Resistant Tomato Cultivars Research Articles

Comparative proteomic analysis of bacterial wilt susceptible and resistant tomato cultivars

To investigate the molecular mechanisms of bacterial resistance in susceptible and resistant cultivars of tomato, a proteomic approach was adopted. Four cultivars of tomato were selected on the basis of their response to bacterial ( Pseudomonas solanacearum) inoculation wherein cultivar Roma and Riogarande, and cultivar Pusa Ruby and Pant Bahr were considered as resistant and susceptible cultivars, respectively. Proteins were extracted from leaves of 3-week-old seedlings of the four cultivars and separated by 2-DE. A total of nine proteins were found to be differentially expressed between the susceptible and resistant cultivars. Amino acid sequences of these proteins were determined with a protein sequencer. The identified proteins belongs to the categories of energy, protein destination and storage, and defense. Of these proteins, a 60 kDa chaperonin and an apical membrane antigen were significantly upregulated in resistant cultivars compared with susceptible cultivars. Application of jasmonic acid and salicylic acid resulted in significant changes in levels of apical membrane antigen and protein disulfide-isomerase. Taken together, these results suggest that apical membrane antigen might be involved in bacterial resistance process through salicylic acid induced defense mechanism signaling in tomato plants.

Crop rotations with Mi gene resistant and susceptible tomato cultivars for management of root-knot nematodes in plastic houses

Four crop rotations including the Mi resistant tomato cv. Monika (R) and the susceptible cv. Durinta (S), were assessed for three consecutive cropping seasons in three unheated plastic houses located in north-eastern (Barcelona), east (Eivissa) and south-western (Cádiz) Spain. Tomatoes were cultivated from March to July 2005 (1), 2006 (2) and 2007 (3) at Barcelona and Eivissa, and from March to July 2006 (1), September 2006 to February 2007 (2) and March to July 2007 (3) at Cádiz. The crop sequences in the rotation were R 1R 2S 3, R 1S 2S 3 S 1R 2S 3 and S 1S 2S 3. Nematode population densities in soil and roots, reproduction indices, and tomato yield were compared for each cropping cycle and rotation. Average pre-plant nematode densities (Pi) at the beginning of the study were 247, 7, and 172 juveniles per 250 cm 3 of soil at Barcelona, Eivissa and Cádiz, respectively. The Mi resistant tomato cv. Monika suppressed M. javanica at Barcelona and Eivissa, and M. arenaria and M. incognita at Cádiz, and prevented nematode population increases by more than 90% compared with the susceptible cv. Durinta. At Barcelona, the 3-year average tomato yield increased by 2.6 kg m −2 in the rotations including at least one resistant tomato crop, and by 6.1 kg m −2 when the resistant cultivar was cropped for two consecutive years. However, no differences in yield were found at Eivissa due to low Pi, nor at Cádiz where the resistant cultivar was poorly adapted to the agroclimatic conditions of the area.

A Pectinase‐deficient Ralstonia solanacearum Strain Induces Reduced and Delayed Structural Defences in Tomato Xylem

AbstractIt has been suggested that oligogalacturonides (OGAs) released by bacterial pectinases can induce plant defence responses. To test this hypothesis, resistant tomato cultivar LS‐89 and susceptible cultivar Ponderosa were inoculated with either wild‐type Ralstonia solanacearum strain K60 or a pectinase‐deficient triple mutant K60‐509, which lacks endo‐polygalacturonase PehA, exo‐poly‐alpha‐d‐galacturonosidase PehB, and pectin methylesterase Pme. K60 induced structural defence responses, including electron‐dense materials (EDMs) in vessels and apposition layers (ALs) in parenchyma cells adjacent to xylem vessels colonized by bacteria in LS‐89 stems. In contrast, LS‐89 infected with K60‐509 did not have any EDMs in vessels at 4 days after inoculation (DAI), and had them only rarely at 7 DAI. In LS‐89 infected with K60‐509, ALs were rarely observed in parenchyma cells adjacent to vessels at 4 DAI, and while they were present at 7 DAI, they were thinner than ALs induced by K60. The bacterial density in LS‐89 stems infected with K60‐509 was lower than in stems infected with K60 at 4 DAI, but the strains reached similar population sizes by 7 DAI, showing the pectinase‐deficient mutant colonized resistant stems more slowly than did the wild‐type strain. Vessels infected with K60‐509 contained fewer EDMs at 7 DAI than were observed at either 4 or 7 DAI in vessels colonized by K60, although bacterial density in the xylem tissues containing K60‐509 at 7 DAI was about the same as in the xylem tissues containing K60 at 4 DAI. Neither the wild‐type strain nor the pectinase‐deficient mutant induced these histopathological changes on susceptible cultivar Ponderosa. These results indicate that R. solanacearum pectinases play some role in eliciting histopathological changes in LS‐89, likely by releasing OGAs that trigger plant structural defences.

