Tipburn (TB) is a physiological disorder that results in necrosis along the margins of lettuce leaves. The disorder is objectionable to consumers and reduces the shelf life of whole and minimally processed lettuce. The objectives were to 1) determine the variation for tipburn resistance in iceberg, romaine, green leaf, and red leaf cultivars; and 2) determine the genotype × location interaction for tipburn resistance. Tipburn incidence was recorded on 10 plants in each of three repetitions in Salinas, Calif., and Yuma, Ariz., trials with 20 iceberg, 21 romaine, 11 green leaf, and six red leaf cultivars. Data were analyzed using analysis of variance type statistics of ranked data. Variation for TB resistance was found in all lettuce types at both locations, although iceberg cultivars (average percentage of TB = 31% Salinas; 77% Yuma) had significantly (P < 0.01) higher levels of resistance at both location than romaine (58% Salinas, 81% Yuma), green leaf (52% Salinas; 88% Yuma), and red leaf (43% Salinas, 89% Yuma). The Yuma, Ariz., trial was more conducive for TB, and had less variation (range of percentage of TB = 33% to 100% Yuma, 0% to 100% Salinas). Four iceberg, one green leaf, and one red leaf genotype with industry acceptable levels of TB (<5%) were identified in the Salinas environment. Genotype × location interaction was present (P < 0.01), and included rank order changes within all lettuce types. The correlation between the locations was low, 0.26, but significant (P = 0.045). A need exists for romaine, green leaf, and red leaf germplasm with improved TB resistance, particularly in the Yuma environment. The presence of variation within each lettuce type and genotype × location indicates that genetic improvement should be possible using within type crosses followed by selection in the Yuma or Salinas target environment.
Big vein is an economically damaging disease of lettuce (Lactuca sativa L.) incited by Mirafiori lettuce big vein virus, which is vectored by the soil-borne fungus Olpidium brassicae (Woronin) P.A. Dang. Resistance to this disease is needed because no feasible cultural control methods have been identified. Partial resistance is available within cultivated lettuce and is expressed as delayed appearance of symptoms in combination with a reduced percentage of symptomatic plants. Complete resistance has been identified only in accessions of L. virosa L., an incongruent wild relative of lettuce. Resistance from L. virosa has not been introgressed into lettuce. The objective of this research was to determine whether big vein resistance from L. virosa can be introgressed into lettuce. Progenies of backcross (BC) hybrids between L. virosa and L. sativa cultivars were greenhouse tested for big vein resistance over four generations of self-pollination. Selected plants from resistant BC families were used as parents to create BC2 progeny from crosses with high partial-resistant cultivars, intermediate partial-resistant cultivars, and susceptible cultivars to test for the presence of transgressive segregants. Experiments were conducted in the greenhouse by infecting seedlings with O. brassicae zoospores collected from big vein symptomatic plants. Plots were evaluated for area under the disease progress curve and the percentage of symptomatic plants; asymptomatic plants from resistant families were retained in every generation. Complete resistance to big vein was not recovered, and may be the result of insufficient sampling of BCF2 progeny or linkage between resistance alleles and alleles causing incongruity. Variation for partial resistance was observed in all BC generations, and transgressive segregants were identified among BC2 families from crosses using partially resistant and susceptible parents. This research suggests that L. virosa contains alleles that confer partial resistance to big vein when introgressed into an L. sativa background, and these alleles are distinct from those present in partially resistant lettuce cultivars. Alternative breeding strategies should be pursued to introgress complete resistance from L. virosa into cultivated lettuce.
