Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 2. Inheritance of the resistance

Summary The inheritance of resistance to Nasonovia ribis nigri in L. sativa was investigated. Parents and F₁ and F₂ populations from crosses between the susceptible cultivar Ravel and two resistant breeding lines were tested. In both breeding lines one dominant gene appeared responsible for resistance.     

The response to saprol (systemic fungicide) in lettuce species and cultivars and its inheritance

Summary The response of Lactuca sativa, L. serriola and L. saligna to saprol was studied. Spraying the whole plant or part of a leaf caused wilting of susceptible plants. A similar effect was observed when leaves were soaked in a solution containing saprol.Inheritance studies with L. sativa and L. serriola showed that resistance to the wilting reaction caused by saprol is controlled by a single recessive gene.Contribution no. 198-E, 1978 series.     

Identifying lettuce species (Lactuca subsect. Lactuca, Asteraceae): A practical application of flow cytometry

The wild lettuce species L. serriola, L. saligna, and L. virosa are important genitors in lettuce (L. sativa) breeding. Identifying these wild species can be problematic because in some cases they look very similar. Flow cytometry was tested for its reliability and general applicability as a tool to distinguish them. Three series of tests were conducted: (1) Tests with three accessions of L. sativa and one accession of each of the wild species, repeated three times throughout the year. In each repeat, the mean relative DNA amount of L. serriola was significantly higher than that of L. saligna, but significantly lower than that of L. virosa. The mean relative DNA amount of L. sativa did not differ from that of L. serriola.(2) Tests with each wild species represented by 10 accessions. Significant differences between the accessions within each species demonstrated the presence of intraspecific variation. Notwithstanding this intraspecific variation, the relative DNA amounts of all accessions of L. serriola were significantly higher than that of all L. saligna accessions, and significantly lower than that of all L. virosa accessions. Therefore, all accessions could be assigned to the appropriate species on the basis of their DNA amounts. (3) Tests with single plants from 10 accessions of each of the wild species. These test revealed that individual plants of L. serriola, L. saligna, and L. virosa can be reliably identified with flow cytometry, when aL. serriola sample of established identity is used as internal reference. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 1. Transfer of resistance from L. virosa to L. sativa by interspecific crosses and selection of resistant breeding lines

Summary Interspecific crosses were carried out between the three Lactuca species L. virosa, L. serriola and L. sativa to transfer resistance to the leaf aphid Nasonovia ribis nigri from L. virosa to the cultivated lettuce. L. sativa, L. serriola was used as an intermediate parent between the other two species. Many irregularities were observed in the interspecific hybrids, ranging from premature dying of F₁ plants resulting from a kind of bastard necrosis till complete male and female sterility and deviating microsporogenesis. Using in vitro culture and after several backcrosses male and female fertile plants were obtained with a L. sativa habit and with resistance to the leaf aphid.     

Differences in resistance of wild Lactuca species to natural infection of lettuce powdery mildew (Erysiphe cichoracearum)

Summary The natural infection of Erysiphe cichoracearum on 29 accessions of five wild Lactuca species (L. serriola, L. saligna. L. virosa, L. aculeata and L. dentata) and on one hybrid of L. serriola x L. sativa has been investigated for two and three years, respectively. No infection was observed on L. serriola (PI 255665), L. saligna (LSA/92/1 and LSA/92/2), L. virosa (LVIR/26 and LVIR/57/1) and L. dentata (PI 234204). The level of susceptibility of the control cultivars of L. sativa was found to be moderate (Fila) to fairly high (Hilde).     

The use of polyacrylamide gradient gel electrophoresis to identify variation in isozymes as markers for Lactuca species and resistance to the lettuce root aphid Pemphigus bursarius

Summary This paper describes simple, routine polyacrylamide gradient gel electrophoresis of allozyme patterns surveyed in wild populations of four Lactuca species, obtained from the Wellesbourne Vegetable Gene Bank. Using multivariate statistical techniques, four allozyme bands were identified, which distinguished the four species, and an additional two bands, which were related to the resistance of Lactuca species to the lettuce root aphid.     