EMERGING STRAINS OF RALSTONIA SOLANACEARUM IN THE FRENCH WEST INDIES RAISE NEW CHALLENGES TO TOMATO BREEDERS

Bacterial wilt (BW) caused by Ralstonia solanacearum is a major biotic constraint to tomato crops in the tropics. The development of universally resistant tomato cultivars has always been hampered by the unusual genetic and phenotypic plasticity of the pathogen. Since 1999 in Martinique, the bacterial wilt situation was dramatically changed after the emergence of strains assigned to a brand new genotype, phylotype IIB/sequevar4NPB (phII/4NPB) which displayed an enlarged host range compared to strains previously reported. These emerging strains have rapidly spread to the main tomato crop areas, causing severe yield losses. The virulence/aggressiveness features of these emerging strains were assessed on a range of 11 tomato lines, including the major resistance sources. Historical strains of phylotype I and phylotype IIA (broad-host range) were used as references. Strains phII/4NPB were more aggressive and virulent compared to the other strains: they wilted the susceptible lines significantly faster, and were able to wilt resistant genitors like Hawaii7996 and CRA66 whereas other strains could not. The field resistance/ tolerance features of tomato commercial varieties and resistant genitors were assessed in two separate trials on a nursery infested by phylotype I strains (2004) and phII/4NPB strains (2006). The wilt indices were higher in the phII/4NPB nursery. Performances of the cultivars were different between the trials, particularly in the case of Craibo and HW7996. This study underlines the necessity to survey and consider the broad diversity of the R. solanacearum species complex in the development of BWresistant tomato cultivars. It is suspected that phII/4NPB strains may also concern other countries in this part of the world as this genetic group was detected in different collections, in Brazil and Trinidad. (Resume d'auteur)

Influence of Fusarium wilt resistant tomato cultivars on root colonization of the mutualistic endophyte Fusarium oxysporum strain 162 and its biological control efficacy toward the root-knot nematode Meloidogyne incognita

The ability of the nematode antagonistic, mutualistic endo-phyte Fusarium oxysporum strain 162, to colonize the endorhiza of different Fusarium wilt resistant and susceptible cultivars of tomato (Solanum lycopersicon L.) was investigated. The fungus colonized the endorhiza of both wilt resistant and susceptible cultivars of tomato. Colonization after 3 weeks ranged from 40% to 100% in trial 1 and from 32% to 75% in trial 2. Percent colonization declined 6 weeks after fungal inoculation in resistant as well as susceptible varieties. Biocontrol presented by nematode penetration in trial 1 and number of galls in trial 2 reached 82% and 55%, respectively. Surprisingly, the level of nematode control in Fusarium wilt resistant cultivars was found similar or even higher in some cases when compared to that in wilt susceptible cultivars. Although endophyte colonization decreased over time, the antagonistic fungus reduced Meloidogyne incognita infestation significantly in the majority of the cultivars tested.