Big vein (BV) disease of lettuce is caused by soil borne fungal vectored viruses, and reduces marketability through head deformation. Tolerant cultivars reduce BV frequency, but no resistant cultivars exist. L. virosa L. is highly resistance. The objectives were to 1) determine if L. virosa P.I.s exhibit variation for resistance, and 2) determine if resistance is transferable to lettuce. Seedlings were inoculated with root macerate of BV infected plants, transplanted to BV infested soil, and greenhouse grown for 3 months. Twelve plants in each of 1,2, or 3 reps of Great Lakes 65 (GL65-susceptible), Pavane (Pav-tolerant), L. virosa (11 accessions), and BC1 F2 through F5 families of lettuce cultivars x L. virosa accession IVT280 were tested. The percentage of BV afflicted plants was recorded. In hybrid families, BV free plants from tolerant families were selected and advanced. No BV was found in L. virosa. Variation for tolerance was observed in BC1 F2 and F3 families; 33% had greater tolerance than Pav (17% afflicted). Additional tests identified 11 BC1 F3 families (14%) with greater tolerance than Pav (42% afflicted). Subsequent BC1 F4 and F5 generations however, were more susceptible than Pav. Lactuca virosa is highly resistant, but resistance did not transfer to hybrid progeny. Variation for tolerance was observed in BC1 F2 and F3 families, but later generations were susceptible. Interactions or linkage of genes for developmental processes and BV resistance may hinder introgression. Introgression will continue using congruity backcrossing and a greater diversity of L. virosa.
Verticillium wilt caused by Verticillium dahliae Kleb. is an economically damaging disease of iceberg lettuce on the Central Coast of California. Foliar wilting symptoms that manifest near or at peak market maturity (MM) lead to collapse of the head, making it unmarketable. Complete resistance to race 1 of the pathogen is known, but adequate levels of resistance are not available against race 2. Additional mechanisms or traits that reduce foliar symptoms (FS) are needed to lessen economic losses from this disease. Since the disease affects leaves, the harvested product, identification of iceberg cultivars that delay the onset of FS past peak MM could reduce yield loss from the disease. The goal of this research was to identify iceberg lettuce germplasm with delayed onset of FS. Diverse iceberg cultivars were evaluated in replicated field experiments for MM, FS severity, and adaptation. A few winter-adapted cultivars showed fewer FS past MM and seem to be promising candidates for breeding. These cultivars are not adapted to the California Central Coast where the disease currently predominates. Further studies will determine the usefulness of this trait for breeding improved cultivars for use in V. dahliae–infested fields. Developing new cultivars that combine currently available sources of partial resistance against race 2 with delayed onset of FS could lead to reduced crop losses should race 2 of V. dahliae become widespread.
Lettuce drop caused by Sclerotinia minor is a damaging disease of romaine lettuce (Lactuca sativa L.) production in California. Introgression of partial resistance from wild, primitive, or heirloom accessions into modern cultivars could improve integrated management approaches to the disease. Breeding methods for lettuce drop resistance are not well developed and hinder the development of new lettuce drop–resistant cultivars. The objective of this work was to develop a pedigree-based breeding method for introgression of lettuce drop resistance into modern romaine germplasm. Progeny from crosses between the partially resistant cultivar Eruption and the susceptible romaine cultivars Darkland and Hearts Delight were selected in a modified pedigree breeding scheme. Families were evaluated for disease incidence and selected for lettuce drop resistance in artificially infested field experiments conducted in the summer and fall. Infected plants of partially resistant lines commonly do not produce seed, and therefore selection of resistant plants from infested nurseries is not possible. Noninfested field experiments were used to select individual plants with improved horticultural characteristics for seed production, but from within resistant families only. Evaluation and selection of progeny using this breeding scheme occurred from the F2:3 through the F5:6 generations. In all generations, superior resistance was identified in the ‘Eruption’ × romaine crosses. The breeding scheme generated eight green romaine-type inbred lines with better resistance than the romaine parent and better head weight than ‘Eruption’. Use of the new romaine lines as parents in backcrosses to romaine produced F2:3 families with high levels of resistance. The pedigree method used in this research can be implemented with any source of resistance, but is constrained by the use of family selection and the inability to select individual plants for resistance directly. Breeding schemes that use single seed descent or molecular markers are alternative approaches that would enable selection for resistance on individual genotypes.
