Pyramiding of genes conferring resistance to Tomato yellow leaf curl virus from different wild tomato species

Tomato (Solanum lycopersicum) production in tropical and subtropical regions of the world is limited by the endemic presence of Tomato yellow leaf curl virus (TYLCV). Breeding programmes aimed at producing TYLCV‐resistant tomato cultivars have utilized resistance sources derived from wild tomato species. So far, all reported breeding programmes have introgressed TYLCV resistance from a single wild tomato source. Here, we tested the hypothesis that pyramiding resistances from different wild tomato species might improve the degree of resistance of the domesticated tomato to TYLCV. We have crossed TYLCV‐resistant lines that originated from different wild tomato progenitors, Solanum chilense, Solanum peruvianum, Solanum pimpinellifolium, and Solanum habrochaites. The various parental resistant lines and the F₁ hybrids were inoculated in the greenhouse using viruliferous whiteflies. Control, non‐inoculated plants of the same lines and hybrids were exposed to non‐viruliferous whiteflies. Following inoculation, the plants were scored for disease symptom severity, and transplanted to the field. Resistance was assayed by comparing yield of inoculated plants to those of the control non‐inoculated plants of the same variety. Results showed that the F₁ hybrids between the resistant lines and the susceptible line suffered major yield reduction because of infection, but all hybrids were more resistant than the susceptible parent. All F₁ hybrids resulting from a cross between two resistant parents, showed a relatively high level of resistance, which in most cases was similar to that displayed by the more resistant parent. In some cases, the hybrids displayed better levels of resistance than both parents, but the differences were not statistically significant. The F₁ hybrid between a line with resistance from S. habrochaites and a line with resistance from S. peruvianum (HAB and 72‐PER), exhibited the lowest yield loss and the mildest level of symptoms. Although the resistance level of this F₁ hybrid was not statistically different from the level of resistance displayed by the 72‐PER parent itself, it was statistically better than the level of resistance displayed by the F₁ hybrids between 72‐PER and any other resistant or susceptible line.     

Resistance in two Lycopersicon species to an Arkansas isolate of tomato spotted wilt virus

Summary Tomato spotted wilt virus (TSWV) was obtained from infected tomatoes in commercial fields in Arkansas in 1985. A greenhouse screening procedure for identifying tomatoes resistant to TSWV was established using an enzyme-linked immunosorbent assay (ELISA) to detect infected plants. Symptom expression was variable and symptom expression was not reliable for identifying infected plants. Germplasm evaluated for resistance to one typical Arkansas isolate (85–9) of TSWV included: twenty cultivars and breeding lines of Lycopersicon esculentum Mill, 52 accessions of L. pimpinellifolium (Jusl.) Mill and 8 accessions of L. peruvianum (L.) Mill. All cultivated accessions and breeding lines evaluated were susceptible. Some individual plants in several accessions of L. pimpinellifolium were resistant and nearly all plants of the L. peruvianum accessions that were evaluated were resistant to isolate 85–9.Dept. of Plant Pathology     

Screening wild Lycopersicon species for resistance to powdery mildew (Oidium lycoperiscum)

Since the late 1980s powdery mildew, designated Oidium lycopersicum, frequently invaded the tomato crop in Western Europe. All commercial cultivars are susceptible. To screen for resistance in wild species a reliable and efficient disease test was developed. Young plants with two to three true leaves are inoculated at high relative humidity by spraying with a freshly prepared suspension of 2×10⁴ conidia, ml^–1. Symptoms are periodically evaluated according to a scale based on the percentage of leaf area with mycelium.One hundred and twenty seven accessions, representing eight wild Lycopersicon species, were screened for resistance to O. lycopersicum. A large variation in resistance was found between species. L. hirsutum was the most resistant species; L. pennellii was moderately resistant; species of the subgeneric group of L. esculentum and of the peruvianum-complex were all susceptible. L. parviflorum was classified separately due to a large variation between accessions. Except for this species, a low variation was found between accessions within species. High levels of resistance were observed in four accessions of L. hirsutum, in one of L. parviflorum and in one of L. peruvianum. This resistance is characterized by a very low disease incidence and a strongly restricted mycelium growth and lack of sporulation.     