Variability in infection and reproduction of Meloidogyne javanica on tomato rootstocks with the Mi resistance gene

The response of 10 commercial or experimental tomato rootstocks with the Mi resistance gene to an initial inoculum of a Mi‐avirulent population of Meloidogyne javanica was determined in pot tests conducted in spring and summer. In a field test, the rootstocks were subjected to continuous exposure to high initial population densities (2050 ± 900 second‐stage juveniles (J2) per 250 cm3 soil) of the nematode. The presence of the Mi locus in the resistant rootstocks and cultivars was confirmed using the PCR co‐dominant markers REX‐1 and Mi23. Nematode infectivity (egg masses) and reproduction (eggs g−1 root) were highly variable in the spring tests. Rootstocks PG76, Gladiator and MKT‐410 consistently responded as highly resistant, with nematode multiplication rate (Pf/Pi) < 1 and reproduction index (RI) < 10%, and they were as efficient as standard resistant tomato cultivars at nematode suppression. The relative resistance levels of rootstocks Brigeor, 42851, 43965, Big Power and He‐Man varied depending on the susceptible standard used for reference or the duration of the test. Rootstocks Beaufort and Maxifort were susceptible to M. javanica (Pf/Pi > 50 and RI > 50%). Rootstocks PG76 and He‐Man, and the resistant tomato cv. Caramba showed high levels of resistance in the test conducted in summer, whereas MKT‐410 and 42851 and the resistant tomato cv. Monika were moderately resistant. In the field, seven rootstocks showed high levels of resistance and one (He‐Man) showed an intermediate level, whereas Beaufort and Maxifort were susceptible.

First Report and Molecular Characterization of DNA A of Three Distinct Begomoviruses Associated with Tomato Leaf Curl Disease in Ghana.

Tomato leaf curl disease is reported to be widespread in Ghana and to cause severe yield losses (4). So far, the causal agent has not been identified. Thirty-three tomato (Solanum lycopersicum L.) samples with symptoms such as curling, yellowing, small leaves, and stunting were collected from the Ashanti Region, the main tomato-production area in Ghana, including three samples from Akumandan in the autumn of 2007 and 30 samples from Kumasi in the spring of 2008. The observed leaf curl disease incidence in the farmer's field in Kumasi was approximately 75%. Viral DNAs were extracted from the 33 samples and tested for the presence of begomoviral DNA-A, DNA-B, and associated satellite DNA by PCR with previously described primers (1,3). The expected 1.4-kb DNA-A begomovirus fragment was obtained from one of the samples from Akumadan and from 25 samples from Kumasi. DNA-B and DNA-beta were not detected by PCR. The 1.4-kb PCR products from all positive samples were cloned and sequenced. Sequence comparison by MegAlign software (DNASTAR, Inc., Madison, WI) showed three distinct virus groups. One isolate from each group was selected and specific primers were designed to complete the DNA-A sequence. The DNA-As of GH5-3 (group 1), GOTB2-2 (group 2), and GHK2 (group 3) isolates consisted of 2,803 (GenBank Accession No. EU350585), 2,794 (GenBank Accession No. EU847739), and 2,792 nt (GenBank Accession No. EU847740) respectively. All contain the geminiviral conserved nonanucleotide sequence TAATATTAC in the intergenic region and the six predicted open reading frames (ORFs V1, V2, C1, C2, C3, and C4). BLASTn analysis was conducted with geminivirus sequences available in the GenBank database at National Center for Biotechnology Information (Bethesda, MD). Further sequence comparisons were performed by Clustal V algorithm of MegAlign software with the representative isolates of begomovirus species reported by Fauquet et al (2) and the sequences that showed high scores in BLASTn search. The DNA-A sequence of isolate GHK2 from Kumasi showed highest sequence identity (96.5%) with Tomato yellow leaf curl Mali virus (TYLCMLV; GenBank Accesssion No. AY502934). The DNA-A sequence of GH5-3 and GOTB2-2 isolates had 87.5% sequence identity with each other. Both had highest sequence identities of 76.7 and 77.6%, respectively, with Tomato leaf curl Antsiranana virus, Madagascar (GenBank Accession No. AM701764). They constitute two distinct begomovirus species based on DNA-A sequence comparisons and the International Committee on Taxonomy of Viruses proposed species demarcation of 89% sequence identity. The names Tomato leaf curl Ghana virus for isolate GH5-3 and Tomato leaf curl Kumasi virus for isolate BOTB2-2 are proposed, respectively. To our knowledge, this is the first report of molecular characterization of begomoviruses associated with tomato leaf curl disease in Ghana and of the presence of three distinct tomato begomoviruses. This presence should be considered for recommending or developing stable begomovirus resistant tomato cultivars for Ghana.

