Screening for Fusarium resistance in seedling populations of Asiatic hybrid lily

A test to select Fusarium resistant seedlings of the Asiatic hybrid lily is described. Young seedlings of 28 populations, obtained from an incomplete diallel between eight parents with different levels of Fusarium resistance, were tested for resistance. Significant differences in Fusarium resistance between and within populations were detected. The average percentage of selected seedlings ranged from 34% in resistant × resistant crosses to 2% in susceptible × susceptible crosses. Although resistant descendants were obtained in susceptible × susceptible crosses, using at least one resistant parent produced higher percentages of resistant seedlings. The resistance level of the parents correlated highly with the general combining ability for Fusarium resistance based on the seedling test. For eight populations, seedlings selected for Fusarium resistance and non-tested (control) seedlings of the same cross were compared, after propagation, in a clonal test. Variation between and within populations, found at seedling level, was confirmed at clonal level. A positive selection response was found for all eight populations. In the seedling test, approximately 18% of the seedlings were selected as resistant of which 15% (2.7% of seedlings tested) appeared to be susceptible escapes. Comparison between selection at seedling level and at clonal level indicated that approximately 25% of the seedlings tested were missed (rejected resistant plants) in the seedling test. The practical use of a seedling test for Fusarium resistance in lily breeding programs is discussed.     

Studies on resistance to bacterial speck (Pseudomonas syringae pv. tomato) in tomato cv. Ontario 7710

Plants of 17 tomato cultivars and four wild Lycopersicon accessions were evaluated for their reaction to Pseudomonas syringae pv. tomato (Pst) in a greenhouse following a leaf‐spray inoculation. The genotypes exhibited a large amount of variation in response to Pst infection, with disease severity index (DSI) ratings from 0.2 to 3.9. The cultivar ‘Ontario 7710’ and two accessions of Lycopersicon hirsutum (LA 1773 and LA 1775) were the most resistant, with DSI values of 0.2, 0.4 and 0.6, respectively. Three varieties, M 1812, Kujawski and Warszawski, also showed a high level of tolerance. The most susceptible was ‘A 100’(DSI = 3.9). The inheritance of resistance to bacterial speck was investigated by disease tests in segregated populations obtained by hybridizing the tomato cv. Ontario 7710 with the susceptible variety ‘A 100′. Plants were rated for disease severity by inspecting each plant and were then evaluated according to phenotypic similarity to ‘Ontario 7710’ or ‘A 100’ in respect of the number and size of the spots. Genetic analysis in F1, F2 and backcross segregations indicated that resistance of'Ontario 7710’ to Pst is conferred by one incompletely dominant gene, Pto.     

柠檬合子胚自交实生苗的鉴定及其对柑橘溃疡病的抗性初步评价

枸橼C-05是抗溃疡病的柑橘种质资源,柠檬是枸橼的后代,其自交后代可能继承枸橼的抗性基因。为分析柠檬自交群体对柑橘溃疡病的抗感性规律,本研究建立了433株的‘尤力克’柠檬自交后代群体,并利用SSR分子标记对合子胚实生苗进行了鉴定,观察合子胚实生苗和珠心胚实生苗叶片形态学的差异,对所有自交后代离体注射接种柑橘溃疡病菌,初步评价了实生后代对柑橘溃疡病的抗病性差异。结果表明‘尤力克’柠檬存在多胚性,从300对SSR引物中筛选到8对可用于‘尤力克’柠檬合子胚鉴定的引物,共鉴定出合子胚实生苗57株,确定珠心胚实生苗4株。叶形指数、叶尖指数、翼叶长、翼叶宽和叶柄长均不能作为区分‘尤力克’柠檬合子胚实生苗与珠心胚实生苗的形态学标记,但合子胚实生苗的翼叶长度变异幅度大于珠心胚。57株合子胚实生苗离体接种柑橘溃疡病菌后,5株表现为抗病,52株表现为感病;4株珠心胚实生苗均表现为感病;在372株未鉴定清楚的自交后代中,8株表现为抗病,364株表现为感病。本研究表明利用SSR分子标记还不能完全鉴定出‘尤力克’柠檬合子胚植株,可能需要更多的引物或更灵敏的分子标记才能把全部实生后代鉴定出来。利用合子胚自交后代接种鉴定,更有利于分析‘尤力克’柠檬对柑橘溃疡病的抗感性遗传规律,挖掘抗性基因,为柠檬的抗病育种提供种质资源。     

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.     