Overcoming unilateral breeding barriers between Lycopersicon peruvianum and cultivated tomato,Lycopersicon esculentum

Summary Treatment of Lycopersicon peruvianum stigmas with an artificial medium analogous to stigmatic exudate allowed pollen germination and growth on immature pistils. Growth of Lycopersicon esculentum pollen tubes to L. peruvianum ovules, an otherwise incompatible cross, was achieved following such treated bud-pollinations. No plantlets were recovered, although a few embryos from this cross at the globularity heart stage of development were excised at 22 days after pollination, indicating the presence of crossing failures as severe as in the reciprocal cross. Hybrid plants were obtained from the reciprocal cross, using as pollen parent an L. peruvianum line selected for congruity with L. esculentum. Bud pollinations to L. peruvianum, using these interspecific F₁ hybrids as the pollen parent, allowed viable embryo development and plantlet recovery. Resulting backcross plants, which possess approximately 1/4 L. esculentum genome in a L. peruvianum cytoplasm, may facilitate further introgression of the L. esculentum nuclear genes into in foreign cytoplasm.     

Developing tomato breeding lines resistant to tomato yellow leaf curl virus

Using controlled whitefly-mediated inoculation techniques, seven Lycopersicon chilense accessions, highly resistant to isolates of tomato yellow leaf curl virus(TYLCV) from Southern Europe, TYLCV-Sr, were selected. All exhibited similar levels of partial resistance, being symptomless and with low levels of viral DNA accumulation. However, a differential response to infection was found in interspecific hybrids with tomato and inbred lines derived from different L. chilense accessions, allowing a precise discrimination among them. This selection procedure which considers the expression of the resistance genes in the tomato genetic background led to the selection of two highly resistant F₁ hybrids derived from L. chilense LA 1932 and LA 1938. A backcrossing programme was initiated, selecting for horticultural characteristics and TYLCV resistance, in field and controlled inoculation conditions. As a result of this programme, six advanced breeding lines (UPV Ty 1, 3, 6, 9, 17 and 53), exhibiting a high level of resistance to TYLCV-Sr, were obtained. Under high inoculum pressure conditions these lines suffered only 30-40% yield loss relative to non-infected control plants, and compared with 90-95% yield loss in susceptible controls. These lines also have horticultural characteristics appropriate for the fresh market tomato cultivation system in this area, and are a good base material for obtaining commercial hybrids highly resistant to different isolates of TYLCV.     

Inheritance of resistance to potato cyst-eelworm (Heterodera rostochiensis Woll.) in the genus Lycopersicon

Summary The presence of resistance to potato cyst-eelworm in wild Lycopersicon species was confirmed.The resistance of two plant accessions, L. pimpinellifolium, B6173, and L. peruvianum, B6001, was compared in screening tests against three eelworm isolates, using the susceptible L. esculentum Ailsa Craig as a control. Both accessions were shown to possess high levels of resistance to the Wren isolate (pathotype A) of H. rostochiensis, and resistance to the Castle Donington (pathotypes A, B, and E) and Hare Lane (pathotype B) isolates. The level of resistance in L. pimpinellifolium was greater than that in L. peruvianum. Because of its greater resistance, and the ease with which it can be hybridised with tomato cultivars, L. pimpinellifolium was chosen as a donor parent for breeding programmes designed to incorporate potato cyst-eelworm resistance into a commercial tomato variety.The resistance of L. pimpinellifolium, B6173, to the Wren isolate of H. rostochiensis was shown to be controlled by a single dominant gene for which the symbol Hero is proposed.     

Identification of somatic hybrids of dihaploid Solanum tuberosum lines and S. brevidens by species specific RAPD patterns and assessment of disease resistance of the hybrids

Summary Symmetric somatic hybrids were produced by electrofusion of protoplasts of two dihaploid tuber-bearing potato (Solanum tuberosum L.) lines and Solanum brevidens Phil., a diploid non-tuber-bearing wild potato species. A total of 985 plants was obtained. Verification of nuclear hybridity of putative hybrids was based on additive RAPD patterns, general morphological characteristics and chromosome counts. 53 (90%) calli regenerated into plants which were identified as somatic hybrids. Most of the hybrids were aneuploids at the tetraploid (4×) or hexaploid (6×) level. The 20 hybrids tested expressed a high level of resistance to potato virus Y (PVY ^N ) characteristic of the S. brevidens parent. Resistance to late blight (Phytophthora infestans (Mont.) de Bary) varied between hybrids, but was on average better than that of the fusion parents. Resistance of hybrids to bacterial stem rot (Erwinia carotovora subsp. atroseptica (van Hall) Dye) was not superior to that of commercial potato cultivars.     