Regulation of defense-related enzymes associated with bacterial spot resistance in Tomato

Twenty tomato (Solanum lycopersicon) cultivars were screened for resistance against bacterial spot disease incited byXanthomonas axonopodis pv.vesicatoria under field conditions with and without pathogen infection. Screening was done by artificially inoculating aX. axonopodis pv.vesicatoria suspension to 4-week-old tomato seedlings and observing them for typical symptoms of the disease. Seedlings were categorized into highly resistant, resistant, susceptible and highly susceptible cultivars on the basis of disease incidence. Tomato cultivars were screened for defense-related enzymes, total phenols and lignin contents. The temporal patterns of all these enzymes were estimated with a moderately susceptible tomato cultivar. Native PAGE analysis of both peroxidase (POX) and polyphenol oxidase (PPO) was carried out for the time course of enzyme activities and also by selecting three different tomato cultivars, following infection with the pathogen. Based on the inducible amounts of these enzymes upon pathogen infection, the tomato cultivars were correlated with the disease incidence under field conditions. A significant (P≤0.05) correlation was observed between the degree of host resistance and the enzyme levels. In highly resistant tomato cultivars the enzyme levels, total phenols and lignin contents were increased in comparison with highly susceptible tomato cultivars. Isoform analysis of POX and PPO enzymes indicated a clear difference between resistant and susceptible tomato cultivars in the number of isoforms and also in the intensity of each isoform in the presence of pathogen infection. The possible regulation of defense-related enzymes in imparting host resistance is discussed.

Quantitative Differences in Aphid Virulence and Foliar Symptom Development on Tomato Plants Carrying the Mi Resistance Gene

The Mi resistance gene in tomato reduces the feeding, fecundity, and survival of certain isolates of the potato aphid (Macrosiphum euphorbiae Thomas). This study compared the performance of two potato aphid isolates, WU11 and WU12, on nearly isogenic susceptible (Mi −) and resistant (Mi+) tomato cultivars. Although Mi significantly reduced the population growth of both aphids, WU12 numbers decreased by only 15% compared with 95% for isolate WU11. These results show that there are quantitative differences in virulence among potato aphid isolates. Compared with WU11 aphids, isolate WU12 caused more necrosis on both resistant and susceptible plants, and this increased damage may play a role in the partial virulence of isolate WU12. However, infestation with aphid isolate WU12 did not compromise plant defenses against isolate WU11 in resistant plants. Prior inoculation with either aphid isolate caused a modest reduction in the survival of WU12 adults, but this form of induced resistance was observed on both resistant and susceptible cultivars. Thus, Mi did not play a role in acquired resistance or mediate any indirect interactions between the two aphid isolates. Notably, the mode of action of Mi-mediated resistance seemed to differ depending on the aphid isolate tested. Mi dramatically deterred feeding by WU11 aphids, whereas the effects of resistance on isolate WU12 seemed to be caused primarily by antibiosis. Tolerance did not seem to be a major component of Mi-mediated responses, although resistant plants showed a modest reduction in the amount of foliar necrosis induced per aphid compared with susceptible plants.

Quantitative Differences in Aphid Virulence and Foliar Symptom Development on Tomato Plants Carrying the <I>Mi</I> Resistance Gene

The Mi resistance gene in tomato reduces the feeding, fecundity, and survival of certain isolates of the potato aphid (Macrosiphum euphorbiae Thomas). This study compared the performance of two potato aphid isolates, WU11 and WU12, on nearly isogenic susceptible (Mi-) and resistant (Mi+) tomato cultivars. Although Mi significantly reduced the population growth of both aphids, WU12 numbers decreased by only 15% compared with 95% for isolate WU11. These results show that there are quantitative differences in virulence among potato aphid isolates. Compared with WU11 aphids, isolate WU12 caused more necrosis on both resistant and susceptible plants, and this increased damage may play a role in the partial virulence of isolate WU12. However, infestation with aphid isolate WU12 did not compromise plant defenses against isolate WU11 in resistant plants. Prior inoculation with either aphid isolate caused a modest reduction in the survival of WU12 adults, but this form of induced resistance was observed on both resistant and susceptible cultivars. Thus, Mi did not play a role in acquired resistance or mediate any indirect interactions between the two aphid isolates. Notably, the mode of action of Mi-mediated resistance seemed to differ depending on the aphid isolate tested. Mi dramatically deterred feeding by WU11 aphids, whereas the effects of resistance on isolate WU12 seemed to be caused primarily by antibiosis. Tolerance did not seem to be a major component of Mi-mediated responses, although resistant plants showed a modest reduction in the amount of foliar necrosis induced per aphid compared with susceptible plants.

