Breeding for resistance to diseases in lettuce: successes and challenges

Lettuce mosaic has been a long-term world wide problem. Resistance is available at a single locus. A single gene, partially dominant, for mild reaction, can be combined with an allele for resistance to produce a higher level of resistance. Breeding work is under way. An additional source of high level resistance has been tentatively identified. A new seed borne strain of LMV that overcomes resistance may become a problem. Big vein has also been a long-term problem. Moderate sources of resistance have been identified, based upon proportion of plants showing symptoms at harvest time. Pavane appears to have the highest level of the identified sources. The species Lactuca virosa appears to have complete resistance and is being used in a breeding program. Verticillium is a new disease in lettuce. No iceberg type lettuce cultivars tested are resistant. Several other cultivars of different type are resistant and are being used in breeding.