Variation for Resistance to Verticillium Wilt in Lettuce (Lactuca sativa L.).

Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (Lactuca sativa). In 2004 and 2005, 107 and 22 lettuce cultivars, respectively, were screened for resistance in a field infested with Verticillium dahliae, and disease progress on resistant and susceptible cultivars was determined. Greenhouse experiments were conducted to evaluate 16 cultivars for resistance to a race 1 and a race 2 isolate. Significant differences for resistance were observed within cultivated lettuce. In susceptible cultivars, disease levels increased through the season, whereas disease in resistant cultivars remained constant. Resistance in greenhouse tests was dependant upon the race used. Seven cultivars were resistant to race 1, whereas all were susceptible to race 2. Cultivar reactions to race 1 in greenhouse and field experiments were correlated, indicating the utility of greenhouse evaluations. The identification of resistance in diverse lettuce types is beneficial to the breeding process. However, because of the existence of resistance-breaking race 2 isolates, this resistance may not be durable. Alternatively, targeted releases of race-1-resistant cultivars to fields with only race 1 pathogen genotypes may extend the life of these cultivars.

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.

Characterization and genetic analysis of a lettuce (Lactuca sativa L.) mutant, weary, that exhibits reduced gravitropic response in hypocotyls and inflorescence stems.

A lettuce (Lactuca sativa L.) mutant that exhibits a procumbent growth habit was identified and characterized. In two wild type (WT) genetic backgrounds, segregation patterns revealed that the mutant phenotype was controlled by a recessive allele at a single locus, which was designated weary. Hypocotyls and inflorescence stems of plants homozygous for the weary allele exhibited reduced gravitropic responses compared with WT plants, but roots exhibited normal gravitropism. Microscopic analysis revealed differences in the radial distribution of amyloplasts in hypocotyl and inflorescence stem cells of weary and WT plants. Amyloplasts occurred in a single layer of endodermal cells in WT hypocotyls and inflorescence stems. By contrast, amyloplasts were observed in several layers of cortical cells in weary hypocotyls, and weary inflorescence stem cells lacked amyloplasts entirely. These results are consistent with the proposed role of sedimenting amyloplasts in shoot gravitropism of higher plants. The phenotype associated with the weary mutant is similar to that described for the Arabidopsis mutant sgr1/scr, which is defective in radial patterning and gravitropism.

New Source of Resistance to Cucumber mosaic virus in Capsicum frutescens.

A small-fruited pungent pepper accession, Capsicum frutescens 'BG2814-6', is resistant to several isolates of Cucumber mosaic virus (CMV). Resistance in BG2814-6 is incompletely penetrant and is controlled by at least two major recessive genes. The accession BG2814-6 and C. annuum 'Perennial', the leading source of CMV tolerance, appear to share one or more CMV resistance genes. CMV was detected in uninoculated leaves in a small percentage of both BG2814-6 and Perennial plants, indicating that resistance is not absolute in either genotype. Enzyme-linked immunosorbent assay absorbance values of samples taken from inoculated leaves corresponded well with visible viral symptoms for parental genotypes. While Perennial plants accumulated CMV antigen in inoculated leaves, CMV antigen was not detected in inoculated leaves of 73% of BG2814-6 plants, suggesting that there may be a mechanistic difference in resistance between the two genotypes.