Response to potyvirus infection and genetic mapping of resistance loci to potyvirus infection in Lactuca

 We have investigated the interaction between two different potyviruses and resistant cultivars of Lactuca sativa. Turnip mosaic virus (TuMV) and lettuce mosaic virus (LMV) were used to inoculate several cultivars under different temperature regimes to characterize the resistance reaction. Resistance conferred by the recessive mo locus against LMV infection did not provide immunity. Virus accumulated in plant tissues to different levels depending on the genetic background of the cultivar, suggesting that several genes were involved in the resistance phenotype. Under temperature regimes that enhanced the hypersensitive reaction, resistant cultivars produced necrotic reactions. In contrast, resistance to TuMV infection conferred by the dominant Tu locus resulted in complete immunity in the plant. No virus accumulated in inoculated leaves nor was any necrotic reaction observed. The resistance loci were characterized at the genetic level by mapping them relative to molecular markers. Only weak linkages could be identified to mo, again supporting the hypothesis that several genes are involved. The Tu locus was mapped in two different crosses relative to several markers, the closest two linked at less than 1 cM. A high-resolution genetic map of the Tu locus was constructed by screening 500 F₂ individuals for recombinants around that locus. Received: 4 June 1996/Accepted: 15 November 1996     

Subterranean clover mottle sobemovirus: its host range, resistance in subterranean clover and transmission through seed and by grazing animals

Subterranean clover mottle sobemovirus (SCMV) was transmitted by manual inoculation of sap to 27 cultivars belonging to three sub-species of subterranean clover. The virus readily infected systemically all inoculated plants of five susceptible cultivars of ssp. subterraneum. Ten others showed partial resistance as not all infected plants developed systemic infection; cold winter conditions further delayed or prevented systemic movement in four of them. Two cultivars of spp. brachycalycinum and four of spp. yanninicum failed to develop systemic infection following inoculation and were considered highly resistant. Resistance to SCMV in three of the spp. yanninicum was further confirmed by the failure to establish detectable primary infections in most of the inoculated leaves. Moreover, when the four ssp. yanninicum cultivars were graft-inoculated with SCMV, systemic infection eventually developed in them but the virus concentration was low. SCMV was also transmitted by manual inoculation of sap to a further 23 species of Trifolium, Medicago or Pisum. Three species were non-hosts, five were infected only in inoculated leaves and 18 others developed systemic infection in some or all plants. SCMV reached very high concentrations and was stable in subterranean clover sap. It was transmitted experimentally between subterranean clover plants by brushing infected leaves against healthy ones and in swards was readily transmitted by the trampling and grazing of sheep, but only poorly by mowing. Seed transmission of SCMV to seedlings of five cultivars of subterranean clover was low (0–0.12%). SCMV was not transmitted by Myzus persicae.     

Study on inheritance of resistance to root aphid, Pemphigus bursarius, in lettuce

The inheritance of resistance to lettuce root aphid, Pemphigus bursarius, was studied in lettuce using the Wellesbourne cultivars Avondefiance and Avoncrisp as resistant parents and Borough Wonder and Webb's Wonderful as aphid-susceptible parents. All four cultivars were crossed in all possible combinations including reciprocals and the response to root aphid of plants in the P₁F₁F₂ and BC generations was assessed using apterae of P. bursarius from the lettuce cv. Iceberg. Resistance to attack was clearly inherited and the parents appeared to be homozygous for their resistance or susceptibility. In the F₁ generation, however, in all crosses between resistant and susceptible parents, segregation into susceptible, resistant and some slightly less resistant plants occurred. This and the highly significant differences in segregation between pairs of reciprocal crosses in the F₁ and other generations indicate that the inheritance of resistance to root aphid is controlled by extra-nuclear factors. Modifying genes might also be involved but there appears to be no linkage of root aphid resistance with resistance to downy mildew, for which the Wellesbourne lettuces were bred.     