Agroinoculation of Tomato Yellow Leaf Curl Virus (TYLCV) Overcomes the Virus Resistance of Wild Lycopersicon Species

Accessions of the wild tomato species Lycopersicon chilense LA 1969 and L. hirsutum LA 1777 which are resistant to tomato yellow leaf curl virus (TYLCV) in field- and in whitefly-mediated transmission tests were agroinoculated with a tandem repeat of the TYLCV genome. Large amounts of viral DNA started to accumulate in the agroinoculated L. chilense and L. hirsutum plants about 10 days after the agroinoculation. Yellowing and narrowing of the upper leaves were observed in the L. chilense plants but no curling as in susceptible L. esculentum cultivars. The agroinoculated L. hirsutum plants showed typical yellowing and curling of young leaves. These findings indicate that TYLCV introduced by means of agroinoculation leads to the breakdown of natural resistance mechanisms which prevent the replication, spread and expression of symptoms in resistant tomato genotypes.     

Resistance to Tomato spotted wilt virus introgressed from Lycopersicon peruvianum in line UPV 1 may be allelic to Sw-5 and can be used to enhance the resistance of hybrids cultivars

The breeding line UPV 1 developed from the PE-18 accession of Lycopersicon peruvianum collected in Huallanca, Ancash, Peru, shows resistance to TSWV. Mechanical inoculation and thrips transmission were used to study the inheritance of TSWV resistance of this line. UPV 1resistance is controlled by a dominant gene. The penetrance of this resistance gene was complete in mechanical inoculation and incomplete when thrips transmission was used. Linkage tests between the resistance genes of lines UPV 1 and RDD (Sw-5), indicated allelism. A molecular analysis using a SCAR marker tightly linked to Sw-5 also supported this hypothesis. In heterozygotes the level of resistance expressed in UPV 1 is higher than that expressed in RDD (Sw-5), indicating that the resistance from UPV 1 may be of higher value for the development of commercial hybrids. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response of tolerant breeding lines of tomato, Lycopersicon esculentum, originating from three different sources (L. peruvianum, L. pimpinellifolium and L. chilense) to early controlled inoculation by tomato yellow leaf curl virus (TYLCV)

Selection of tomato plants supposedly tolerant to tomato yellow leaf curl virus (TYLCV), based solely on the absence of symptoms in an infested field can be misleading. An inoculation routine was therefore established to avoid escapes and to overcome difficulties associated with the age of the plant at the time of infection. The inoculation routine was applied to a selection of resistant/tolerant individuals generated through a diallel F¹ cross and to F² segregating populations originating from three wild tomato species described as tolerant to TYLCV: Lycopersicon peruvianum EC 104395, Lycopersicon pimpinellifolium Hirsute and Lycopersicon chilense LA 1969. Clear differences were observed between susceptible symptomatic and tolerant symptomless tomato genotypes, indicating that the uncertainty resulting from escapes, from different levels of inoculum, and from the time of inoculation, can be eliminated. The genes involved in tolerance provided different levels of protection; combinations of various tolerant sources and levels in a single genotype gave a higher level of tolerance. Differences in level of protection were found between genes from the same source and between sources; none of the sources tested had complete dominance. The results obtained with the F² segregating population showed that tolerance from L. pimpinellifolium is controlled by one major gene, that from L. chilense by two genes, and that from L. peruvianum by three genes with no dominant effect. The combination of sources for resistance can thus have positive or negative synergistic effects, or no effect. We suggest that a maximal level of tolerance can be obtained by the additive effect of the partly dominant genes from L. pimpinellifolium and L. chilense.     

Low temperature seed germination of Lycopersicon species evaluated by survival analysis

Summary Low temperature germination responses were evaluated for 18 high altitude accessions representing five wild Lycopersicon species and 19 accessions of L. esculentum which have reputed ability to germinate in the cold. Survival analysis indicated that one accession of L. chilense germinates better at 10°C than PI 120256, the fastest-germinating L. esculentum genotype, and that PI 120256 germinates as well as PI 126435 (L. peruvianum). Additional wild ecotypes exhibiting rapid germination at 10°C were identified from L. peruvianum and L. hirsutum. These ecotypes may possess genetic potential for introgressing cold germination ability into L. esculentum cultivars.     