Development of Molecular Markers Linked to Cladosporium fulvum Resistant Gene Cf-6 in Tomato by RAPD and SSR Methods

Leaf mold, caused by the fungus Cladosporium fulvum, is a serious disease of tomato. In the current study, the main physiological races of C. fulvum collected from three northeastern provinces of China were identified using a set of identification hosts. The results showed that the prevalent pathogenic physiological races were 1.2.3, 1.3, 3, 1.2.3.4, and 1.2.4. F1, F2, and BC1 tomato plants were obtained by crossing C. fulvum-resistant cultivar 03748 carrying the Cf-6 gene and susceptible cultivar 03036. Three 10-mer oligonucleotide random amplified polymorphic DNA (RAPD) primers and two simple sequence repeat (SSR) primers were selected for the further molecular marking analysis after 210 RAPD primers and 50 SSR primers were screened using the bulked segregate analysis method. The polymorphic DNA bands were amplified among parents, 10 F1 plants, 184 F2 plants including 145 resistant plants and 39 sensitive plants using three RAPD primers and two SSR primers so that three RAPD molecular markers and two SSR molecular markers linked to the Cf-6 loci were identified. Three RAPD markers were linked to the Cf-6 resistant locus separated with 8.7 cM, 20.3 cM, and 33.4 cM. Also, one RAPD codominant marker S374619/559 was found. The locations of the two SSR markers were 12.6 cM and 9.7 cM away from the Cf-6 locus. After cloning and sequencing two specific DNA fragments closely connected to the Cf-6 resistant and susceptible alleles respectively, in the RAPD codominant marker S374619/559 and one codominant sequence characterized amplified region marker S674619/559 was converted from RAPD marker S374619/559. In the RAPD marker S374619/559, the length difference of two specific fragments, 619-bp fragment and 559-bp fragment, is the result of one insertion (60 bp) in the 619-bp fragment. These markers will facilitate the selection of resistant tomato germplasm containing the Cf-6 gene and cloning of Cf-6 to breed new C. fulvum resistant tomato cultivars.

Three QTLs for Botrytis cinerea resistance in tomato

Tomato (Solanum lycopersicum) is susceptible to grey mold (Botrytis cinerea). Partial resistance to this fungus was identified in accessions of wild relatives of tomato such as S. habrochaites LYC4. In order to identify loci involved in quantitative resistance (QTLs) to B. cinerea, a population of 174 F(2) plants was made originating from a cross between S. lycopersicum cv. Moneymaker and S. habrochaites LYC4. The population was genotyped and tested for susceptibility to grey mold using a stem bioassay. Rbcq1, a QTL reducing lesion growth (LG) and Rbcq2, a QTL reducing disease incidence (DI) were identified. Rbcq1 is located on Chromosome 1 and explained 12% of the total phenotypic variation while Rbcq2 is located on Chromosome 2 and explained 15% of the total phenotypic variation. Both QTL effects were confirmed by assessing disease resistance in two BC(2)S(1) progenies segregating for either of the two QTLs. One additional QTL, Rbcq4 on Chromosome 4 reducing DI, was identified in one of the BC(2)S(1) progenies. F(2) individuals, homozygous for the Rbcq2 and Rbcq4 alleles of S. habrochaites showed a reduction of DI by 48%. QTLs from S. habrochaites LYC4 offer good perspectives for breeding B. cinerea resistant tomato cultivars.

