Wild Lactuca Species Research Articles

Differential Sources of Resistance from Lactuca serriola Against Three Races of Xanthomonas hortorum pathovar vitians (Brown, 1918) Morinière et al. 2020 Causing Bacterial Leaf Spot of Lettuce.

Bacterial leaf spot (BLS) of lettuce (Lactuca sativa L.) is caused by the bacterium Xanthomonas hortorum pv. vitians which is hypothesized to have at least three races of the pathogen present in North America as defined by their differential resistance phenotypes in lettuce cultivars/accessions. Though resistance to X. hortorum pv. vitians race 1 has been identified in cultivated lettuce, numerous other X. hortorum pv. vitians strains cause disease on cultivars carrying this resistance locus. Thus far, resistance to these 'additional' X. hortorum pv. vitians strains has not been adequately described in L. sativa or in any other wild Lactuca species sexually compatible with cultivated lettuce. We have performed an extensive screening of approximately 500 Lactuca accessions from L. sativa, L. serriola, L. saligna, L. virosa, L. aculeata, L. altaica, and L. perennis species to identify accessions resistant to these additional X. hortorum pv. vitians races. Following the initial screenings, greenhouse tests confirmed that X. hortorum pv. vitians race 2 and race 3 could be defined using Lactuca sativa accessions. Race 2 strain BS3217 had an incompatible response (hypersensitive response) on ten Lactuca serriola accessions including PI491114 and PI491108, while race 1 (BS0347) and race 3 (BS2861) strains of X. hortorum pv. vitians showed a compatible response (disease) on these genotypes. L. serriola accession ARM09-161 (and selections derived from it) was the only genotype resistant to the race 3 strain BS2861. L. serriola accessions identified in this study to be resistant to race 2 and race 3 of X. hortorum pv. vitians, together with race 1 resistant cultivars, can be used for pyramiding resistance loci against the three races of the BLS-causing pathogen.

Bioengineered intestinal tubules as a tool to test intestinal biological efficacy of lettuce species

Lettuce (Lactuca sativa) is one of the most consumed and cultivated vegetables globally. Its breeding is focused on the improvement of yield and disease resistance. However, potential detrimental or beneficial health effects for the consumer are often not targeted in the breeding programs. Here, a bioengineered intestinal tubule was used to assess the intestinal efficacy of extracts from five plant accessions belonging to four Lactuca species. These four species include the domesticated L. sativa, closely related wild species L. serriola, and phylogenetically more distant wild relatives L. saligna and L. virosa. We assessed the epithelial barrier integrity, cell viability, cell attachment, brush border enzyme activity, and immune markers. Extracts from L. sativa cv. Salinas decreased cell attachment and brush border enzyme activity. However, extracts from the non-edible wild species L. saligna and L. virosa reduced the epithelial barrier functions, cell attachment, cell viability, and brush border enzyme activity. Since wild species represent a valuable germplasm pool, the bioengineered intestinal tubules could open ways to evaluate the safety and nutritional properties of the lettuce breeding material originating from crosses with wild Lactuca species.

Distribution and ecology of wild lettuces Lactuca serriola L. and Lactuca virosa L. in central Chile

Abstract Wild lettuces (Lactuca L.) provide valuable genetic resources for crop breeding, but are also significant invasive weeds. We explored the distributions, habitats, and ecological characteristics of populations of wild Lactuca species in central Chile. We documented two species – Lactuca serriola L. (prickly lettuce) and Lactuca virosa L. (opium/bitter lettuce) in 204 localities. These observations indicate that: i) both allochthonous (Euroasian) Lactuca species occur and are able to regenerate in central Chile; ii) L. serriola forms dense populations in urbanized areas; iii) both species can expand along transport corridors to high elevations; iv) the spread of L. virosa and persistence of dense populations in elevations above 2,000 m a.s.l. prove the invasiveness of this species in extreme climates; v) both species may contain novel traits of interest for germplasm conservation.