Identification of sources of resistance in Lycopersicon species to Tomato leaf curl geminivirus (ToLCV) by agroinoculation

A total of 90 genotypes of Lycopersicon species were tested forresistance to the Tomato leaf curlgeminivirus (ToLCV) by agroinoculation andthe vector whitefly (Bemisia tabaciGenn.) inoculation techniques underinsect-proof glasshouse conditions. Therate of infection in the inoculated plantswas determined by detection of the viralDNA in individual plants by the nucleicacid spot hybridization (NASH). Of the 38cultivars and 11 breeding lines of L.esculentum Mill. tested, none was highlyresistant or resistant while three andseven were moderately resistant whenexposed to the cloned virus DNAs byagroinoculation and whitefly inoculationrespectively. On the other hand, among the38 commercial cultivars screened, 16(42.1%) were highly susceptible in vectorinoculations and 31 (81.6%) inagroinoculation. Among the exoticcollection (EC) accessions six were highlyresistant, eleven resistant to whiteflyinoculation and none was highly susceptiblein either of the two tests, indicating thepresence of resistance among the ECaccessions. A higher degree of resistancewas observed in other species of Lycopersicon. While only one accession ofL. cheesmanii Riley was tested, itcould not be infected by either of the twomethods. L. pimpinellifolium (Jusl.)Mill. genotype EC 251580 was similarlyresistant. In L. peruvianum (L.)Mill., five EC accessions could not beinfected by whitefly inoculation, withthree of these being resistant and twomoderately resistant in agroinoculation.This study demonstrates the importance ofthe agroinoculation technique in the virusresistance screening programs andidentifies several good sources ofresistance to the Tomato leaf curlvirus in Lycopersicon species.     

Diversity of heat-stable genotype specific resistance to Meloidogyne in Maranon races of Lycopersicon peruvianum complex

Accessions of Lycopersicon peruvianum complex and their F₁progenies were screened for genotype specific resistance to Mi-1-avirulent M. incognita and M. javanica biotypes at25 ^°C and at 32 ^°C (a temperature at which Mi-1resistance is not expressed), and to Mi-1-virulent M.incognita at 25 ^°C. All entries of the L. peruvianumChotano-humifusum race accessions LA2157 and LA2334 were resistantto Mi-1-avirulent biotype at 25 ^°C and at 32 ^°C,indicating that the accessions are homozygous for the heat-stableresistance. The L. peruvianum Maranon race accessions LA1626,LA1708, LA2172, LA2185, LA2326 and LA2328 segregated for heat-stableresistance to Mi-1-avirulent biotype. The F₁ progeny tested ofLA392 × LA2157, LA2334 × LA2157, LA2328 × LA2326,LA2328 × LA2185, LA1708 × LA2328 andLA1626 × LA2172 were resistant to Mi-1-avirulent biotype at32 ^°C. There were differences in the segregating accessions andF₁ hybrids for expression of heat-unstable and heat-stable resistanceto Mi-1-avirulent Meloidogyne spp. The L. peruvianumLA392 and LA2163 and L. chilense LA1968, LA1972, LA2404, LA2405,LA2406, LA2748, LA2930, and the L. peruvianum × L.chilense hybrids were homozygous susceptible with all entries testedsusceptible at 32 ^°C. Cuttings of these L. peruvianumaccessions and their F₁ progenies were susceptible to Mi-1-virulent M. incognita biotype at 25 ^°C.     

Crossability and cytological studies in Solanum macrocarpon and Solanum linnaeanum (Solanaceae).

Summary Crossing experiments involving the various groups of Solanum macrocarpon L. complex, including wild (dasyphyllum), semi-wild and cultivars produced fully fertile F1 and F2 hybrids. This confirmed earlier findings that these groups belong to the same biological species. S. macrocarpon complex and S. linnaeanum Hepper ex Jaeger are isolated by reproductive barriers. F1 and F2 hybrids between various groups of S. macrocarpon showed heterosis. F2 superior hybrids were isolated as candidates for the future breeding programmes. Most wild-type traits like prickliness, hairiness were dominant to those possessed by cultivars. Relatively resistant hybrids to pest attack were noted. S. macrocarpon complex and S. linnaeanum are diploids (2n=24). The overall nature of the karyotype suggests that the two are related. The F1 hybrids between the cultivars of S. macrocarpon and the wild group (S. dasyphyllum) showed normal meiosis. A lack of isolating barriers confirmed that S. macrocarpon complex constitute a single biological species.     

