Edward J. Ryder

Weedborne Reservoirs and Seed Transmission of Verticillium dahliae in Lettuce

The seed transmission of Verticillium dahliae was evaluated in lettuce (Lactuca sativa). Seed collected from lettuce plants infected with V. dahliae were plated with or without surface sterilization on Sorensons modified NP10 medium. Of the seed plated with or without surface sterilization, 90 and 66%, respectively, yielded colonies of V. dahliae. The incidence of Verticillium wilt ranged from 55 to 80% among lettuce plants grown from seed harvested from infected plants. All evaluated isolates of V. dahliae were capable of seed transmission in lettuce. A V. tricorpus isolate failed to cause significant disease in lettuce or to become seedborne. Storage of contaminated seed at seven temperatures ranging from -20 to 15°C for up to 72 weeks did not reduce the incidence of V. dahliae in seed, whereas storage at room temperature (23 ± 2°C) for 20 to 52 weeks reduced the incidence of V. dahliae without affecting seed viability. Of the 11 weed species collected from fields with a known history of Verticillium wilt of lettuce, four yielded V. dahliae. Pathogenicity tests demonstrated that isolates of V. dahliae from Sonchus oleraceus, Capsella bursa-pastoris, and Solanum sarrachoides were as virulent as or more virulent than an isolate of V. dahliae from lettuce. These results demonstrate the potential of seedborne and weedborne inoculum to disseminate V. dahliae.

The Genes of Lettuce and Closely Related Species

Nomenclature rules are proposed for naming and symbolizing genes for lettuce (Lactuca sativa L.). To date, 59 loci have been identified, including 6 influencing anthocyanin, 10 chlorophyll genes, 11 affecting leaf morphology, 4 genes influencing heading, 7 genes for flower and seed characteristics, 7 male sterile genes, 1 gene affecting sensitivity to chemicals, and 13 genes for disease resistance. Several cases of multiple alleles and gene linkage are known.

Host Resistance to Mirafiori lettuce big-vein virus and Lettuce big-vein associated virus and Virus Sequence Diversity and Frequency in California

Big vein is an economically damaging disease of lettuce (Lactuca sativa) caused by the Olpidium brassicae-vectored Mirafiori lettuce big-vein virus (MLBVV). Lettuce big-vein associated virus (LBVaV) is also frequently identified in symptomatic plants, but no causal relationship has been demonstrated. Although big vein is a perennial problem in the United States, the extent of MLBVV and LBVaV infection and diversity is unknown. Lettuce cultivars partially resistant to big vein reduce losses, but do not eliminate disease. While Lactuca virosa does not develop big vein symptoms, it has not been tested for infection with MLBVV or LBVaV. Lettuce cultivars Great Lakes 65, Pavane, Margarita, and L. virosa accession IVT280 were evaluated for big vein incidence and virus infection in inoculated greenhouse trials. Additional lettuce samples were collected from field sites in California, classified for symptom severity, and evaluated for virus infection. Reverse transcription-polymerase chain reaction and nucleotide sequencing were used to determine infection with MLBVV and LBVaV, and sequence diversity among viral isolates, respectively. Infections with MLBVV and MLBVV/LBVaV were dependent on big vein symptom expression in California production areas, and isolates were closely related to those found in Europe and Japan. Partial big vein resistance was identified in Margarita and Pavane; however, MLBVV infection was found in asymptomatic plants. L. virosa IVT280 remained symptomless and virus free, suggesting that it is immune to MLBVV and LBVaV.

Genetic analysis and mapping of resistance to lettuce dieback: a soilborne disease caused by tombusviruses

A diverse collection of modern, heirloom and specialty cultivars, plant introduction (PI) accessions, and breeding lines of lettuce were screened for susceptibility to lettuce dieback, which is a disease caused by soilborne viruses of the family Tombusviridae. Susceptibility was evaluated by visual symptom assessment in fields that had been previously shown to be infested with Lettuce necrotic stunt virus. Of the 241 genotypes tested in multiple field experiments, 76 remained symptom-free in infested fields and were therefore classified as resistant to dieback. Overall, resistant genotypes were as prevalent among modern cultivars as in heirloom cultivars or primitive germplasm. Within modern germplasm, however, all crisphead (iceberg) cultivars were resistant, while all romaine cultivars were susceptible. Using enzyme-linked immunosorbent assay, tombusviruses were detected in leaves of some plants of resistant genotypes that were grown in infested fields, suggesting that symptom-free plants are not immune to viral infection. The inheritance of resistance was studied for ‘Salinas’, a modern iceberg cultivar, and PI 491224, the progenitor of recently released romaine germplasm with resistance to lettuce dieback. Resistance was conferred by a dominant allele at a single locus in both genotypes. The tombusvirus resistance locus from ‘Salinas’, Tvr1, was mapped in an intraspecific Lactuca sativa population to a location that corresponds to linkage group 2 on the consensus map of Lactuca. The largest cluster of resistance genes in lettuce, the Dm1/Dm3 cluster, is found on this linkage group; however, the precise position of Tvr1 relative to this cluster has not yet been determined. To our knowledge, Tvr1 is the first tombusvirus resistance gene identified for any plant host.

Genetic mapping in lettuce

Cultivated lettuce (Lactuca sativa L.) is a diploid (2n = 18) species in the Cichoreae tribe of the Compositae (Asteraceae) family. There are three well-established wild species in the subsection Lactuca, L. serriola, L. saligna, and L. virosa; all are 2n = 18 and self-fertilizing. These species can be crossed to L. sativa with increasing difficulty in the order listed. Several other sexually compatible species have been described (Ferakova 1977) but their validity as distinct species remains unclear. L. serriola is closely related to L. sativa and may be conspecific (Lindqvist 1960a; Kesseli et al. 1991). These wild species, especially L. serriola, have been sources of several disease resistance genes (see below; Crute 1988); however, they remain a rich source of variation that has yet to be accessed systematically.