Identification and mapping of new genes for resistance to downy mildew in lettuce

Key messageEleven new major resistance genes for lettuce downy mildew were introgressed from wild Lactuca species and mapped to small regions in the lettuce genome.Downy mildew, caused by the oomycete pathogen Bremia lactucae Regel, is the most important disease of lettuce (Lactuca sativa L.). The most effective method to control this disease is by using resistant cultivars expressing dominant resistance genes (Dm genes). In order to counter changes in pathogen virulence, multiple resistance genes have been introgressed from wild species by repeated backcrosses to cultivated lettuce, resulting in numerous near-isogenic lines (NILs) only differing for small chromosome regions that are associated with resistance. Low-pass, whole genome sequencing of 11 NILs was used to identify the chromosome segments introgressed from the wild donor species. This located the candidate chromosomal positions for resistance genes as well as additional segments. F2 segregating populations derived from these NILs were used to genetically map the resistance genes to one or two loci in the lettuce reference genome. Precise knowledge of the location of new Dm genes provides the foundation for marker-assisted selection to breed cultivars with multiple genes for resistance to downy mildew.

Genetic relationships and structured diversity of Lactuca georgica germplasm from Armenia and the Russian Federation among other members of Lactuca L., subsection Lactuca L., assessed by TRAP markers

We studied the genetic relationships of Lactuca georgica samples originating in Armenia and the Russian Federation with samples representing four other predominantly self-pollinating wild Lactuca species (L. serriola, L. aculeata, L. saligna, and L. virosa) originating in various countries, as well as with samples representing cultivated lettuce, L. sativa by using 48 TRAP markers. We also visualized their genetic diversity and structure. The present study is likely the first molecular phylogenetic evaluation of a detailed screening of L. georgica germplasm. Data analysis of the three major wild species in this study, L. georgica (134 samples), L. virosa (57 samples), and L. serriola (40 samples) showed that allele frequencies of all 47 polymorphic loci varied significantly among the species. A total of 11, 9, and 10 alleles were unique to L. georgica, L. serriola, and L. virosa, respectively; 71% of TRAP marker diversity was between species. The Neighbor-Joining tree clearly clustered the whole set of 238 samples according to their taxonomic determination. It also reflects the gene diversity as well as the genetic distance values among samples representing the between and within variance of the various species. The L. georgica samples clustered most distantly from the L. sativa samples. The interspecies comparisons between samples belonging to L. georgica with those belonging to L. sativa displayed a high distance, lower only from the interspecies comparisons between samples belonging to L. virosa (in the tertiary gene pool of cultivated lettuce) with those belonging to L. sativa. Thus, additional molecular data with more hybridization experiments are necessary to reconsider if L. georgica is indeed a constituent of the primary gene pool of cultivated lettuce. The L. georgica samples were divided into two sub-clusters, with samples collected in southeast and central Armenia grouping together while all those collected in the north and Dagestan grouped together.

Genetic structure and diversity in natural populations of three predominantly self-pollinating wild Lactuca species in Israel

It is important to optimize strategies for collecting wild germplasm from their natural habitats for the successful conservation of plant genetic resources. We studied the population structure of three predominantly self-pollinating wild Lactuca species (Lactuca serriola, L. saligna, and L. aculeata). Seeds for this study were collected from individual plants in northern Israel, along a line transect, and two populations per each Lactuca species. The distance between neighboring plants sampled for seeds varied from 1.5 to 37 m. The transect length at single sites ranged from 47.2 to 151.8 m. The taxonomic status of 67 individual plants was morphologically validated during greenhouse multiplication. Both genetic structure and diversity were analyzed by using 11 EST–SSR loci and 230 AFLP markers. Relatively low genetic diversity values were observed, increasing in the following order: L. aculeata < L. serriola < L. saligna. Network analysis clearly separated samples according to their taxonomic determination; also reflecting the gene diversity as well as the genetic distance values among the three species. Nevertheless, given the predominantly selfing character of these species, populations were not uniform (genetically and morphologically). It seems that overall genetic variation in a population increases at its periphery, due to the presence of plants with “non-indigenous” alleles, which are most likely coming from migration and subsequent interpopulation or interspecific hybridization. Mantel tests generally indicated a positive association between genetic distance and micro-geographical distance of a particular population, primarily due to the “outlier” samples collected at a population’s periphery.

