Definitions

mosaic-disease

Lettuce mosaic virus

Lettuce mosaic virus (LMV) is a typical potyvirus (genus Potyvirus, family Potyviridae), which causes one of the major virus diseases of lettuce crops world-wide.

LMV is seed-borne to a low but significant rate (1-10% of the seeds produced by an infected mother plant germinate into infected seedlings). This provides the primary inoculum in lettuce crops. LMV, and thus the mosaic disease, is then spread locally from plant to plant by the feeding stylets of aphids.

Like all plant viruses, LMV is totally harmless to the consumer but causes defects in heading, leaf distortions and leaf colour anomalies, which altogether result in the infected lettuce plants to be unmarketable. Disease rates can reach 100% locally, and therefore cause complete loss of the harvest. LMV can also infect other crops such as spinach and peas, as well as ornamentals (especially the Cape Daisy Osteospermum spp) and wild plants (especially the prickly lettuce Lactuca serriola and the oxtongue Helminthia echioides). All these plants, and probably others, can serve as local sources from which LMV can spread to crops.

Prevention

Like other diseases caused by viruses in plants, there is no mean to cure plants from LMV once they are infected. Prevention of LMV can however be relatively efficient based on:

  • control of the absence of LMV in seed lots before trading; typically, ELISA is used and a rejection rate of 1 out of 30,000 seeds is applied;
  • the use of the recessive resistance gene mo1; mo1 has two commercial alleles (mo1¹, formerly known as g, and mo1², formerly known as mo), both of which protect plants against LMV accumulation and, in some cases where LMV nevertheless accumulates, protect against symptom expression as well as prevents transmission through seed.

In the late twentieth century, a novel type of LMV appeared in lettuce crops around the world. It was called LMV-Most (mo1-breaking, Seed-Transmitted) to account for its ability to infect and to spread through seeds even in the presence of mo1. Therefore, LMV-Most can only be prevented by controlling seeds, which consumes time, effort and money.

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