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.     

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.     

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.     

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.     

Evaluation of tomato entries with different combinations of resistance genes to tomato yellow leaf curl disease in Tunisia

Tomato yellow leaf curl virus (TYLCV) is one of the most widespread begomoviruses transmitted by the whitefly Bemisia tabaci that cause tomato yellow leaf curl virus diseases (TYLCDs). TYLCD losses can be especially severe in open‐field tomato (Solanum lycopersicum L.) crops. TYLCV is particularly well known and widespread in the Mediterranean (Med) countries where TYLCD control is mostly based on insecticidal control of B. tabaci populations. Unfortunately, Tunisian B. tabaci populations include Middle East‐Asia Minor I and the Med species that have developed resistance to many classes of insecticides. Therefore, TYLCD‐resistant cultivars are essential for sustainable disease management. Six TYLCD resistance genes (Ty) have been introgressed from wild species into cultivated tomato and are available for breeding. Information on the Ty genes or gene combinations is useful for breeding resistant cultivars. To this end, 14 tomato lines carrying different Ty gene combinations and two susceptible tomato entries were evaluated for TYLCD incidence and severity in two field trials during late season in Tunisia. Entries with Ty‐1/Ty‐3 + Ty‐2 offered the highest levels of resistance in Tunisia.     

Evaluation of greenhouse inoculation techniques to screen sorghum for resistance to downy mildew

Summary Six inoculation techniques were compared for the artificial promotion of downy mildew (Peronosclerospora sorghi) in sorghum. These were (1) sprouted seeds incubated between sporulating infected leaves, (2) sprouted seeds depped in conidial suspension, (3) sprouted seeds sprayed with conidial suspension, (4) seedlings at plumule stage inoculated with drops of a conidial suspension, (5) seedlings at plumule stage sprayed with a conidial suspension, and (6) seedling showered with conidia falling from infected leaves. Seedlings at the one-leaf stage sprayed with a conidial suspension (6 × 10⁵ ml^-1) showed the highest systemic infection (100%) in the susceptible lines IS 643 and IS 18433. This technique is effective, repeatable, and allows the deposition of a conidial suspension as a fine mist on the entire seedling surface. In the greenhouse, the technique was used to test the downy mildew reaction of genotypes previously reported as resistant (< 5% incidence) in 3–4 years of field screenings. Of the 61 genotypes tested, 21 were free from downy mildew, 14 had less than 5% incidence, and the rest showed variable susceptible reactions. Therefore, the technique can be reliably and effectively used in the greenhouse to detect disease escapes and to indentify resistance.     

Testing resistance to TSWV introgressed from Lycopersicon peruvianum by artificial transmission techniques

Three lines of Lycopersicon esculentum (RDD, UPV 1 and UPV 32) with resistance to TSWV introgressed from L. peruvianum have been tested. RDD is a carrier of the SW5 gene and the other two lines have been developed at the Universidad Politécnica de Valencia (UPV) from accessions collected in the Andean region. Two methods of artificial inoculation, mechanical and by Frankliniella occidentalis populations, and three highly virulent Spanish isolates of TSWV have been used. Inoculation by populations of thrips proved to be more efficient than mechanical transmission independently of the considered isolate. The three lines were completely resistant when mechanical transmission was used. Also RDD and UPV 1 showed high resistance levels by thrips inoculation method; the isolates had the same effect on these two genotypes. Nevertheless, UPV 32 shows a partial resistance depending on the isolate utilized. The inoculation of virulent isolates by thrips under a high pressure of inoculum caused infection in a few plants of the three genotypes tested. In spite of being the most efficient at present for their unspecificity to isolates (and so the most used by breeders), resistance conferred by SW5 does not guarantee the absence of infection depending on the prevailing crop conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic Analysis of Resistance to Phytophthora Root Rot in Tomato (Lycopersicon esculentum Mill.)

The resistant accession, LA1312, and the susceptible cultivar ‘Peto 343′, were crossed to develop F₁, F₂ and BC₁ populations for genetic analysis of resistance in tomatoes to Phytophthora parasitica Dastur, the causal agent of Phytophthora root rot. There was no maternal effect on resistance. Generation means analysis indicated that tolerance to Phytophthora root rot was under genetic control with both simple (additive and dominance) and digenic interaction (additive × additive and additive × dominance) effects contributing to the total genetic variation among generation means. Weighted least square regression analysis indicated that the majority (ca. 96 %) of the genetic variation could be explained by simple additive effects alone. Narrow sense heritability was estimated as 0.22. Based on effective factor formulae, at least five effective factors controlled the resistance. Implications for breeding procedures are discussed.     