Lettuce drop caused by Sclerotinia spp. is an economically important disease of lettuce (Lactuca sativa L.), and cultivars with resistance to mycelial infection by Sclerotinia sclerotiorum (Lib.) de Bary and S. minor Jagger as well as to S. sclerotiorum ascospores are needed. Assessing resistance in field experiments can be complicated by fast bolting or small stature lettuce lines that may escape rather than resist the pathogens. Therefore, methods to select resistant lines from morphologically variable populations are needed. We used S. sclerotiorum and S. minor-infested field experiments, regression analysis, field experiments with artificially high plant densities, and S. sclerotiorum ascospore inoculations to identify lettuce lines with resistance to both pathogens. Three replicated experiments in S. sclerotiorum-infested fields were conducted in Yuma, AZ, and three replicated experiments in a S. minor-infested field were conducted in Salinas, CA, using diverse populations of iceberg, romaine, leaf, butterhead, Latin, oilseed lettuce, and wild relatives of lettuces; and genetic variation for the incidence of lettuce drop from mycelial infections was identified. In two S. minor field experiments, a quadratic regression model was developed that related rapid bolting with reduced lettuce drop. Regression residuals were calculated, and eight cultivars or PIs had negative residuals in two independent field experiments, indicating higher resistance than predicted by their rate of bolting. Eruption, a small-statured Latin cultivar, had significantly lower disease levels than susceptible cultivars in experiments with high plant densities, indicating that its small size did not facilitate disease escape. Ascospore inoculations confirmed resistance in ‘Eruption’ and L. virosa SAL012, whereas the oilseed lettuce PI 251246 may have partial resistance to infection. These lines will likely be useful for development of Sclerotinia spp.-resistant lettuce cultivars.
Bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians (Xcv) is an important lettuce disease in California. No adequate control measures have been found, although resistance exists in several heirloom cultivars. Deployment of cultivars resistant to bacterial leaf spot will reduce these periodic and costly disease events. The objectives of this research were to 1) identify new sources of resistance within modern crisphead cultivars and 2) select for resistance in `Salad Crisp' × `Iceberg' progeny. Field plots were established and grown with overhead irrigation, and a three-strain mixture of Xcv was applied until runoff 1 week after thinning at 1 × 109 CFU/mL. Twenty-six crisphead cultivars were tested in unreplicated field trials and rated on a 1 (susceptible) to 4 (resistant) scale. Selection was carried out between and within families from the F2 to F4 generation. Sixteen F3 families were evaluated in unreplicated plots, and 12 F5 families were tested in replicated plots for disease incidence and severity. No usable levels of resistance were identified in the modern crisphead cultivars tested to date. All F3 families had resistance greater than `Iceberg', and 19 plants from eight families were selected for further breeding. Subsequently, 12 plants from two F4 families were selected. Replicated trials of 12 F5 families indicated that all lines have disease severity comparable to both parents. Breeding lines from crosses to `Salinas 88' are currently being developed.
Verticillium wilt of lettuce caused by Verticillium dahliae can cause severe economic damage to lettuce producers. The pathogen exists as two races (Races 1 and 2) in lettuce, and complete resistance to Race 1 is known. Resistance to Race 2 isolates has not been reported, and production of Race 1-resistant cultivars will likely increase the frequency of Race 2 strains. The objective of this research was to select lettuce accessions for resistance to Race 2 isolates of V. dahliae. Two independent populations totaling 314 randomly sampled PIs were evaluated for Verticillium wilt disease incidence (DI) caused by V. dahliae isolate VdLs17 in one unreplicated and two replicated greenhouse experiments. Selection for PIs with reduced DI was conducted between each experiment and plant stems were plated on semiselective media to identify colonized plants that remained non-symptomatic. No accession with complete resistance was identified, although accessions with partial resistance were selected. Genetic variation for the frequency of V. dahliae-colonized plants that remain symptomless was detected. Four PIs (169511, 171674, 204707, and 226641) were selected for further testing in three replicated greenhouse experiments and demonstrated significantly lower disease incidence than the susceptible control cultivars. The results indicate that lettuce has genetic variation for partial resistance to a Race 2 isolate of V. dahliae. The resistant PIs selected in this research are morphologically diverse, and no dependence between rate of bolting and resistance was found. PIs with partial resistance may be useful for breeding lettuce cultivars with resistance to Race 2 isolates of V. dahliae.