Resistance in natural populations of three wild Lactuca species from Israel to highly virulent Californian isolates of Bremia lactucae

Seedlings of 213 accessions representing 9, 14, and 10 Israeli natural populations of the wild Lactuca serriola, L. saligna, and L. aculeata, respectively, were initially screened for their resistance to a pathotype CAVIII isolate of Bremia lactucae. All 60 L. serriola accessions were susceptible while all 83 accessions of L. saligna were resistant. Out of the 69 L. aculeata accessions, 36 (52.2 %) were resistant. From those resistant accessions, 56 L. saligna and 23 L. aculeata accessions were then tested at the seedling stage for their reaction against five highly virulent isolates originating from California and representing the two current major pathotypes and a novel type of B. lactucae; true leaves of adult plants were also tested with two out of these five isolates. Our study supports previous observations that L. saligna is highly resistant to B. lactucae. However, our results provide additional evidence that L. saligna may not be an absolutely non-host plant for B. lactucae at least at a seedling stage, which is in agreement with other recent data for this species. Sixteen (69.6 %) out of the 23 L. aculeata accessions expressed resistance against all isolates tested, even in seedling stage as well as in true leaves of adult plants. This study is probably the first report of detailed screening of resistance to some B. lactucae isolates in natural populations of L. aculeata. These patterns of resistance reactions show that L. aculeata, a species within the primary lettuce gene pool, should be considered as an attractive source of germplasm for resistance breeding of cultivated lettuce (L. sativa).

Wild Lactuca species, their genetic diversity, resistance to diseases and pests, and exploitation in lettuce breeding

Current knowledge of wild Lactuca L. species, their taxonomy, biogeography, gene-pools, germplasm collection quality and quantity, and accession availability is reviewed in this paper. Genetic diversity of Lactuca spp. is characterized at the level of phenotypic and phenological variation, variation in karyology and DNA content, biochemical traits, and protein and molecular polymorphism. The reported variation in reaction to pathogens and pests of wild Lactuca spp. is summarized, including the viral pathogens (Lettuce mosaic virus-LMV, Mirafiori lettuce virus/Lettuce big vein virus-LBV, Beet western yellows virus-BWYV, Tomato spotted wilt virus-TSWV, Cucumber mosaic virus-CMV, Lettuce necrotic stunt virus-LNSV), bacterial pathogens (corky root-Rhizomonas suberifaciens, bacterial leaf spot-Xanthomonas campestris pv. vitians), fungal pathogens (downy mildew-Bremia lactucae, powdery mildew-Golovinomyces cichoracearum, anthracnose-Microdochium panattoniana, stemphylium leaf spot-Stemphylium spp., sclerotinia drop-Sclerotinia spp., verticillium wilt-Verticillium dahliae, fusarium wilt-Fusarium spp., pythium wilt-Pythium tracheiphylum, P. uncinulatum), nematodes (potato cyst nematode-Globodera rostochiensis, root-knot nematode-Meloidogyne spp., incognita, hapla, javanica, enterolobii), insects and mites (the green lettuce aphid-Nasonovia ribisnigri, the green peach aphid-Myzus persicae, the potato aphid-Macrosiphum euphorbiae, leafminer-Liriomyza spp., L. langei). The approaches used to exploit wild Lactuca spp. in lettuce breeding (interspecific hybridization, cell and tissue culture, transformation) are dicussed, and known examples of lettuce cultivars with traits derived from wild Lactuca spp. are described.

Wild and weedy Lactuca species, their distribution, ecogeography and ecobiology in USA and Canada