Variation in forage grasses as hosts of the root-knot nematode Meloidogyne naasi (Franklin) and selection for resistance in ryegrass

Summary Reactions of 13 grasses to Meloidogyne naasi varied with species; ryegrasses, fescues and their hybrids were generally susceptible and cocksfoot and timothy resistant. Marked variation in host resistance levels occurred between genotypes within cultivars.Selection of single plants, followed by tests on replicate tillers, identified resistant and susceptible genotypes in both Italian and perennial ryegrass cultivars. Resistant plants had few nematode-induced galls and fewer females and eggs than susceptibles. There was more or less continuous variation, with many genotypes intermediate between extremes of resistance and susceptibility. Selected resistant and susceptible genotypes are of use in assessing variation in nematode populations and as controls for breeding and selection programmes.     

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.     

Segregation distortion of Brassica carinata derived black rot resistance in Brassica oleracea

Three segregating F₂ populations were developed by self-pollinating 3 black rot resistant F₁ plants, derived from across between black rot resistant parent line 11B-1-12 and the susceptible cauliflower cultivar ‘Snow Ball’. Plants were wound inoculated using 4 isolates ofXanthomonas campestris pv. campestris (Xcc) race 4, and disease severity ratings of F₂ plants from the three populations were scored. A total of 860 arbitrary oligonucleotide primers were used to amplify DNA from black rot resistant and susceptible F₂ plants and bulks. Eight RAPD markers amplified fragments associated with completely disease free plants following black rot inoculation,which segregated in frequencies far lower than expected. Segregation of markers with black rot resistance indicates that a single, dominant major gene controls black rot resistance in these plants. Stability of this black rot resistance gene in populations derived from 11B-1-12 may complicate introgression into B. oleracea genotypes for hybrid production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Screening for Resistance to Bacterial Blight in Upland Cotton (Gossypium hirsatum L.)

Fifty four varieties of upland cotton (Gossypium hirsutum L.) were screened for resistance to bacterial blight caused by Xanthomonas malvacearum E. F. Smith, (Dowson) under natural field conditions and artificial Inoculation. In general, moderately high levels of field infection provided useful information on susceptibility of varieties but 11 needed to be1 supplemented with artificial inoculation to confirm resistance. Hypocotyl regions of one week-old seedlings were subjected to artificial inoculation with bacterial blight crude isolate using hypodermic needles. None of the fifty lour varieties tested was immune to disease 18.5 % showed a high level of resistance, 29.6 % were recorded as resistant while 20.4 % were susceptible. Another 18.5 % were highly susceptible and 13.0 % showed inconsistent disease reaction under natural field conditions. However, under artificial inoculation the seedlings showed lesions of varied sizes but none of the varieties was found to be resistant. Statistical analysis showed non-significant (P = 0.05) disease interaction indicating susceptibility to disease at the seedling stage. This may be due to lower levels of resistance in seedlings than in mature cotton plants. The resistant varieties were mainly of African origin and well adapted to local conditions. They are therefore potentially useful as commercial varieties in their own right or as donor parents for blight resistance.     

Identification of a single dominant allele for resistance to blackleg in Brassica napus'Surpass 400'

The inheritance of resistance to blackleg (caused by Leptosphaeria maculans) was examined in the F1 and F2 of a cross between highly resistant canola ‘Surpass 400’ and susceptible ‘Westar’ in the field. Blackleg‐infected canola straw was collected from the field and scattered among plants to increase disease development with the aid of natural rainfall. Disease severity on seedlings was assessed as the average number of lesions on leaves 1 and 2, and on adult plants as the percentage necrosis on a cross‐section of stems immediately above the crown. All ‘Westar’ plants were susceptible (S) and all ‘Surpass 400’ and F1 plants were resistant (R) at both growth stages. Disease severity on F2 plants segregated 3 : 1 (R : S) as expected for a single dominant resistance allele in both the seedling and adult plant stages. There was a high proportion (91.1%) of matching reactions (R‐R and S‐S) between seedling and adult plants. ‘Surpass 400’ is the source of a single dominant allele for blackleg resistance in Brassica napus that is expressed strongly in both seedlings and adult plants.     