Identification of sources of resistance in lettuce to the lettuce root aphid, Pemphigus bursarius

In a series of glasshouse and laboratory experiments, lettuce varieties and wild Lactuca species were screened for resistance to the lettuce root aphid, Pemphigus bursarius (Ron.). Several new sources of resistance to P. bursarius development and survival were identified. Extremely high levels of resistance were found in the wild species L. saligna, L. perennis and L. virosa. Resistance was also identified in the lettuce variety Grand Rapids. This resistance is controlled by one or two genes. At least one of these genes is not allelic to the existing Lra gene. Grand Rapids therefore represents a new source of resistance to P. bursarius. This revised version was published online in August 2006 with corrections to the Cover Date.     

New sources of major gene resistance in Lactuca to Bremia lactucae

Summary A total of 1789 accessions of several lettuce collections was screened to find new major gene resistance to the downy mildew fungus Bremia lactucae Regel. The accessions belonged to the species Lactuca sativa (N=1288), L. serriola (N=399), L. saligna (N=52) and L. virosa (N=50). A total of 20 races of B. lactucae were used, 14 of which were NL-races, isolated from cultivated lettuce in the Netherlands. The other six races were isolated from wild L. serriola in Czechoslovakia. The accessions were initially screened with two races: NL1 and NL3. Accessions with resistance to one or both of these races were tested with the other races. Phenotypes with new resistance were found in accessions of all four Lactuca species. Of L. sativa, four accessions were found with resistance phenotypes that could not be explained by combinations of known major genes. Many accessions of L. serriola had resistance phenotypes that indicated the presence of unknown resistance genes. All interactions between accessions of L. saligna and races of B. lactucae were incompatible in leaf disc tests, except for four accessions, which showed some sporulation with race NL6. Several accessions of L. virosa were resistant to all races used. Other accessions of L. virosa gave a race-specific interaction with B. lactucae.     

The location and characteristics of novel sources of resistance to Bremia Lactucae Regel (downy mildew) in wild Lactuca L. Species

Summary Twenty-one accessions of 3 wild Lactuca species which could be hybridised with L. sativa, the cultivated lettuce, were inoculated at different stages of plant development with 3 multivirulent isolates of Bremia lactucae. Nineteen sources of resistance to B. lactucae, not attributable to the previously recognised resistance factors 1–11 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying R11. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific.Tests on segregating F₂ populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/18) appears to be inherited as a single dominant gene, which is sometimes incomplete in expression and allelic to either Dm6 or R7. The segregation patterns for resistance in PI 281876 did not give readily interpretable ratios.To assess the frequency of occurrence in B. lactucae populations of virulence factors to overcome this novel resistance, 11 of the novel sources of resistance were inoculated with numerous collections of the pathogen from the UK, Czechoslovakia and elsewhere and found to show a high level of resistance.     

Comparison of sources of resistance to leaf aphids in lettuce (Lactuca sativa L.)

Summary Three lines of lettuce with resistance to Nasonovia ribisnigri, based on the dominant Nr-gene, and four lines selected for partial resistance to Myzus persicae were tested against three species of leaf aphid: N. ribisnigri, M. persicae and Macrosiphum euphorbiae. The effect of the Nr-gene was also studied in a segregating F2 population.In the material tested, resistance to N. ribisnigri was exclusively based on the Nr-gene, lines selected only for resistance to M. persicae showed no resistance to N. ribisnigri. The Nr-gene also induces partial resistance to M. persicae, but the level of this resistance is influenced by other genes, because the lines with Nr-gene differed significantly from each other for reproduction of M. persicae. The Nr-gene had no effect on the resistance of lettuce to M. euphorbiae.In lines with the Nr-gene, levels of resistance to M. persicae and to M. euphorbiae were correlated, suggesting that the resistance may be determined by the same genes. The Nr-line with highest resistance to M. persicae was comparable for this characteristic to the lines selected for resistance to M. persicae.The cultivars Taiwan and Ravel possess a resistance factor to M. euphorbiae that has no effect on M. persicae or N. ribisnigri. Lines selected for resistance to M. persicae also showed partial resistance to M. euphorbiae. Based on the present results no conclusions can be drawn whether this resistance is based on the same genes that provide resistance to M. persicae, or on a resistance factor comparable to that found in Taiwan and Ravel.     

