Photo 1. Small yellow lettuce with down-turned leaves infected with Lettuce mosaic virus. Compare with green healthy plant behind.
Lettuce mosaic virus; the abbreviation is LMV.
Worldwide. Asia, Africa, North and South America, the Caribbean, Europe, Oceania. It is recorded in Australia and New Zealand.
Lettuce, and many other members of the Asteraceae family. The main hosts, other than lettuce, are, chicory, and several ornamentals (e.g., Osteospermum and Coreopsis species), and common weeds: Sonchus (sowthistle) and Stellaria (chickweed).
Light and dark green patches on the leaves, puckering, sometimes with browning of the veins (Photos 1&2). Generally, the plants are stunted, yellow, with the edges of the leaves rolling downward, and unmarketable.
There are two ways that the virus is spread. It is spread in seed and also by aphids. Seedborne infections are important as they can be very common, depending on the time that the mother plant was infected, the virus strain, the lettuce variety, and the temperature. Up to 40% of the seed can be infected. The virus can be spread long distances in seed, possibly between countries.
Spread by aphids is usually over shorter distances, between plants in the same crop or between crops in the same region. There are many species that spread viruses naturally, although some species are more efficient than others. In Pacific islands, Aphis gossypii is a common species and is known to transmits the virus in a non-persistent way (from research done elsewhere); this means that aphids pick up the virus on their mouth parts after a short feed on an infected plant (less than a minute), then infect a healthy plant the next time they feed, but soon after lose the ability to infect.
Apart from spread between lettuces, it also occurs from several ornamental daisies and weeds.
The virus is also present in pollen, and seed becomes infected at fertilisation.
Lettuce mosaic is an important virus disease, with varieties differing in their reaction to infection. Affected plants do not produce marketable heads. There have been several major epidemics of the virus, and because it is seedborne, there are schemes in many countries to produce seed certified free from infection.
In Australia and the USA, seed is certified free from infection if there is only one seed infected per 30,000. In Europe, it is less than this. Experience has shown that levels as low as 0.1% are sufficient to start an epidemic if there are aphids migrating through the crop, probing the plants as they stop to feed.
Look for light and dark green patterns on young leaves that have ragged edges.
As this disease is not recorded in Fiji, Samoa, Solomon Islands and Tonga, biosecurity authorities should consider the potential pathways for entry. Note that it is seedborne.
There are commercial varieties available that are resistant to this disease; check to see if they are available from retailers in your area.
The use of insecticides for the control of aphids that spread the virus is not recommended. The time is too short between an aphid sucking up the virus when it feeds on a diseased plant and spreading the virus as it feeds again on a healthy plant. By the time the insecticide has killed the aphid it has spread the virus.
Should growers wish to use an insecticide to kill aphids, use those given under Aphids (see Fact Sheet no. 38):
AUTHORS Grahame Jackson
Information from CABI (2014) Lettuce mosaic virus Crop Protection Compendium. (http://www.cabi.org.cpc/); and information (and Photo 1) from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing. Photo 2 Kohler F, Pellegrin F, Jackson G, McKenzie E (1997) Diseases of cultivated crops in Pacific Island countries. South Pacific Commission. Pirie Printers Pty Limited, Canberra, Australia.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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