Penetrance of gene I for Fusarium resistance in the tomato

Tomato seedlings were inoculated, from one to ten days after emergence, with the tomato Fusarium wilt fungus race 1. The penetrance of gene I for Fusarium resistance in the homozygous resistant variety Homestead 24 was almost complete. In the F₁ (Ii) between Homestead 24 and the susceptible Marmande penetrance was incomplete and ranged between 66.3% and 100% in different experiments. The age of seedlings at time of inoculation did not affect the final percentage of diseased plants while it influenced the nature and the time of appearance of disease symptoms. Possible consequences of incomplete penetrance for the resistance of F₁ hybrids are discussed.     

Bacterial wilt and canker of tomato: fundamentals of a complex biological system

Tomato (Solanum lycopersicum) is well-known as a model for study of plant–pathogen interactions, since it is a crop of global relevance and susceptible to multiple bacterial, fungal, viral and nematode pathogens. Among bacterial phytopathogens, the actinomycete Clavibacter michiganensis subsp. michiganensis (Cmm) is the causal agent of bacterial wilt and canker of tomato, considered a quarantine disease at international level. The tomato–Cmm interaction has been studied to decipher the pathogenicity mechanisms in Cmm, susceptibility mechanisms in tomato, molecular basis of resistance to Cmm in wild species relative to domesticated tomato, and the level of genetic variability in Cmm. The objective of this review is to discuss recent advances in tomato–Cmm compatible interaction, which can be integrated for application in early diagnosis and biological control of bacterial wilt and canker of tomato. Further study of plant–microorganism interactions is a promising field for improvements in tomato pathogen resistance.     

Analysis of genetic variability in various tissue culture-derived lemon plant populations using RAPD and flow cytometry

Five populations of lemon plants [Citrus limon (L.) Burm] obtained from undeveloped ovules through different tissue culture procedures were examined for the presence of somaclonal and irradiation-induced genetic variation. Tested groups were: (1) nucellar seedlings; (2) organogenic, regenerated via adventitious buds from nucellar seedling internodes; (3) embryogenic population, regenerated from non-irradiated nucellar callus via somatic embryogenesis; (4) embryogenic population, regenerated from irradiated nucellar callus via somatic embryogenesis; and (5) protoplast-derived, regenerated via somatic embryogenesis. Genomic DNA samples from 360 plants (72 from each group) were screened for polymorphism among RAPD fingerprints amplified by 10 decamer primers. Among all tested plants, genetic variation was detected only within the group of plants recovered from irradiated embryogenic calli. Out of 72 plants from that group, three had RAPD fingerprints different from the rest of the population, and fourth plant was found to be cytochimeric, consisting of diploid and tetraploid cells as revealed by flow cytometry. In all other populations of regenerated plants, we did not come across any plants with changed ploidy level.     

Genetic characterization of reniform nematode resistance for Gossypium arboreum accession PI 417895

Rotylenchulus reniformis is an important root pathogen of cotton in the south‐eastern United States, and management is hindered by the lack of host‐plant resistance in upland cotton (Gossypium hirsutum). The G. arboreum accession PI 417895 is highly resistant to R. reniformis, and a segregating population of 300 F₂ plants was developed for phenotypic characterization of resistance. The population showed quantitative variation for nematode infection. Twenty plants showed no infection and were classified as escapes. Fifty‐four plants were classified as resistant or moderately resistant, whereas, 226 were classified as moderately susceptible or susceptible based on the nematode response of the susceptible parent, indicating resistance is a recessive trait, but these data did not support the single recessive gene model. Alternatively, this model would be supported if the 77 plants with a similar nematode response as observed for PI 417895 were classified as resistant. Twelve plants showed high levels of resistance and these data would support a two recessive gene model. Accession PI 417895 represents a new source of R. reniformis resistance with two major genes conferring resistance. Introgression of multiple resistance genes into G. hirsutum will require the development of larger populations to recover the resistant phenotype.     

