. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, Page 315. doi.org/10.1002/mrd.1080280418. Supplemental photographs: Fig. 1 Symptoms of sisal black spot disease a, b, c, d showed symptoms in the field, e and f were symptoms after inoculating Neoscytalidium dimidiatum JMHB1. a, c, and e were the front of the lesions, b, d, and f were the back of the lesions. Fig. 2 Primary colony (a) and old colony (b) of Neoscytalidium dimidiatum JMHB1 Fig. 3 Arthrospores (a), conidia and chlamydospores (b) of Neoscytalidium dimidiatum JMHB1