Inheritance of tomato leaf curl virus resistance in Lycopersicon hirsutum f. glabratum

Summary Inheritance of resistance to tomato leaf curl virus (TLCV) was studied in the progenies derived from interspecific crosses between TLCV resistant Lycopersicon hirsutum f. glabratum line B 6013 and five susceptible cultivars (HS 101, HS 102, HS 110, Pusa Ruby and Punjab Chhuhara) of L. esculentum. P₁, P₂, F₁, F₂, B₁ and B₂ progenies of the five crosses were artificially inoculated with local strains of TLCV by means of the vector whitefly, Bemisia tabaci (Genn.). and the disease reaction was studied in all the crosses. Reaction of parents, F₁, F₂ and backcrosses suggests that resistance derived from L. hirsutum f. glabratum B 6013 is based on two epistatic genes, one from the wild parent and one from the cultivated one, resulting in a 13:3 segragation in the F₂.     

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.     

New sources for high resistance of tomato to the tomato spotted wilt virus from Lycopersicon peruvianum

Mechanical inoculation and transmission by thrips in a growth chamber were used in order to screen Lycopersicon peruvianumand Lycopersicon chilense germplasm for tomato spotted wilt virus (TSWV) resistance. Two highly aggressive Spanish TSWV isolates (HA-931100 and T-941117), having different restrictotypes were used. L. peruvianum accessions PI-126935, PI-126944, CIAPAN 16, PE-18 and CIAPAN 17 showed high resistance to both isolates in mechanical and thrips transmission. Their resistance appears useful in breeding programmes.     

Transfer of Tomato Leaf Curl Virus Resistance from Lycopersicon hirsutum f. glabratum to L. esculentum

Six lines, i.e., H-2, H-11, H-17, H-23, H-24, and H-36, resistant to Tomato Leaf Curl Virus (TLCV) have been developed with controlled introgression of L. hirsutum f. glabratum into Lycopersicon esculentum. The disease incidence, 120 days after inoculation, of those lines derived from L. hirsutum f. glabratum ranged from 8.3 to 35.0 %, whereas in susceptible varieties it ranged from 95.0 to 100 %. The coefficient of infection (CI) values in the resistant lines were very low, ranging from 0.25 to 4.55, whereas in susceptible varieties CI values ranged from 60.56 to 88.96. Line H-2 had the highest resistance by showing the least disease incidence and CI values. The fruit size and days to maturity in resistant lines were close to those of cultivated susceptible varieties. These lines have the scope for being used as varieties in the TLCV infested areas or as foundation lines for further genetic improvement.     

Evaluation and inheritance of the Lycopersicon hirsutum resistance against potato virus Y

Summary Potato virus Y (PVY) infects most Solanaceous crops grown in Mediterranean countries in open fields and in greenhouses. Necrogenic strains, which have been isolated from diseased tomatoes in France since the 1980's, seriously cause yield and quality loss of tomato fruits. Lycopersicon hirsutum PI 247087 was found to be resistant to PVY. Virus could not be detected in inoculated leaves by ELISA and/or by back-inoculation on susceptible plants. This resistance was efficient against the 16 tested isolates or strains. Temperature and inoculum concentration did not affect its expression. All the F₁ plants of (Momor × PI 247087), (PI 134417 × PI 247087) and (PI 247087 × PI 134417) had symptom scores and ELISA values similar to those of the susceptible parents. The mechanism of resistance could be immunity-like or inhibition of virus migration from cell to cell. The resistance of L. hirsutum PI 247087 appeared to be governed by two independent recessive genes. In a few F₂ plants of (PI 134417 × PI 247087) and F₂ (Momor × PI 247087), virus was able to multiply in the inoculated leaves but could not establish a systemic infection. This finding may suggest a mechanism which interfers with the long distance migration of the virus in the plant.     

Genetics of resistance to early blight (Alternaria solani Sorauer) in tomato (Lycopersicon esculentum L.)