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.     

Screening for new sources of resistance to Clavibacter michiganensis subsp. michiganensis (Cmm) in tomato

Bacterial canker of tomato, caused by Clavibacter michiganensis subsp. michiganensis (Cmm), is considered the most serious bacterial threat, resulting in high damages in production areas. Worldwide, Cmm is subjected to quarantine regulations.There is no cultivar in market containing Cmm resistance genes. This project aimed to screen tomatoes or wild relatives of tomato for resistance to Cmm, to be used for starting breeding programs. We have screened 24 different wild accessions of tomato and found several new tolerant sources: Solanum pimpinellifolium GI.1554, S. parviflorum LA735 and S. parviflorum LA2072. We also confirmed the tolerance which was reported previously in S. peruvianum LA2157, S. peruvianum PI127829, S. peruvianum LA385, S. habrochaites LA407 and S. lycopersicum cv. IRAT L3. No immunity was found. Also accessions showing a low disease score still contained high titers of bacteria as determined by a dilution plating method, using tow selective media. These results were confirmed with a TaqMan real time PCR assay, which was developed to determine and quantify Cmm in planta.     

Inheritance of tree and fruit characters in progenies from crosses of sweet cherry (Prunus avium L.) cultivars

Summary Approximately 1000 seedlings from 20 combinations crossed in 1979, 1980 and 1981 (Theiler-Hedtrich 1985a) were tested for several characters: fruit set (yield), fruit size, fruit colour, formation of abscission layer and bleeding after fruit removal from fruit stalks, bacterial canker resistance, flowering and harvesting time. From progeny of crosses with Stella as pollinator, 56% (Vittoria × Stella) and 46% (Schüttler × Stella) of the seedlings were self-compatible, of which 14 were high yielding with good fruit size and quality. From the data recorded it can be concluded:fruit set is a recessive character; only 5 to 20% of very good yielding seedlings were obtained in different progeny, even if the parental plants were both very good croppers.Fruit juice and skin colour was in most progenies ‘black’ even if they were from combinations with ‘white’ varieties, e.g., Merton Glory or Schüttler. Only from the combination Schiittler (‘white’) × Stella (‘black’), 50% of the seedlings were ‘white’; Stella therefore is heterozygous for the character of fruit juice and skin colour.Fruit size is evenly distributed in progeny with respect to the fruit size of their parent plants.Abscission layer formation and non-bleeding is a genetically complex character. In combinations where both parent plants formed fruits with complete abscission layers and which were not bleeding after fruit removal from the stalk, this character was inherited only to 50% (Vittoria × Schüttler) or 85% (Vittoria × Frühe von der Weid) in the progeny. For the genetical control of this character further studies are necessary.Bacterial canker susceptibility was evenly distributed in seedlings from all combinations even if the highly resistant cv. Vittoria was used as one parent plant, thereby not confirming the expected results of a higher proportion of resistant seedlings from combinations with Vittoria.Flowering and harvest time of the seedlings from different combinations was within the range of the parent plants. Only in the combination of Vittoria × Stella (mid-to late-ripening season) one seedling out of 99 was found to form ripe fruits two weeks earlier than the parental plants. From the seedlings tested 40 have been chosen for further evaluation or genetical studies.     

Contribution of host-pathogen interactions to the expression of the blackleg disease of spring rape (Brassica napus L.) caused by Leptosphaeria maculans (Desm.) Ces. et de not

Variation in the responses of selected genotypes of spring rape to infection by a wide range of isolates of Leptosphaeria maculans was analyzed in this study.Differences in severity of disease damage to seedlings between host and pathogen genotypes were highly significant. Interactions between host and pathogen were also highly significant. The generally resistant cultivar Wesreo was susceptible to a number of isolates whereas the normally susceptible cultivars Zollern-gold and Ceska were resistant to some isolates. Overall, however, Wesreo was resistant to a far greater number of isolates than were the susceptible genotypes.Significant differences in severity of disease damage were also observed between groups of isolates from different sites. All host genotypes were resistant to a substantially greater proportion of isolates from certain sites than from others. Interactions were again detected in that the ranking of sites on the basis of the proportion of isolates causing severe damage varied between host genotypes.The significance of host-pathogen interactions in the B. napus-L. maculans relationship is discussed with special reference to current views on the nature of horizontal resistance.     