During 2002, 2004, 2006 and 2008, trips were undertaken in the United States and Canada by members of the Department of Botany, Palacký University in Olomouc (Czech Republic) to record distributions and sample the diversity of wild and weedy Lactuca species. In that period, 16 states in the USA (Arizona, California, Colorado, Idaho, Iowa, Minnesota, Montana, Nevada, New York, North Carolina, Oregon, South Dakota, Utah, Washington, Wisconsin, Wyoming) and two provinces in Canada (Ontario, Quebec) were visited. Seven wild and weedy Lactuca species (L. serriola, L. saligna, L. virosa, L. canadensis, L. biennis, L. floridana, L. ludoviciana), an interspecific hybrid (L. canadensis × L. ludoviciana), and an undetermined Lactuca species were recorded, and 343 seed samples were collected from 200 locations. The largest number of wild Lactuca species that we observed was recorded in Iowa, and the largest number of samples was collected in California. The most common habitats of weedy species (L. serriola, L. saligna and L. virosa) were along transport corridors, such as roadsides, road ditches, parking sites and petrol stations, grassy slopes and ruderal places. The most frequent species was L. serriola, an Old World introduction, which we observed in all states and provinces visited, except for New York, and in broad range of elevations (up to 2,358 m a.s.l.). Lactuca saligna was recorded only one time in Salinas, California. Lactuca virosa was found repeatedly in Washington State along the road to the Mt. St. Helens and in Redwoods, California. Native North American taxa (L. canadensis, L. floridana, L. ludoviciana) were recorded only in Iowa. Lactuca biennis was found only in Canada in the southern part of Quebec. Records of diseases and pests showed only occasional occurrences of downy mildew (Bremia lactucae) and powdery mildew (Golovinomyces cichoracearum). Most plant-disease findings were made in Canada. Results of these efforts enrich our knowledge of the distribution, ecogeography and ecobiology of Lactuca species occurring spontaneously in North America. Seed collections (accessions) will be conserved in the working collection of the genebank at Palacký University to support future research in ecogeography, ecobiology, resistance and genetic polymorphisms, and to enrich of germplasm diversity available for lettuce breeding.

Representation of wild Lactuca spp. (Asteraceae, Lactuceae) in world genebank collections

The spread of the genus Lactuca worldwide includes 16 species in Europe, 51 in Asia, 43 in Africa and 12 in America (mostly North American subcontinent). Natural distributions of Lactuca spp. are compared with the representation of wild Lactuca spp. in world genebank collections as recently summarized in the “International Lactuca database” (ILDB). A total of 27 wild Lactuca species are reported in world genebank collections in the ILDB, however, due to incorrect taxonomic determination the real number of species is lower. The substantial part (92%) of the collections is represented only by three species (L. serriola, L. saligna, L. virosa) and from a geographic viewpoint they are mostly European in origin. The autochthonous species originating from other continents (Asia, Africa, America), which form c. 83% of known Lactuca spp. richness, are very poorly represented in collections (only c. 3%). The majority of accessions originate from Europe (59%) and Asia (37%), nevertheless the whole area of natural distribution is not well covered. An extremely low number of accessions is available from Africa and America (2% each). Thus, the global biodiversity of Lactuca spp. germplasm is represented very poorly and is biased in genebank collections. For future studies of taxonomy, phytogeography, ecology, phylogenetic relationships, genetic diversity, inter- and intra-population structure, resistance research and practical breeding exploitation of wild Lactuca spp. germplasm, plant material from a wider ecogeographic distribution must be collected and introduced into genebank collections more intensively.

Description of morphological characters of wild Lactuca L. spp. genetic resources (English-Czech version)

Of about 100 Lactuca species, there are only 22 represented in world gene bank collections. The description of morphological and other important features of wild lettuce accessions and a correct taxonomic ranging increase their potential successful utilization. A broad study of 22 wild Lactuca species, their morphology, anatomy, karyology, DNA content and isozyme variation, and a search of large number of literature sources (description keys, floras, monographs) provided a base for an elaboration of a descriptor set. This set consists of 88 descriptors and 24 of them are elucidated by figures. It provides a tool for Lactuca species characterization and determination and for a discrimination of an infraspecific variation. Obtained data can be used for description of wild Lactuca genetic resources and also for research purposes.

Lactuca saligna, a non-host for lettuce downy mildew ( Bremia lactucae), harbors a new race-specific Dm gene and three QTLs for resistance.

Lactuca sativa (lettuce) is susceptible to Bremia lactucae (downy mildew). In cultivated and wild Lactuca species, Dm genes have been identified that confer race-specific resistance. However, these genes were soon rendered ineffective by adaptation of the pathogen. Lactuca saligna (wild lettuce) is resistant to all downy mildew races and can be considered as a non-host. Therefore, L. saligna might be an alternative source for a more-durable resistance to downy mildew in lettuce. In order to analyze this resistance, we have developed an F(2) population based on a resistant L. saligna x susceptible L. sativa cross. This F(2) population was fingerprinted with AFLP markers and tested for resistance to two Bremia races NL14 and NL16. The F(2) population showed a wide and continuous range of resistance levels from completely resistant to completely susceptible. By comparison of disease tests, we observed a quantitative resistance against both Bremia races as well as a race-specific resistance to Bremia race NL16 and not to NL14. QTL mapping revealed a qualitative gene ( R39) involved in the race-specific resistance and three QTLs ( RBQ1, RBQ2 and RBQ3) involved in the quantitative resistance. The qualitative gene R39 is a dominant gene that gives nearly complete resistance to race NL16 in L. saligna CGN 5271 and therefore it showed features similar to Dm genes. The three QTLs explained 51% of the quantitative resistance against NL14, which indicated that probably only the major QTLs have been detected in this F(2) population. The perspectives for breeding for durable resistance are discussed.