Symptomatic Evidence for Differential Root Invasion by Fusarium Crown and Root Rot Pathogens Between Common Tomato Lycopersicon esculentum and Its Varieties

AbstractThe pathogenic isolates (Kin2001a, Kin2001b and Kin2003) of Fusarium oxysporum f. sp. radicis‐lycopersici were obtained from hydroponically cultured seedlings of pear tomato (Lycopersicon esculentum var. pyriforme) infected at different times and their pathogenicity examined in an in vitro assay system on cotyledonal seedlings of pear tomato, cherry tomato (L. esculentum var. cerasiforme) and common tomato (L. esculentum). With the in vitro assay, infection and subsequent disease progress could be microscopically observed. Pear and cherry tomatoes suppressed invasion by all isolates at the junctions of epidermal cells along the root, comparable with the resistant cultivars of common tomato. The pathogen entered pear and cherry tomatoes at the tips of lateral roots and tap roots, in contrast to infection of susceptible cultivars of common tomato. In Kin2003‐inoculated roots, the top of the lateral rootlets first became discoloured, followed by the cortical parenchyma, central xylem vessel and finally the crown. This dark‐brown discolouration expanded rapidly and severe rot developed in the discoloured regions. In contrast, the dark‐brown discolouration in Kin2001b‐infected roots expanded into the cortical parenchyma cells abutting the originally infected lateral rootlets and at a much slower rate. Kin2001a was in a new group that entered via the cortical cleavage formed by the emergence of lateral rootlets, in addition to the tips of taproots and lateral roots. In this in vitro assay system, the Japanese pathogenic isolates collected from different districts of Japan were characterized and classified by the mode of host invasion. Of 13 isolates, four were placed with Kin2003, six with Kin2001a and three with Kin2001b.

Tomato chlorosis crinivirus

<i>Tomato chlorosis crinivirus</i>

Inoculation conditions of tomato leaf curl virus (TYLCV) and resistance of foreign resistant tomato cultivars to TYLCV strain in Aichi prefecture

Inoculation conditions of tomato leaf curl virus (TYLCV) and resistance of foreign resistant tomato cultivars to TYLCV strain in Aichi prefecture

ACQUIRED AND R-GENE-MEDIATED RESISTANCE AGAINST THE POTATO APHID IN TOMATO

We examined the effects of three forms of host plant resistance in tomato, Lycopersicon esculentum, on the potato aphid, Macrosiphum euphorbiae. Mi-1.2, a resistance gene (R-gene) in tomato that deters aphid feeding, reduced the population growth of both potato aphid isolates tested, although it appeared to have a greater impact on isolate WU11 than on isolate WU12. The results suggest that there may be quantitative differences in virulence between these two aphid isolates. We also examined two distinct forms of acquired resistance in tomato, jasmonic acid (JA)-dependent and salicylic acid (SA)-dependent induced defenses. Exogenous foliar application of JA triggered expression of a JA-inducible proteinase inhibitor in tomato cultivars with and without Mi-1.2, although the effects of treatment on aphid performance differed between these cultivars. JA-treatment reduced aphid population growth on a susceptible tomato cultivar that lacks Mi-1.2, but did not significantly enhance or inhibit aphid control on a near-isogenic resistant tomato cultivar that carries this gene. Foliar application of an SA analog, benzothiadiazole (BTH), was used to induce SA-dependent defenses. BTH treatment reduced the population growth of both aphid isolates on a susceptible tomato cultivar, and also enhanced aphid control on a resistant cultivar. The results indicate that both SA- and JA-dependent acquired resistance in tomato have a direct negative effect on a phloem-feeding insect. Furthermore, this study demonstrates that acquired resistance and R-gene-mediated resistance can interact for enhanced suppression of insect herbivores.

