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Melon necrotic spot virus

  • Black spot on watermelon leaves
    Black spot on watermelon leaves
  • Browning of rind on watermelon
    Browning of rind on watermelon
  • Necrosis on watermelon stems
    Necrosis on watermelon stems
  • Symptoms inside rockmelon fruit.
    Symptoms inside rockmelon fruit.

Have you seen Melon necrotic spot virus (MNSV)?

Be on the lookout and report it.

Under Queensland legislation, if you suspect the presence of Melon necrotic spot virus, you must report it immediately to Biosecurity Queensland on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

Early detection and reporting are key elements in controlling this priority plant disease.

Fruit is not considered to be a pathway for the spread of MNSV. However, Queensland has regulated entry conditions for the import of MNSV carriers from other states in which MNSV is present.

MNSV carriers include cucurbit plants (rockmelon, watermelon, cucumber and honeydew melon), soil or other growing mediums that have come into contact with a cucurbit plant, and appliances that have been used for planting, producing or harvesting a cucurbit plant.

View the entry conditions for MNSV carriers in the Queensland Biosecurity Manual .

General information

Melon necrotic spot virus (MNSV) causes a serious disease of cucurbit crops, including cucumber, honeydew melon, rockmelon (also known as cantaloupe or muskmelon) and watermelon.

MNSV has been detected sporadically in Australia, with outbreaks in New South Wales in 2012 and in Victoria in 2016.  Unlike overseas detections, MNSV infection has not persisted after detection in Australia. The reasons for this are unclear, but may be related to climate and/or the behaviour of its main vector Olpidium bornovanus (a microscopic soil-inhabiting fungus-like organism).

MNSV has also been reported to be transmitted by the western spotted cucumber beetle (Diabrotica undecimpunctata undecimpunctata) and the banded cucumber beetle (Diabrotica balteata) (Coudriet et al, 1979).

Cause

Melon necrotic spot virus (MNSV) is a plant virus from the genus Carmovirus, in the family Tombusviridae; and is comprised of tiny particles.

MNSV particles are very robust, and are able to survive in soil without plant host material or the vector (Olpidium bornovanus) for several years.

A number of MNSV strains with differing symptoms have been reported.  A strain reported in rockmelon from Japan (Ohki et al, 2008) did not infect the entire rockmelon plant (Cucumis melo).

MNSV does not affect human or animal health.

Species name

MNSV, muskmelon necrotic spot virus, necrotic spot of melon

Description

Melon necrotic spot virus (MNSV) infection produces a range of different necrotic spots and lesions on host plant leaves, stems and fruit. Different host plants and virus strains can cause varied symptoms.

In Australia, MNSV has only been detected on watermelons and rockmelons.

Cooler temperatures increase the severity of symptoms and reduce host plant resistance.

Fruit

Rockmelon fruit surface lesions are relatively large, sunken and make the fruit misshapen. On watermelon, skin lesions tend to remain small, and can give the fruit a rough looking appearance.  Internal fruit quality is often severely impacted, with internal rotting or ‘mushiness’ common.  In severe cases, fruit may start to rot prior to harvest.

The most distinctive feature of MNSV infection is the brown stains/patches that develop throughout the watermelon and rockmelon fruit flesh and the seed cavity.

Leaves

Small transparent spots appear on new leaves of both rockmelon and watermelon. These spots turn brown and grow in size as the leaf matures. Leaves curl and wilt and may die. Brown necrotic lesions may also develop on leaf petioles and plant stems.

What it can be confused with

Some individual symptoms produced by MNSV infection may be confused with other cucurbit diseases and/or physical damage.

Leaf lesions may appear similar to those caused by bacterial fruit blotch of melon (Acidovorax avenae subsp. citrulli). However, bacterial fruit blotch lesions have a water-soaked edge to their lesions.  Bacterial fruit blotch is usually associated with heavy rain.

Internal fruit rotting can appear similar to fruit flesh softening caused by Cucumber green mottle mosaic virus (CGMMV) or post-harvest cold storage issues.

