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Date: January, 2008
Source: British Society for Plant Pathology, New Disease Reports (NDR) vol. 16 [edited]
<http://www.bspp.org.uk/ndr/jan2008/2007-77.asp>

Melon necrotic spot virus newly reported in China

During April 2007, systemic necrotic spots were observed on melon (_Cucumis melo_) grown in plastic greenhouses in Haimen city of Jiangsu Province, China. By mid May [2007] disease rates in different plastic houses ranged from 19 to 100 percent with symptoms characteristic of _Melon necrotic spot virus_ (MNSV). These symptoms included many necrotic spots on the young leaves (Fig. 1a) followed by coalescence of spots to form larger irregular lesions on older leaves (Fig. 1b). Necrotic stripes appeared on stems, and the infected leaves also became curled and wilted.

Sap extracts from diseased plants were mechanically inoculated to melon, watermelon (_Citrullus lanatus_), and cucumber (_Cucumis sativus_). Melon, watermelon, and cucumber all showed local necrotic spots on cotyledons 3 to 5 days post inoculation in the greenhouse, and 6 days post inoculation under 20 deg C (68 deg F) in a growth
chamber. Systemic necrotic spots were observed only on melon plants and began to appear 6 to 7 days post inoculation in the greenhouse and 9-14 days post inoculation at 20 deg C (68 deg F) in a growth chamber (Fig. 2a and Fig. 2b).

For further confirmation of the causal agent, polymerase chain reaction (PCR) was used to amplify a 710 basepair region of the MNSV coat protein gene. The expected PCR product was obtained from virus infected fresh leaves, but not from healthy plants. The alignment of a 673 basepair region of the amplified nucleotide sequence showed 93 and 92 percent identity to MNSV isolates from Spain and the Netherlands, respectively.

MNSV has been previously reported in Japan, the Netherlands, USA, UK, Greece, Korea, Spain, and other countries. To the best of our knowledge, this is the 1st report of MNSV in China.

Figure 1a: Necrotic spots on young leaves
<http://www.bspp.org.uk/ndr/jan2008/2007-77-1a.jpg>
Figure 1b: Coalesced spots forming large irregular lesions on older leaves
<http://www.bspp.org.uk/ndr/jan2008/2007-77-1b.jpg>
Figure 2a: Systemic necrotic spots on muskmelon 7 days post inoculation
<http://www.bspp.org.uk/ndr/jan2008/2007-77-2a.jpg>
Figure 2b: Systemic necrotic spots on honeydew melon 7 days post inoculation
<http://www.bspp.org.uk/ndr/jan2008/2007-77-2b.jpg>

[Q.-S. Gu et al]

--
Communicated by:
ProMED-mail
<promed@promedmail.org>

[Melon necrotic spot virus (MNSV; genus _Carmovirus_, family _Tombusvirus_) was first reported in Japan in 1959. It has since spread widely resulting in worldwide economic losses in production of melons and other cucurbits. Recently, serious outbreaks of disease associated with new strains of MNSV have occurred at various sites in Japan where the virus is reported to be an especially serious threat to glasshouse crops. Symptoms may vary at different growth stages of host plants and include chlorotic or necrotic spots on leaves, necrotic streaks in stems, and occasionally plant death. The virus also causes sponge-like symptoms and crevices in melon fruits that greatly decrease their commercial value. External appearance of affected fruits remains normal so that it is impossible to recognise and remove them prior to sale.

MNSV is transmitted through the soil by the fungal vector _Olpidium bornovanus_ (and/or _O. radicale_, the taxonomy of these species is apparently still under dispute) via zoospores. Fungal resting spores can survive in the soil for years. The virus (directly or via its vector fungus) can also be spread by seed (for example at a rate of 10-40 percent in melon), plant-to-plant contact, and mechanical means. Since the virus may be both seed-borne and soil-borne, it may be readily distributed through the cucurbit seed market to become endemic in a district, where it can then spread locally and persists for a long period in association with its soil-borne fungal vector. Soil fumigation by methyl bromide was used previously to sanitise the soil between cultivations. The use of this chemical is now prohibited in many countries for environmental reasons. Disease management therefore relies on use of certified virus-free seed and phytosanitary cultural practices. Melon germplasm with a single
recessive resistance gene preventing the accumulation of viral RNAs has been identified and is used in developing cucurbit varieties resistant to MNSV.

_O. bornovanus_ is also the vector for other cucurbit viruses in the same family of _Tombusvirus_, for example _Cucumber leaf spot virus_ and _Cucumber necrosis virus_. A number of viruses of different crops are spread by the related fungus _O. brassicae_.

Maps
China:
<http://www.chinapage.com/map/map.html> and
<http://healthmap.org/promed?v=36.5,103.9,4>
Provinces of China:
<http://www.sacu.org/provmap.html>
Jiangsu Province:
<http://www.maps-of-china.com/jiangsu-s-ow.shtml>

Pictures
MNSV leaf symptoms:
<http://www.nogyo.tosa.net-kochi.gr.jp/byoki/boujosho/library/siryou/jpg/esohantenha.jpg>
Internal fruit symptoms on watermelon:
<http://www.nogyo.tosa.net-kochi.gr.jp/byoki/boujosho/library/siryou/jpg/esohantenka.jpg> 
and
<http://www.nogyo.tosa.net-kochi.gr.jp/byoki/boujosho/library/siryou/jpg/esohantenkabubun.jpg>

Links
Disease information (subscription only):
Kubo et al., Plant Pathology (2005) 54, 615-620
<http://www.blackwell-synergy.com/doi/full/10.1111/j.1365-3059.2005.01253.x> 
and
Kazutaka et al., European J. Plant Pathology 2007, published online
<http://www.springerlink.com/content/c43960j7g54p4457/>
New severe MNSV strains:
<http://www.cababstractsplus.org/google/abstract.asp?AcNo=20053166574>
MNSV taxonomy:
<http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.074.0.02.009.htm>
_O. bornovanus_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=480421>
_O. radicale_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=276024>
_Olpidium_ accepted species names:
<http://www.umaine.edu/chytrids/Chytrid%20Bibliography/M-O.html>
Cucurbit viruses transmitted by strains of _O. bornovanus_:
<http://cat.inist.fr/?aModele=afficheN&cpsidt=3530657>
List of cucurbit diseases and pathogens:
<http://www.apsnet.org/online/common/names/cucurbit.asp>. - Mod.DHA]

[see also in the archive:
2007
----
Big vein virus, lettuce mosaic virus - USA 20070325.1034
2006
----
Black scorch virus, sugar beet - USA: 1st report 20060606.1575
2004
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Cucumber leaf spot virus - Poland: 1st report 20041204.3238
Mirafiori lettuce virus - Chile (Chacabuco Province) 20041105.3002
2003
----
Mirafiori lettuce virus - Brazil (Sao Paulo State) 20030614.1462
2001
----
Cucumber leaf spot, cucumber - Spain 20011107.2762]
 

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