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.
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.