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.

Biodiversity and ecogeography of wild Lactuca spp. in some European countries

Staff members of the Department of Botany of Palacký University in Olomouc and Gene Bank Department – Workplace Olomouc, Research Institute of Crop Production in Prague, Czech Republic, conducted an expedition in seven European countries (Austria, Czech Republic, France, Germany, Italy, the Netherlands, Switzerland) in August/September 1999 to collect wild Lactuca spp. germplasm and study its geographic distribution, ecology and biodiversity. During the mission, more than 600 locations were visited resulting in the collection of 602 seed samples (accessions) of wild Lactuca species and 13 seed samples of related genera (Chondrilla and Mycelis). Lactuca serriola f. serriola, L. serriola f. integrifolia, L. saligna and L. viminea subsp. chondrilliflora were prevalent in southern Europe (Italy, France), however, only L. serriola was common in central and western Europe (Austria, Czech Republic, Germany, Netherlands, Switzerland). The greatest diversity of Lactuca species was found in France, where also the most seed samples (165) were collected. The most characteristic habitats with a high density of Lactuca spp. populations were observed along roads and highways, grassy ditches, ruderal communities, and dust-heaps. Natural infections by powdery mildew (Erysiphe cichoracearum) and downy mildew (Bremia lactucae) on some wild Lactuca spp. were observed. Recent observations concerning the geographic distribution, population structure, habitats, and natural occurrence of diseases of Lactuca spp. are discussed. This assemblage of genetic resources of Lactuca spp. can serve as the basis of future studies of species diversification, spatial population structure, plant microevolution, domestication processes, and genetic variability of host-parasite interactions.

Sesquiterpene Lactones in Virus-Resistant Lettuce

Lactuca saligna and Lactuca virosa are two wild lettuce species that show multiple virus resistance, and both have been successfully used in breeding programs for the introduction of virus resistance into commercial lettuce, Lactuca sativa. These two wild Lactuca species contained three major sesquiterpene lactones, i.e. lactucin, 8-deoxylactucin, and lactucopicrin, as measured by gradient RP-HPLC. L. saligna contained 103, 372, and 79 μg/g, respectively, and L. virosa contained 257, 173, and 1733 μg/g, respectively, of these three lactones that are present in both free and glycoside bound forms. The commercial lettuce varieties Montello (L. sativa) and Saladcrisp, derived from L. sativa×L. saligna, had very low levels of lactucin and lactucopicrin and were devoid of 8-deoxylactucin. Five breeding lines of lettuce, bred from the same interspecies cross as var. Saladcrisp, were also devoid of 8-deoxylactucin and contained between 0 and 60% as much lactucin or lactucopicrin as their parent strains. One lettuce breeding line, 93-426, contained only 18 μg/g of lactucin and was devoid of both lactucopicrin and 8-deoxylactucin, but this line is resistant to three viruses. This suggests that the level and types of these sesquiterpenene lactones present in lettuce have little effect on virus resistance. Nevertheless, lettuce breeders using wild Lactuca as resistant parents should be cognizant of their high levels of sesquiterpene lactones

Further observations on variation of lettuce mosaic virus in relation to lettuce (Lactuca sativa), and a discussion of resistance terminology

