New Disease Reports (2019) 39, 7. [http://dx.doi.org/10.5197/j.2044-0588.2019.039.007]
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First report of Phomopsis asparagi causing stem blight on asparagus in Vietnam

L.D. Thao 1* and N.T. Dung 2

*thaoledinh.ppri@mard.gov.vn

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Received: 25 Jan 2019; Published: 23 Feb 2019

Keywords: alpha conidia, ITS region, Phoma

Asparagus (Asparagus officinalis) has become an economically important crop in many regions of Vietnam. A survey conducted in the asparagus growing areas of North Vietnam in 2018 revealed that more than 70% of asparagus plants were exhibiting disease symptoms. Oval-shaped lesions with dark brown or black pycnidia were found on all aerial parts of the plant, extending and covering large areas of stems (Figs. 1-2). These symptoms were similar to those caused by Phomopsis asparagi (syn. Phoma asparagi), the most serious pathogen of cultivated asparagus in many parts of the world (Elena, 2006; Zaw et al., 2017).

Diseased samples were collected from Hanoi, Bac Ninh and Bac Giang regions. Pathogens were isolated on water agar (2%) and purified by single spore isolation. A representative isolate PPRI1809.1 was chosen for further study and stored in the culture collection of the Plant Protection Research Institute of the Vietnam Academy of Agricultural Sciences. Fungal mycelium on potato dextrose agar was white to grey and produced aggregated pycnidia after three weeks of incubation at 25°C (Fig. 3). Alpha conidia were formed inside pycnidia, and were hyaline, biguttulate, aseptate, spindle-shaped and tapering towards both ends and measured 8.9 ± 1.0 µm x 3.2 ± 0.2 µm (n=30), (Fig. 4). Based on these morphological characters, the fungus was initially identified as P. asparagi (Uecker & Johnson, 1991; Thao et al., 2018).

The identity of the causal pathogen was confirmed by the analysis of the ITS region sequence using the primers ITS1 and ITS4 (White et al., 1990). The raw sequence was assembled with MEGA 7.0 and the consensus sequence deposited in GenBank (Accession no. MK400353). A BLAST analysis showed that the ITS sequence from PPRI1809.1 shared 99% identity with that of P. asparagi (JQ619528, LC203583 and KY16055).

To fulfil Koch's postulates, isolate PPRI1809.1 was used to inoculate shoots of healthy asparagus plants. A conidial suspension in sterile distilled water (10 conidia/ml) was sprayed on three asparagus plants (one shoot per plant). One plant was inoculated with sterile distilled water as a control. Inoculated plants were placed in a greenhouse at 25°C for two weeks and the experiment was repeated three times. Typical stem blight symptoms (Fig. 5) were observed 10 days post inoculation and the pathogen was successfully re-isolated from symptomatic tissue. No symptoms developed and no pathogens were isolated from the control plants.

This is the first report of P. asparagi infecting asparagus in Vietnam. The disease was widely spread in the northern regions of Vietnam in August and September 2018, when the temperature is around 25-30°C and the humidity is high. This disease has the potential to become a major threat to the asparagus production of Vietnam.

Figure1+
Figure 1: Stem blight symptoms observed in asparagus field.
Figure 1: Stem blight symptoms observed in asparagus field.
Figure2+
Figure 2: Typical symptoms of asparagus stem blight.
Figure 2: Typical symptoms of asparagus stem blight.
Figure3+
Figure 3: Three-week-old colony of Phomopsis asparagi (isolate PPRI1809.1) on potato dextrose agar.
Figure 3: Three-week-old colony of Phomopsis asparagi (isolate PPRI1809.1) on potato dextrose agar.
Figure4+
Figure 4: Alpha conidia of Phomopsis asparagi. Bar = 20 µm.
Figure 4: Alpha conidia of Phomopsis asparagi. Bar = 20 µm.
Figure5+
Figure 5: Symptoms developed ten days after infecting healthy plants with Phomopsis asparagi (isolate PPRI1809.1) in a pathogenicity assay.
Figure 5: Symptoms developed ten days after infecting healthy plants with Phomopsis asparagi (isolate PPRI1809.1) in a pathogenicity assay.

References

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  2. Thao LD, Zaw M, Matsumoto M, 2018. Diaporthe species complex occurring on Asparagus kiusianus in Japan. Journal of Plant Pathology 101, 161-167.
  3. Uecker FA, Johnson DA, 1991. Morphology and taxonomy of species of Phomopsis on Asparagus. Mycologia 83, 192-199. [http://dx.doi.org/10.1080/00275514.1991.12025995]
  4. White TJ, Bruns T, Lee S, Taylor J, 1990. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. In: Innis MA, Gelfand DH, Sninsky JJ, White TJ, eds. PCR Protocols: A Guide to Methods and Applications. San Diego, CA, USA: Academic Press, 315-322. [http://dx.doi.org/10.1016/B978-0-12-372180-8.50042-1]
  5. Zaw M, Naing TAA, Matsumoto M, 2017. First report of stem blight of asparagus caused by Phomopsis asparagi in Myanmar. New Disease Reports 35, 17. [http://dx.doi.org/10.5197/j.2044-0588.2017.035.017]

To cite this report: Thao LD, Dung NT, 2019. First report of Phomopsis asparagi causing stem blight on asparagus in Vietnam. New Disease Reports 39, 7. [http://dx.doi.org/10.5197/j.2044-0588.2019.039.007]

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