Melon necrotic spot virus

MNSV is a plant virus from the genus Gammacarmovirus in the family Tombusviridae.

Up to 5 strains of MNSV have been reported based on host range and symptoms produced. Two of these strains, MSNV I and MNSV IV, have been reported from Australia.

MNSV does not affect human or animal health.

MNSV infection produces a range of necrotic spots on host plant leaves, stems and fruit. Depending on the host plant and virus strain, symptom type and appearance can vary.

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 spots are relatively large, sunken and can distort fruit shape. On watermelon, fruit surface spots tend to remain small and can give the fruit a rough 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 in watermelon fruit rind and throughout rockmelon fruit flesh and seed cavities.

Leaves

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

Stems

Necrotic spots can be produced on host plant stems. Stem spots are always associated with either leaf spots or fruit symptoms, and do not occur on their own.

Some individual symptoms produced by MNSV infection can readily be confused with those caused by other cucurbit diseases and/or physical damage.

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

Necrotic leaf spots may appear similar to those caused by bacterial fruit blotch of melon (Acidovorax citrulli). However, bacterial fruit blotch spots tend to have a water-soaked edge and are usually associated with heavy rain.

Necrotic stem spots can also be caused by other cucurbit viruses such as papaya ringspot virus (Papaya ringspot virus—type W), watermelon mosaic (Watermelon mosaic virus) and zucchini yellows (Zucchini yellow mosaic virus) or other fungal diseases such as gummy stem blight (Stagonosporopsis spp.).

MNSV is present in horticultural production areas including:

• Europe (Greece, Italy, Netherlands, Spain, Sweden)
• the Americas (Brazil, Guatemala, Honduras, Mexico, Panama, United States of America, parts of the Caribbean)
• Asia (China, Japan)
• parts of Africa (Tunisia).

In Australia, MNSV has been detected in isolated outbreaks in New South Wales (2012), Victoria (2016) and Queensland (2018).

MNSV is not considered to be widely distributed in Queensland or Australia.

MNSV infects several cucurbit species, including:

• cucumber (Cucumis sativus)
• honeydew melon (Cucumis melo cv. inodorus),
• rockmelon (or cantaloupe or muskmelon) (Cucumis melon var. cantalupo)
• watermelon (Citrullus lanatus).

MNSV infects both glasshouse and field-grown crops.

Resistance to MNSV has been bred into many modern cucurbit varieties. However, some MNSV strains are resistance breaking and can cause symptoms, especially during periods of low temperatures and low light (e.g. extended periods of cloudy weather).

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 systemically infected by MNSV (and the virus can be detected from all plant tissues), but not rockmelon and cucumber.

Vectors are:

• Olpidium bornovanus
• Olpidium radicale.

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

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

In 2018–19, 229,809 tonnes of watermelons were produced in Australia for a total value of AUD$107 million: 73,539 tonnes (or 33%) were grown in Queensland. Queensland producers exported 5,160 tonnes (worth $12.7m) representing 70% of total Australian exports. In 2018–19, 30% of watermelons from Australia went to New Zealand, and 57% to the United Arab Emirates.

In 2018–19, 61,280 tonnes of rockmelon and honeydew melons were produced in Australia for a total value of AUD$74.5 million: 23,287 tonnes (or 38%) were grown in Queensland. Queensland producers exported 7,399 tonnes (worth $12m) representing 52% of total Australian exports.

MNSV causes necrotic spots on leaves and stems, fruit deformation and reduces the eating quality of fruit. Internationally, significant impacts have been reported with whole crop losses, and death of plants. However, in Australia outbreaks have been sporadic and very limited in distribution.

MNSV-infected fruit are not considered a pathway for further spread of the virus so there are no MNSV-related market access and trade implications for the sale of cucurbit fruit within Australia. However, some international markets are sensitive to the presence of MNSV in fruit and apply restrictions.

MNSV is spread by:

• infected seed
• infected soil and water
• contaminated tools and equipment
• direct contact between plants.

MNSV is easily distributed through cucurbit 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.

In the 1970s, MNSV was reported to be transmitted by the western spotted cucumber beetle (Diabrotica undecimpunctata) and the banded cucumber beetle (Diabrotica balteata). However, transmission of MNSV by insects has not been reported since then.

Visit Plant Health Australia for fact sheets on how to monitor for specific exotic melon pests and diseases.

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

Protect your farm from plant pests and diseases:

• Learn how to protect your farm from emergency plant pests
• Understand good farm biosecurity
• Visit the farm biosecurity website
• Download the farm biosecurity planner for the melon industry (PDF, 1.6MB).

MNSV is restricted matter under the Biosecurity Act 2014.

Report suspected MNSV 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 MNSV, you must take all reasonable and practical steps under your control to minimise any associated risks. This is called a general biosecurity obligation (GBO).