Trichome characteristics of F1Lycopersicon esculentum × L. cheesmanii f. minor and L. esculentum × L. pennellii hybrids and effects on Myzus persicae

Summary The green peach aphid (Myzus persicae) is a serious pest of the tomato (Lycopersicon esculentum). Trichome-based host plant resistance of wild Lycopersicon species may offer an alternative to pesticides for management of this pest. Two wild species, Lycopersicon cheesmanii f. minor and L. pennellii, were crossed with L. esculentum and the types and densities of trichomes possessed by the F₁ hybrids measured. The effects of these trichomes on M. persicae were assessed by placing nymphs on leaves and, 96 hr later, designating each nymph as either dead, alive, emigrating off the leaf or unaccounted for. A generalised linear model was used to detect relationships between the numbers of nymphs in each designation and the densities of trichomes and leaf area. Mortality of nymphs on L. pennellii hybrids was associated with high densities of type IV and low densities of type I trichomes. Mortality of nymphs on L. cheesmanii f. minor hybrids was associated with high densities of type III trichomes. An increase in the densities of type III trichomes on L. pennellii hybrids was associated with greater numbers of nymphs emigrating off the leaf whilst unaccounted nymphs on L. pennellii hybrids were associated with increased densities of type VII and type I trichomes. Further experimentation using excised leaflets found mortality on L. pennellii hybrids may be due to nymphs being hindered by type I trichomes and increased densities of type V trichomes were associated with greater numbers of unhindered nymphs. Implications of results are discussed in the context of introducing trichome-based host plant resistance into L. esculentum.     

Nematode resistance in bananas: screening results on some wild and cultivated accessions of Musa spp.

Bananas cultivated for export all belong to Cavendish cultivars and are all recognized as very susceptible to nematodes, particularly to the burrowing nematode Radopholus similis and the lesion nematode Pratylenchus coffeae. Even if there have been many changes in the management of banana nematodes in large commercial banana plantations, chemical control still remains most often the last resort method to manage the nematodes, although the number of registered products is definitely declining. Therefore, nematode control though genetic improvement is gaining new interest worldwide. In this study, 55 banana accessions mostly diploids from the Musa acuminata genome group (AA) but including some triploid accessions (AAA), some diploids of the Musa balbisiana genome group (BB) and some interspecific hybrids (AAB, AB) were evaluated for resistance to four nematode species R. similis, P. coffeae, Meloidogyne incognita and M. arenaria. These experiments were conducted in a growth chamber under controlled conditions. All banana accessions were susceptible to nematode species, although many different levels of susceptibility were detected. This study confirmed the good resistance status to R. similis of some cultivars from the Pisang jari buaya and Pisang batuau subgroups and the partial resistance of 17 diploid accessions significantly different from the susceptible reference cv. Grande Naine. This study also showed that 12 diploid accessions exhibited a partial resistance to P. coffeae, including some usual or potential genitors belonging to the wild diploids subspecies burmannica (cvs. Long Tavoy 1 and 2) and burmannicoides (cv. Calcutta 4). No source of resistance to Meloidogyne spp. was found. These screening results, combining for the first time four nematode species, are discussed within the scope of banana breeding in order to produce parental diploid lines with single or combined nematode resistances and further develop triploids that can substitute existing susceptible commercial cultivars.     

Levels of host plant resistance to banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), in African Musa germplasm

Forty-five Musa clones, including endemic and introduced cultivars plus hybrids, were evaluated for resistance against the banana weevil, Cosmopolites sordidus, in a field trial in Uganda. The predominant groups of staple crops, East African highland bananas (Musa spp. AAA) and plantains (Musaspp. AAB), as well as plantain-derived hybrids (AAB × AA), showed the highest levels of susceptibility to this pest. These were followed by dessert bananas (Musa spp. AAA), exotic bananas (Musa spp. ABB) and finally diploids of M. acuminata (AA). Hybrids of banana origin were highly resistant. Some East African highland cultivars, especially brewing types (e.g., Kabula, Bagandeseza, Ediirira), showed intermediate levels of resistance. Among the non-highland bananas, high levels of resistance were observed in Yangambi-Km5 (AAA), Cavendish (AAA), Gros Michel (AAA), Kayinja (ABB, Pisang Awak subgroup), Ndiizi (AB, Ney Poovan subgroup)and Kisubi (Ney Poovan subgroup). The highest resistance was observed in banana hybrids TMB2×7197-2, TMB2×8075-7 and the wild banana Calcutta-4 (AA). These were considered the best sources of resistance for a weevil resistance-breeding programme with the two hybrids commonly used as improved male parents. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reaction of Lycopersicon cultivars and wild accessions to tomato leaf curl virus