The genetic nature of early blight resistance in tomato was studied in three crosses at seedling and adult plant stages. A six generation mean analysis of the cross Arka Saurabh (susceptible) × IHR1939 (resistance) and its reciprocal cross revealed that the resistance to early blight was conferred by recessive polygenes at both seedling and adult plant stages. This polygenic early blight resistance revealed the importance of additive and additive × additive gene effects at seedling stage and higher magnitude of dominance and dominance× dominance gene effects at adult plant stage. Evaluation of parents, F₁, F₂ and backcross generations of IHR1816 (resistance) × IHR1939 (resistance) revealed that the early blight resistance genes in IHR1816 (Lycopersicon esculentum NCEBR-1) and IHR1939 (Lycopersicon pimpinellifolium L4394) are independent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of tomato hybrids resistant to tomato leaf curl virus disease in South India

Resistance to tomato leaf curl virus disease (ToLCVD) in tomato (Solanum lycopersicum) is scarce but was developed recently in three open pollinated (OP) varieties (Sankranthi, Nandi and Vybhav), which are now available for cultivation in South India. Hybrids with superior yield capabilities, however, are a preferred choice of cultivation by farmers but hybrids are highly susceptible to ToLCVD. In order to develop virus resistant tomato hybrids, the three OPs were crossed with 12 tomato genotypes with superior agronomic characteristics. From the crosses, 20 hybrids were selected (named BLRH-1 to BLRH-20, Bangalore leaf curl virus-resistant hybrid) and evaluated for their resistance to ToLCVD at the University of Agricultural Sciences, Bengaluru, South India, both through whitefly-mediated inoculations in the glasshouse and natural infection in the field during summer 2005. Their growth and fruit yield parameters were compared against the popularly grown hybrids and OPs. Differences occurred between hybrids in disease incidence, spread, symptom severity and fruit yield. Of the 20 hybrids evaluated, 11 were found resistant to ToLCVD in the field, but only three (BLRH-3, BLRH-9 and BLRH-16) remained resistant when challenged with high virus inoculum pressure in the glasshouse through whitefly-mediated inoculations. None of the six commercially available hybrids tested was found resistant to ToLCVD. An examination of the extent of heterosis in resistant hybrids for both quantitative and qualitative characters over the popularly grown hybrid, US-618, indicated a major improvement in virus resistance, yield and fruit quality. All the twenty hybrids yielded significantly more than US-618, and of these, BLRH-3 and BLRH-16 were considered the best with yields over ca. 60 t/ha and resistant to virus both in the glasshouse and field.     

Evaluation of resistance to melon yellow spot virus in a cucumber germplasm collection

Melon yellow spot virus (MYSV), a member of the genus Tospovirus, is a serious thrips‐transmitted virus of cucurbits in Japan. Resistant cultivars provide an effective means for reducing the impact of the disease; however, no MYSV‐resistant cucumber has been reported. Susceptibilities of 398 cucumber accessions originating from 26 countries were evaluated by mechanical inoculation of MYSV. Thirteen accessions from South Asia, South East Asia and unknown origin had low disease severity indices (DSIs), and 10 of them showed no necrotic lesions on inoculated leaves. No or little positive reaction was detected by DAS‐ELISA analysis of inoculated leaves of these accessions. 27028930 showed resistance to a melon isolate (MYSV‐S) of MYSV, and ‘Yamakyuri‐1’ showed moderate resistance to a cucumber isolate (FuCu05P).     

Inheritance of resistance to the bipartite Tomato chlorotic mottle begomovirus derived from Lycopersicon esculentum cv. ‘Tyking’