Vascular colonization patterns in susceptible and resistant tomato cultivars inoculated with Fusarium oxysporum f.sp. lycopersici races 0 and 1

The vascular colonization pattern of Fusarium oxysporum f.sp. lycopersici races 0 and 1 in tomato was studied in five susceptible and five resistant cultivar–fungus combinations during a 26‐day period after inoculation by root immersion. Propagules spread discontinuously along the stems in all five cultivars 1 day after inoculation, irrespective of cultivar resistance. Five days later the fungus was limited to the stem bases in all cultivars. Between the fifth and 12th days, stem colonization by the fungus stopped in all cultivar–race combinations. Thereafter, the situation remained stable in resistant combinations, with inoculum distributed discontinuously, and no disease symptoms were apparent. By contrast, in the susceptible combinations a gradual upward colonization of the stems was seen such that fungal distribution was no longer discontinuous and disease symptoms appeared. These results suggest that a fungal ‘incubation’ period in the base of the vascular system is required before a secondary invasion of tissues occurs in susceptible genotypes. The slope of the regression line fitted between the height reached by the fungus up the stem (y) and the time after inoculation (x) provides a measure of the horizontal (polygenic) resistance in tomato cultivars

Field Isolates of Tomato spotted wilt virus Overcoming Resistance in Pepper and Their Spread to Other Hosts in Italy.

Isolates of Tomato spotted wilt virus (TSWV) severely and systemically infecting commercial pepper cultivars with resistance introgressed from Capsicum chinense PI152225 were found in Albenga (northwestern Italy) in July 2000. Experimentally, these resistance-breaking (RB) isolates overcame the resistance in C. chinense PI152225, but they produced infection in other hosts similarly to non-RB isolates from the same area. The RB isolates were indistinguishable from TSWV by serology and electron microscopy, and they were efficiently transmitted by Frankliniella occidentalis. Such isolates were recovered on the same farm in tomato, pepper, and artichoke 2 and 12 months later, suggesting natural spread from the resistant plants and survival. The RB isolates survived in experimental mixed infections with a non-RB isolate in susceptible pepper and C. chinense, but cross-protection in pepper acted against them. Commercial TSWV-resistant pepper but not resistant tomato cultivars from different companies were susceptible to these RB isolates after mechanical inoculation. Similar isolates were not detected among TSWV samples collected from 1993 to 2000 in the area. The management of TSWV and thrips using resistant pepper cultivars is discussed.

Parent-offspring correlation estimate of heritability for early blight resistance in tomato, Lycopersicon esculentum Mill.

This study estimated the heritability (h2) of early blight (EB) resistance in filial progeny of a cross between a susceptible (`NC84173';mid-season maturity) and a resistant (`NC39E'; late-season maturity)tomato breeding lines. It addition, it examined the potential of identifying progeny with mid-season maturity and EB resistance. A total of 162F2 plants were grown under field conditions in 1998 and evaluated for disease symptoms three times during the season, and the area under the disease progress curve (AUDPC) and final percent defoliation (disease severity) were determined. The F2 plants were self-pollinated and F3 seeds produced. The 162 F3 progeny families, consisting of 20 plants per family, were grown in a replicated field trial in 1999 and evaluated for EB resistance (final percent defoliation) and plant maturity(days to 50% ripe fruit). The distributions of the final percent defoliation values in the F2 and F3 generations indicated that resistance from `NC39E' was quantitative in nature. Estimates of h2 for EB resistance, computed as the correlation coefficients between F3progeny family means and F2 individual plant values, ranged from0.65 to 0.71, indicating that EB resistance of `NC39E' was heritable. Across F3 families, a negative correlation (r = –0.46, p< 0.01) was observed between disease severity and earliness in maturity, indicating that plant maturity affected disease severity. However, several F3 families were identified with considerable EB resistance and mid-season maturity, indicating that resistance from `NC39E' might be useful for the development of commercially acceptable EB resistant tomato cultivars.

Regulation of a fungal avirulence gene

The fungal pathogen, Cladosporium fulvum, has a limited host range on tomato, and the disease, tomato leaf mold, is confined to susceptible cultivars. Products of avirulence (Avr) genes present in C. fulvum play a major role in restricting its host range because Avr gene products are recognized by resistant tomato cultivars that carry matching resistance (R) genes, and infection is suppressed by a hypersensitive response (HR), the localized death of host cells. In the absence of either one or both members of the Avr-R gene pair, the tomato plant fails to recognize C. fulvum, HR is not triggered, and the host becomes diseased.