Summary 1201 tomato cultivars, breeding lines and accessions of Lycopersicon species were screened for tomato leaf curl virus (TLCV) under field and laboratory conditions during summer seasons of 1986 to 1989. Two lines of L. hirsutum (PI 390658 and PI 390659) and 2 lines of L. peruvianum (PI 127830 and PI 127831) were resistant to TLCV infection. These accessions did not produce any leaf curl symptoms either in field or after inoculation by whitefly Bemisia tabaci with TLCV. Adult whiteflies died within 3 days after releasing on resistant accessions (PI 390658, PI 390659, PI 127830), whereas the whiteflies survived upto 25 days on susceptible tomato cultivars. Under field conditions 0–4 and 5–25 adult whiteflies were observed on resistant and susceptible cultivars respectively. Hybridization was effected using the popular tomato cultivars Arka Sourabh, and Arka Vikas, as the female parents and the resistant Lycopersicon wild species as the pollen parents, to incorporate the resistant gene(s) into the edible tomatoes.     

Somatic hybridization as a tool for tomato breeding

Protoplast fusion can be used to produce somatic hybrids of species that cannot be obtained by sexual hybridization. The possibility to introgress genes from Solanum species into the cultivated tomato species Lycopersicon esculentum, and to obtain novel cytoplasm-nucleus combinations (cybrids) was considered as an important strategy to extend the genetic variation available for tomato breeding. Somatic hybrids between L. esculentum and other Lycopersicon species, as well as between L. esculentum and Solanum or Nicotiana species, have been produced. Specific mutants, genotypes with antibiotic resistances, and metabolic inhibition by iodoacetate or iodoacetamide and irradiation were used for the selection of hybrids. In addition, the improvement of protoplast culture techniques and the use of the favourable tissue culture traits derived from species such as L. peruvianum, which have been introduced into tomato by classical breeding, allowed the efficient recovery of somatic hybrids. However, the occurrence of somatic incongruity in fusion combinations of L. esculentum and Solanum and even more in L. esculentum and Nicotiana, did not allow the production of true cybrids and/or fertile hybrids, indicating the importance of both cytoplasm-nucleus and nucleus-nucleus interactions in somatic incongruity. Another problem with fusions between distantly related species is the strongly reduced fertility of the hybrids and the very limited homoeologous recombination between chromosomes of the parental species. Partial genome transfer from donor to recipient through microprotoplast (+) protoplast fusion, and the production of monosomic or disomic chromosome addition lines, light overcome some of these problems. In symmetric somatic hybrids between L. esculentum and S. tuberosum the occurrence of limited somatic and meiotic recombination was demonstrated. Fertile progeny plants could be obtained, though at a low frequency, when embryo rescue was performed on a large scale after backcrossing hexaploid somatic tomato (+) potato hybrids with a tetraploid potato genotype. The potential value of genomic in situ hybridization (GISH) and RFLPs for the analysis of the genome/chromosome composition of the hybrids has been demonstrated for intergeneric somatic hybrids between Lycopersicon and Solanum.Abbreviations cpDNA chloroplast DNA - mtDNA mitochondrial DNA     

Breeding for tolerance to beet-cyst eelworm Heterodera schachtii Schm. in sugarbeet

Summary Successive mass selections were made in sugarbeet varieties (Beta vulgaris L.) and in B. vulgaris × B. maritima L. hybrids for tolerance to wilting caused by beet-cyst eelworm attack. The selected lines showed improved tolerance to wilting, but no evidence of resistance to eelworm infection was demonstrated. By repeated back-crossing of the selected lines with the commercial varieties and concurrent selection root yield could be improved without substantially decreasing the tolerance to eelworm attack. These results indicate that tolerance is partly conditioned by complete and incomplete dominant genes.