Severe outbreaks of bipartite begomoviruses (family Geminiviridae) have been observed on tomatoes after the introduction of the whitefly Bemisia tabaci (biotype B) in Brazil. The Lycopersicon esculentum line ‘TX 468-RG’ was identified as one of the best sources of broad-spectrum resistance to species comprising the tomato-infecting Begomovirus complex in Brazil. The genetic basis of resistance to one Begomovirus isolate was investigated using populations from the cross between ‘TX 468-RG’ (P₁) and the susceptible line ‘Ohio 8245’ (P₂). Parental lines, F₁, backcross (BC) to P₁ and BC to P₂ and F₂ generations were inoculated at the two true-leaf stage using 20 viruliferous whiteflies per plant. Assessment was done two weeks after inoculation based upon visual analysis of symptom expression. The ratio of resistant to susceptible plants closely fit to a single recessive gene (locus) model. The sequence analysis indicated that the Begomovirus isolate used in this assay was closely related to the bipartite Tomato chlorotic mottle virus. Therefore, this gene/locus, was tentatively named tcm-1 (tomato chlorotic mottle virus resistance-1). This locus has been transferred to distinct tomato cultivars and levels of resistance similar to that of ‘TX 468-RG’ were observed in advanced (F₈ and F₉) generations. In addition, breeding lines carrying the tcm-1 locus were also resistant to other Brazilian bipartite tomato-infecting Begomovirus species.     

Evaluation of screening techniques and determination of criteria for assessing resistance to Corynebacterium michiganense in tomato

Summary When screening populations segregating for resistance to Corynebacterium michiganense a rapid and reliable inoculation technique is needed. From the several techniques tested, inoculation of the petiole was found to give better differentiation of resistant and susceptible plants than did inoculation of the root or leaf. When petiole-inoculation was compared with stem-inoculation the latter showed the lowest environmental variation and is suggested for use in genetic studies. Stem-inoculation involved clipping the stems of 4 week old seedlings 1 cm above the cotyledonary leaves with scissors which had been dipped in inoculum.Petiole inoculation and, to a lesser degree, stem inoculation failed to produce uniform infection, resulting in variation in the expression of the disease. The scoring was therefore based on the presence, and not the degree, of wilting and stunting, or the size of the cankers. Plants free of all these symptoms were rated resistant. The coefficient of variation for vascular discoloration on individual plants was calculated to be 50%, which in part may be explained by the histopathology of the host.The virulent isolates cm 21 caused death of seedlings resistant to isolate H. However, cultivars resistant to isolate H in the seedling stage did not decrease in fruit weight as compared to susceptible cultivars when inoculated with isolate cm 21 at anthesis.Michigan Agricultural Experiment Station Journal No. 7424.     

Screening for resistance in wild Lycopersicon species to Fusarium oxysporum f.sp. lycopersici race 1 and race 2

Wild Lycopersicon accessions were screened for resistance to the Fusarium wilt disease caused by Fusarium oxysporum f.sp. lycopersici (Fol) race 1 and race 2. In total, four isolates of each race were used. Among 17 accessions of six Lycopersicon species tested, a wide genetic variation for wilt resistance was observed. Most accessions were highly susceptible, some showed intermediate resistance, but one accession of L. cheesmanii (G1.1615 = PI 266375) and two accessions of L. chilense (G1.1556 and G1.1558) were highly resistant to Fol races 1 and 2. The resistance in the latter three accessions equalled or was higher than the resistance determined by the known I-genes, that have been widely used in breeding programmes. These newly found resistant accessions provide breeders with more opportunities for Fusarium disease resistance and may contribute to our understanding of Fusarium disease resistance gene organisation and evolution.     

Evaluation of seven Lycopersicon species for resistance to tomato spotted wilt virus (TSWV)

Summary One hundred eighty-eight accessions of Lycopersicon cheesmanii, L. chilense, L. chmielewskii, L. hirsutum, L. parviflorum, L. pennellii, and L. peruvianum were screened for resistance to three isolates of tomato spotted wilt virus (TSWV). All plants in an accession were initially screened for resistance to TSWV using isolate 85–9 from Arkansas. Visual symptoms were used to cull obviously infected plants, followed by enzyme-linked immunosorbent assay (ELISA) to identify uninfected plants. Cuttings were taken from uninfected plants in the first screening and the resulting plants were inoculated with isolates Glox and T-2 from Texas and Hawaii, respectively. No resistance was identified in L. cheesmanii, L. chmielewskii, L. hirsutum, L. parviflorum, and L. pennellii. However, 33 of 63 L. chilense accession produced 91 of 1268 plants that were uninfected with isolate 85–9 and 20 accessions that produced 40 of 257 plants that were not infected with any of the isolates. After screening with isolate 85–9 9 of 12 L. peruvianum accessions tested had 38 plants uninfected and 8 accessions had 25 plants that were not infected with any of the isolates.