The most distinctive feature of MNSV infection is the brown stains/patches that develop throughout watermelon and rockmelon fruit flesh and the seed cavity.

Lifecycle

Once a plant is infected by MNSV, infection continues for the life of the plant. Depending upon the host species/variety and the strain of MNSV, infection does not always affect the entire plant.  Watermelon plants are often reported as being entirely infected by MNSV, but rockmelon and cucumber are not.

Vectors are Olpidium bornovanus (Lange and Insunza, 1977; Campbell and Sim, 1994) and Olpidium radicale (Gonzalez-Garza, 1979) and western spotted cucumber beetle (Diabrotica undecimpunctata, undecimpunctata) and the banded cucumber beetle (Diabrotica balteata) (Coudriet et al, 1979).

MNSV clings to the outer covering of Olpidium bornovanus spores, and enters the plant through the wounds caused by Olpidium bornovanus infection.

Host

MNSV infects a number of cucurbit species, including:

  • cucumber (Cucumis sativus),
  • honeydew melon (Cucumis melo subsp inodorus),
  • rockmelon (also known as cantaloupe or muskmelon; Cucumis melon supsp. melo)
  • watermelon (Citrullus lanatus).

Internationally, MNSV is a significant disease of glasshouse cucumber and melon crops, but also causes disease in field crops.

Distribution

MNSV is present in horticultural production areas including (but not limited to): Europe (Netherlands), the Americas (United States of America; Panama; Mexico), Asia (China; Japan) and parts of Africa and the Caribbean.

In Australia, MNSV is considered to be present in New South Wales and Victoria after isolated outbreaks in 2012 and 2016, respectively.

MNSV is exotic to Queensland.

How it spreads

MNSV is spread by:

  • infected seed
  • infected soil and water
  • tools and equipment
  • direct contact between plants
  • cucumber beetles (although this may just be a form of mechanical transmission rather than a biological relationship).

MNSV is easily distributed through melon production areas as it can remain present in the soil for several years without host plants present.

MNSV has also been reported to persist in association with its vector - the soil-borne, fungus-like organism Olpidium bornovanus; which inhabits the roots of some plants. The current distribution of Olpidium bornovanus in Australia is unknown.

MNSV can also be transported in surface water when it flows over infected soil or plant material.

Impacts

MNSV-infected fruit are not considered a pathway for further spread of the virus, so there should be no market access and trade implications within Australia.

MNSV causes necrotic lesions on leaves and stems, fruit deformation and reduces the quality of fruit.

Prevention

There is no cure for MNSV; once infected, plants are infected for life. Therefore the most effective means of control is prevention.

There are simple steps you can take to protect your farm:

  • Source seeds from reputable suppliers
  • Purchase healthy seedlings from reputable nurseries
  • Keep records of where plants/seeds are sourced from and where they are planted on your property
  • Check new plants on arrival to make sure they are healthy and are pest and disease free
  • Regularly check your farm and report any unusual or unfamiliar symptoms.

Melon growers are reminded about the need to implement and maintain stringent on-farm biosecurity practices to help prevent pests and diseases getting onto their property. Find out how farmers can protect their farms from plant pests and diseases at:

https://www.business.qld.gov.au/industries/farms-fishing-forestry/agriculture/crop-growing/pest-diseases-plants/protecting-farm

https://www.daf.qld.gov.au/business-priorities/plants/health-pests-diseases/farm-biosecurity

http://www.farmbiosecurity.com.au/

Monitoring and action

Refer to the Melons Australia webpage for a range of resources including the Melon Industry Biosecurity Plan and the Farm Biosecurity Planner for the Melon Industry:

Refer to the Plant Health Australia website for factsheets on how to monitor for specific exotic melon pests and diseases.

Report suspected Melon necrotic spot virus to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

Legal requirements

Melon necrotic spot virus is a prohibited plant pest under the Biosecurity Act 2014.

Report suspected Melon necrotic spot virus to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

If you think you have found Melon necrotic spot virus, you must take all reasonable and practical steps under your control to minimise any associated risks. This is called a general biosecurity obligation (GBO).