Evaluation of quantitative resistance to yellow rust (Puccinia striiformis f. sp. hordei) in the ICARDA/CIMMYT barley breeding programme

The resistance to yellow rust (Puccinia striiformis f. sp. hordei) of 500 advanced barley lines from the ICARDA/CIMMYT breeding programme in Mexico was evaluated on seedlings in the greenhouse and on adult plants in the field. A high frequency of advanced lines (85.8%) showed a susceptible reaction (infection type ≥ 7) on seedlings after inoculation with isolate Mex-1, representing a Mexican variant of race 24. This indicates the absence of effective hypersensitive resistance. In addition, the same advanced lines showed a large variation in disease severity in the field, ranging from 0 to 95%. More than 76% of the advanced lines with a susceptible reaction in the seedling stage demonstrated low disease severity (10% or less in the adult plant). Consequently, these advanced lines possess high levels of quantitative resistance. Two aspects in the ICARDA/CIMMYT barley breeding programme may explain the large number of advanced lines with high levels of quantitative resistance. First, a recurrent selection approach is applied when advanced (F₅) lines reaching homozygosity are intercrossed. Second, low levels of disease are accepted in the selection process instead of selecting the ultimate green plant. Both aspects combined allow the accumulation of quantitative resistance. Certain cultivars released from South-American national programmes in the late 1970s and early 1980s in Peru (UNA-80), Bolivia (IBTA 80) and Ecuador (Teran) are still resistant, demonstrating the durable nature of quantitative resistance to yellow rust.     

Immunity to Clavibacter michiganensis subsp. sepedonicus: Inheritance of immunity in Solanum acaule

Summary Bacterial ring rot of potato (Solanum tuberosum) has not been successfully controlled through management of certified seed. Therefore, the identification of immunity to this pathogen and its introgression into the cultivated potato is vitally important to the potato industry. Immunity was detected in the disomic tetraploid 2EBN (Endosperm Balance Number) species S. acaule. Immune and nonimmune parents were crossed in a 4 × 4 mating scheme that consisted of four immune × immune crosses, four immune × nonimmune, four nonimmune × immune, and four nonimmune × nonimmune crosses. Analysis was performed on the 16 F₁ populations and 54% of the progeny was found to exhibit an immune response to inoculation with Clavibacter michiganensis subsp. sepedonicus (Spieck & Kotth.) Davis et al. Immunity appears to be conferred by two dominant alleles, one at each locus, and may be associated with minor or modifying genes. Phenotypic expression of nonimmune progeny ranged from susceptible to resistant, probably due to minor or modifying genes. These results indicate that it may not be difficult to transfer immunity into the cultivated potato.     

Resistance of barley genotypes to brown leaf spot

Resistance of 59 barley cultivars to three distinct isolates of Bipolaris sorokiniana, causing brown leaf spot was tested on seedlings and adult plants in a screen house and in a growth room, by determination of disease incidence, disease severity, and rate of lesion expansion. The disease reaction and severity at the seedling stage differed among cultivars. The cultivars MNS Brite, NDB 112, Kindred, Parkland, and Oderbrucke were moderately resistant to brown leaf spot, but the resistance was partial to all three isolates. None genotype with complete resistance was found. The cultivars CI 9539, Svanhals, and Chevron, considered as sources of resistance, were classified as moderately susceptible, and susceptible, respectively. Disease severity, disease reaction, and incidence of B. sorokiniana in grains also differed among the cultivars at the adult plant stage. Nevertheless, none of the genotypes was resistant to B. sorokiniana. The rate of lesion expansion differed among cultivars BR 2, MN 698, Kitchin, Svanhals, and NDB 112, with NDB 112 having the slowest expansion rate (0.194–0.205 mm²/day).