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.
Packaged salad-cut lettuce (Lactuca sativa L.) for food service and salad mixes is an increasingly important component of the lettuce industry. The product is highly perishable; cold storage and modified-atmosphere (MA) packaging are used to extend its shelf life. Given the importance of this market, lettuce cultivars, breeding lines, and populations should be selected for increased shelf life in MA environments. The objectives of this research were to determine the genetic variation in lettuce for shelf life in low-O2 MA environments and to develop rapid evaluation methods suitable for a lettuce breeding program. Lettuce was processed from field-grown plants of 33 romaine and three crisphead cultivars over 2 years. Shelf life was evaluated after storage in MA bags and in CO2-free controlled-atmosphere (CA) chambers with gas ratios of 0.2% O2:99.8% N2, 1.0% O2:99.0% N2, or 5.0% O2:95.0% N2. Deteriorated leaf blade tissue was water soaked and wilted with a dull to dark or black color, and midrib tissue and heart leaves were water soaked with a translucent to dark brown color. Genetic variation for shelf life was detected using MA bags or CA chambers, and the results from both years and testing methods were significantly correlated. Oxygen concentration did not affect shelf life in the CA chamber experiment, which indicated that the observed symptoms in the majority of cultivars were probably not from low O2 damage or CO2 injury, although multiple mechanisms of deterioration may be involved. Selection for lettuce cultivars, breeding lines, and populations with extended shelf life is possible using MA bags or CA chamber testing methods and could facilitate a consistent release of germplasm with stable shelf life in MA environments.
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.
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.
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.
Fresh-cut lettuce (Lactuca sativa) packaged as salad mixes are increasingly popular to consumers but are highly perishable. Cultivars bred with extended shelf life could increase overall production efficiency by reducing the frequency of product replacement in the marketplace. Understanding the inheritance of shelf life is needed to develop efficient breeding strategies for this trait. A population of 95 recombinant inbred lines (RILs) from slow-decaying ‘Salinas 88’ × rapidly decaying ‘La Brillante’ was grown in four field experiments. Cut lettuce was evaluated for decay in modified atmosphere (MA) packages flushed with N2 or air (control). Correlations between field experiments ranged from 0.47 to 0.84 (P < 0.01). Three quantitative trait loci (QTL) for decay of cut lettuce were detected on linkage groups (LGs) 1, 4, and 9 with ‘Salinas 88’ alleles associated with slower decay. The QTL on LG 4 (qSL4) was a major determinant of decay explaining 40% to 74% of the total phenotypic variance of the trait. The greatest effect of this QTL was observed between 29 and 50 days after harvest. QTL × environment interactions contributed less than 14% to the total variation. RILs with the ‘Salinas 88’ allele of qSL4 had slower decay when packaged in air compared with N2, whereas no difference between air and N2 packaging was detected with the ‘La Brillante’ allele. A subset of RILs with either the ‘Salinas 88’ or ‘La Brillante’ allele of qSL4 was grown in two field experiments and evaluated for decay of whole heads. Genetic variation among RILs for whole-head decay was found but could not be attributed to qSL4. Decay of cut lettuce in ‘Salinas 88’ × ‘La Brillante’ is a highly heritable trait conditioned by a few QTL and phenotypic selection is likely to be effective. However, shelf life evaluations are time-consuming, destructive, and require large amounts of field-grown lettuce. Therefore, qSL4 is a good QTL to develop molecular markers for marker-assisted selection. The mechanism of decay controlled by qSL4 is unknown but appears to be specific to cut lettuce and may have allele specific interactions with packaging atmospheric compositions.