A demonstration of the inheritance of field resistance to lettuce downy mildew (Bremia lactucae Regel.) in progeny derived from cv. Grand Rapids

Summary The inheritance of field resistance to downy mildew in lettuce derived from cv. Grand Rapids was studied in F₂, F₃ and F₄ progeny from a cross with the more susceptible cv. Lobjoits Green Cos. The results for 32 randomly produced F₃ families and the F₂ population corresponded closely although the absolute level of resistance varied from season to season according to disease pressure and environmental conditions. The field resistance character showed a quantitative pattern of inheritance but the rapid response to selection at F₃ implied that only a few genes may be involved. F₄ families raised from families selected from the two extremes of the F₃ distribution fell into distinct categories with resistance levels corresponding to that of their F₃ parents. An association between resistance and morphological features of the resistant parent was present at F₂ but not in subsequent generations when individuals and families which were as resistant as cv. Grand Rapids and yet bore little morphological resemblance to this cultivar were readily recovered.     

The inheritance of nitrate content in lettuce (Lactuca sativa L.)

Summary The inheritance of nitrate content in lettuce was analysed using 16 F₂ populations and three F₃ populations. Frequency distributions of nitrate content in F₂ and F₃ populations were unimodal and symmetrical, indicating a quantitative inheritance. Both significant positive and negative deviations of the F₂ mean from the mid-parent value were found, indicating dominance or epistasis. Deviations towards low nitrate content were more frequent than deviations in the other direction. Estimates of heritabilities for nitrate content in the F₂ populations ranged from 18% to 69% and were in most cases above 50%. Crosses between low nitrate cultivars did not have lower estimates of heritability in the F₂ than crosses between cultivars with larger differences in nitrate content. In one case a genotype x experiment interaction for nitrate content of parental cultivars was found.Three F₃ populations of crosses between cultivars with low nitrate content were analysed. Estimates of heritabilities for F₃ line means ranged from 78% to 91% and estimates of the genetic standard deviation of nitrate content in unselected advanced generations ranged from 0.24–0.33 g kg^–1. The estimates of heritabilities and of genetic variation in advanced generations offer good prospects of selection of low nitrate genotypes in lettuce. A comparison of efficiency of selection in the F₂ generation and F₃ line selection is made.     

Heterosis in crisphead lettuce (Lactuca sativa L.) hybrids

Summary Six of 16 F₁ hybrids from diverse crosses of crisphead lettuce with butterhead, cos or other crispheads significantly outyielded their respective better parent. Four were crosses between crisphead and cos parents, one was a butterhead × crisphead cross and one, a winter crisphead × winter crisphead cross. This latter hybrid plus one other also proved heterotic in a subsequent trial of ten crosses involving five winter crisphead parents, with yield excesses over better parent of 16 and 19%. It appeared significant that the heterotic crosses were given by parents of dissimilar origin. Trials of F₂ segregants from one of these heterotic hybrids enabled predictions to be made for 10.2 to 24.1% yield increases over better parent by the retention and clonal multiplication of the 10% highest yielding selections. Heterosis demonstrated in these trials may constitute the first reported cases in lettuce.     

An instant bioassay for resistance of lettuce to the leaf aphid Myzus persicae

Summary A reliable impression of the resistance of lettuce plants to the leaf aphid Myzus persicae can be obtained via aphid honeydew production. Under controlled temperatures, the number of honeydew droplets produced by these aphids per plant in 180 minutes with five plants per genotype offers a good criterion of this resistance.     

Increase of genetic variation in some lettuce cultivars by induction of mutations: A technical note

Summary Colchicine treatments of 13 lettuce cultivars for production of polyploid lettuce also resulted in rapid growing diploid mutants.     