Two potyvirus isolates from endive, originating from southern France (Ls252) and from the Netherlands (Ls265), that were highly and poorly pathogenic on lettuce, respectively, were compared with a common isolate (Ls1) of lettuce mosaic virus (LMV) and with two highly deviant Greek isolates fromHelminthia (Picris) echioides (Gr4) and endive (Gr5), earlier recognized as LMV. The isolates could not be distinguished by particle morphology and serology, and were all identified as LMV. Leaf curling, plant stunting and necrosis were more characteristic of the virus than mosaic. The isolates studied varied considerably on differential host species and a range of lettce cultivars including pathotype differentials of Pink et al. [1992b]. Ls1 and Ls265 reacte largely as pathotype II, including the ‘common strain’ of the virus, but Ls265 was least pathogenic on lettuce. Ls252 fitted pathotype IV and was very similar to LMV-E (the ‘Spanish strain’). The Greek isolates were very similar to each other in causing very severe symptoms on some non-lettuce hosts and a number of lettuce cultuvars. In lettuce variectal reaction Gr4 resembled pathotype I, but Gr5 severely affected ‘Salinas 88’, resistant to pathotypes I, II and III, and it appears to be a novel pathotype. Genetic interaction between lettuce and LMV is not following a simple yes-or-no pattern, and it is not a mere matter of resistance versus susceptibility. Adoption of a more realistic resistance terminology is proposed. None of the lettuce cultivars tested was resistant to the most pathogenic isolate Ls252, but resistance to it was found in 2 out of 12 wildLactuca species tested (Lactuca perennis andL. tatarica) while the symptomless plants ofL. perennis clearly reacted in ELISA.

A Lactuca universal hybridizer, and its use in creation of fertile interspecific somatic hybrids.

A Lactuca sativa cv. Ardente line heterozygous for a gene encoding resistance to kanamycin, a positive and dominant trait, was crossed with cv. Girelle, which is heterozygous for a recessive albinism marker. The resulting seeds yielded 25% albino seedlings, of which 50% were also resistant to kanamycin. Such plantlets (KR, a) grown in vitro were used for preparation of universal hybridizer protoplasts, since green buds that can develop on kanamycin containing-medium should result from fusion with any wild-type protoplast. To test the practicability of this selection scheme, we fused L. sativa KR, a protoplasts with protoplasts derived from various wild Lactuca as well as various other related species. Protoplast-derived cell colonies were selected for resistance to kanamycin at the regeneration stage. Green buds were regenerated after fusion with protoplasts of L. tatarica and of L. perennis. So far, 9 interspecific hybrid plants have been characterized morphologically. In addition, random amplified polymorphic DNA (RAPD) analysis with selected primers confirmed that these plants are indeed interspecific hybrids. Some plants are female-fertile and production of backcross progenies with L. sativa is in progress. Since many desirable traits such as resistances to viruses, bacteria and fungi (Bremia lactucae) have been characterized in wild Lactuca species, the use of somatic hybridization in breeding programmes now appears a practical possibility.

Evaluation of wild Lactuca species for resistance of natural infection of powdery mildew (Erysiphe cichoracearum)

The 99 accessions of seven wild Lactuca species (L. serriola, L. saligna, L. virosa, L. viminea, L. perennis, L. tenerrima, L. tatarica) and Mycelis muralis (syn. L. muralis) were tested for natural infection of Erysiphe cichoracearum L. serriola accessions were highly susceptible. L. saligna showed highly variable levels of resistance. Other Lactuca species were mostly resistant or only moderately susceptible.

The wild genetic resources of cultivated lettuce (Lactuca sativa L.)

Seven wild Lactuca species: L. serriola L., L. aculeata Boiss. & Ky., L. scarioloides Boiss., L. azerbaijanica Rech., L. georgica Grossh., L. dregeana DC. and L. altaica Fisch. & C.A. Mey., are taxonomically closely related to the cultivated lettuce. Together with L. sativa they form a distinct natural group. Only scanty information is available on genetic affinities among the various members of this group. However, the combined taxonomic and genetic evidence indicates that all these 7 wild lettuces are very likely interfertile (or largely interfertile) with the crop, and constitute its primary wild gene pool. Also the taxonomically more distant L. saligna L. is partly cross-fertile with the crop.

Potential sources of resistance to lettuce downy mildew (Bremia lactucae) in differentLactuca species

More than 200 accessions of three wildLactuca species were screened in the laboratory for race-specific resistance toBremia lactucae. Only a fewLactuca entries showed resistance both as seedlings and in a leaf disc test. Accessions ofL. serriola andL. sativa had a low degree of resistance. The best entries were also compared under field conditions. TheL. saligna entries were totally free from disease in the field test. High resistance was also recorded in a fewL. serriola entries and in lettuce cultivars such as Saffier and Mariska.