Partial resistance in lettuce to downy mildew (Bremia lactucae). 1. Search for partially resistant genotypes and the influence of certain plant characters and environments on the resistance level

Summary In three bicyclic inoculation experiments in the field 756 genotypes from the IVT Lactuca gene bank were screened for differences in partial resistance against Bremia lactucae. Significant differences between genotypes were found. No linkage was found between the presence or absence of R-genes and partial resistance nor was there a clear relationship between plant characters such as size, erectness and lettuce type and resistance level. Comparison of test results for genotypes which were tested twice and in different years and/or environments revealed a rather good relationship. So no clear genotype × environment interactions for resistance level were found. Results imply that the bicyclic inoculation experiments are useful to determine levels of partial resistance.     

Genotypical differences in nitrate content inLactuca sativa L. and related species and correlation with dry matter content

Summary Nitrate content was measured in 135 genotypes of cultivated lettuce (Lactuca sativa L.) and 21 genotypes of wildLactuca spp. grown in two experiments. In Experiment 1 plants were grown on nutrient film. In Experiment 2 plants were grown in large pots of potting soil. Within cultivated lettuce five plant types were distinguished and within each of them genotypes were found with low nitrate content. The coefficient of variation in the experiment with plants growing in large pots was not larger than in the experiment with plants growing on nutrient film. In butterhead genotypes nitrate content was negatively correlated with dry matter content and positively with plant fresh weight.     

Resistance of wheat to Fusarium spp. in an in vitro seed germination assay and preliminary investigations into the relationship with Fusarium head blight resistance

Resistance of wheat cultivars from the UK, 2003 recommended list were evaluated against Fusarium culmorum, F. graminearum, Microdochium nivale var. majus and var. nivale, F. avenaceum and F. poae using a seed germination assay and related to previously published data on detached leaf assays, using M. nivale, and UK recommended list Fusarium head blight (FHB) resistance ratings. Significant cultivar differences were found after inoculation with all fungal species with the exception of F. poae where the decline in the percentage of seed germinating relative to the controls was small. Correlations of the percentage seed germinating inoculated with the remaining Fusarium spp. and M. nivale were high (r = 0.68; P < 0.01 to r = 0.94; P < 0.001). Overall, F. graminearum caused the greatest reduction with a mean seed germination of 61.7% relative to the controls followed by F. avenaceum (65.5%), M. nivale var. majus (67.2%), F. culmorum (76.6%), M. nivale var. nivale (89.2%) and was least for F. poae (92.5%). The resistance detected in the germination assay was significantly correlated to whole plant FHB resistance ratings (r_s = 0.45; P < 0.05) but was not correlated to partial disease resistance (PDR) components detected using the detached leaf assay, namely, incubation period, latent period and lesion length. The results suggest that while resistances detected in the seed germination and detached leaf assays may in part share a common genetic basis to FHB resistance, resistances detected in both in vitro assays are independent of each other. Resistances detected in the seed germination assay had a lower correlation with FHB resistance ratings against F. culmorum, than those detected by latent period in the detached leaf assay.     

Stem and head reaction of winter wheat cultivars to artificial inoculation by Microdochium nivale under controlled environment and field conditions

Summary Experiments to assess variation in the resistance of winter wheat to infection by Microdochium nivale were conducted over two consecutive years. Resistance was evaluated using an agar disk technique to reproduce stem lesions and by spraying a conidial suspension to reproduce head blight symptoms. Significant variation for stem reaction measured as stem lesion area (SLA), and head reaction measured as disease severity (DS) was found in the 33 winter wheat genotypes tested. Data obtained over two years in controlled environment conditions were significantly correlated (r=0.713 for SLA and r=0.738 for DS), whereas field data showed a significant genotype x year interaction for disease severity. Quantitative variation for susceptibility to stem and head infection by M. nivale was found among the 33 genotypes tested. The majority of genotypes expressed moderate susceptibility, with cultivar Goupil being very susceptible to both stem and head infection, and the remainder, Renan, Arminda, Munstertaler and Saint-Johann were the most resistant. Resistance to stem and head to M. nivale were not correlated (r=0.358).