Cytology of infection,development and expression of resistance to Plasmodiophora brassicae in canola

The timing and expression of resistance to four isolates of Plasmodiophora brassicae, collected from research sites where pathotypes 2, 3, 5 and 6 (Williams' system) had been dominant when characterised in 2006, were assessed in four new commercial cultivars of canola (Brassica napus) with resistance to clubroot. Each of the resistant cultivars was highly resistant to all four of the isolates, and there was no difference in their response to infection. Root hair infection occurred at high levels, but pathogen development occurred more slowly than in a susceptible cultivar (control). Secondary infection and development in cortical cells was severely inhibited in each of the resistant cultivars; only a few bi‐nucleated plasmodia were observed at 12 days after inoculation (DAI), and plasmodia were rarely observed at 18 and 24 DAI. In contrast, development in the susceptible cultivar had progressed to resting spores by 24 DAI. A dense ring of accumulated reactive oxygen species (ROS) was observed in the endodermis, pericycle and vascular cambium of non‐inoculated controls and inoculated plants of the resistant cultivars. However, the ROS ring disappeared rapidly in infected plants of the susceptible cultivar. Plasmodia invaded the stele of susceptible roots by preferentially colonising the xylem parenchyma cells. Expansion and enlargement of lignified xylem cells was observed by 35 DAI. The absence of any specific points of ROS accumulation or lignification of epidermal or cortical cells in the resistant cultivars indicates that a hypersensitive response is not the main mechanism of resistance in these lines. The uniform response of these resistant cultivars to the four isolates of P. brassicae indicates that the resistance in each cultivar may be conditioned by a gene(s) from a single source that confers broad resistance, because most sources of resistance to P. brassicae are pathotype specific.     

Resistance to crown rot (Phytophthora cactorum) in strawberry cultivars and in offspring from crosses between cultivars differing in susceptibility to the disease

During a period of five years (1998‐2002), 26 strawberry cultivars and five selections were tested for resistance to crown rot. Cold stored plants inoculated with zoospores of Phytophthora cactorum were used in all experiments. The results showed that resistance to P. cactorum varies greatly between cultivars, and the most resistant ones were Senga Sengana, Induka, Melody, Glima and Bogota, while the most susceptible were Tamella, Inga, Evita and Jonsok. The results were compared to those obtained by other authors, and there were clear indications that the genetic background affects the degree of susceptibility to P. cactorum. Many of the most resistant cultivars descend from Senga Sengana (e.g. Bounty, Glima, Induka and Melody), and several of the most susceptible cultivars have common parentage (e.g. Inga, Tamella, Elsanta and Evita). In a resistance test of the progeny from two separate crosses between a resistant and a susceptible cultivar 61% and 65% of the offspring were intermediately to very susceptible, and 17% and 13% were resistant at the level of Senga Sengana. Hence, there is a high risk of losing offspring with high levels of resistance if selection for resistance to crown rot is not performed at an early stage in a breeding programme.     

Components of resistance to Cercospora arachidicola and Cercosporidium personatum in groundnuts*

The reactions of four groundnut cultivars to inoculation with two leaf spot pathogens (Cercospora arachidicola and Cercospodirium personatum) were investigated. Resistant cultivars showed longer latent periods, reduced sporulation and less defoliation; however, none of the cultivars was resistant to both pathogens. Numbers of lesions, time to leaflet loss and the percentage of spores which produced lesions were influenced by the concentration of C. personatum inoculum applied to the leaves and there were significant cultivar differences in lesion numbers at the lowest inoculum level. Only the first two characters were affected by applied spore concentration in the study with C. arachidicola. Generally it was found that lower inoculum levels gave a better separation of cultivars. The level of resistance was similar to that found in the slow resting reaction of cereal crops and should be useful in a groundnut breeding programme.     

Detection of resistance to aphids in cereals

Resistance to Sitobion avenae and Metopolophium dirhodum was assessed by caging aphids on the upper part of mature tillers. Despite variation in the results, considered to be due mainly to differences amongst plants of different ages, certain cultivars consistently supported fewer aphids. Winter wheats Anna Migliori and Marsters Al and Indian spring barleys EB 921 and DL 107 were identified as resistant cultivars useful for reference in further work.     

Antibiosis and Non-preference of Sitobion avenae (F.) (Hemiptera: Aphididae) on Leaves and Ears of Commercial Cultivars of Wheat (Triticum aestivum)

Little is known on the resistance of wheat cultivars to Sitobion avenae (F.) in Brazil. The goal of this work was to assess the behavior and biology of S. avenae on four commercial wheat cultivars to verify the existence of resistance by antibiosis in leaves and ears and non-preference in the ears. The smallest net fecundity rates of S. avenae in wheat leaves have been found in the cultivars Embrapa 22 and BRS264, which did not differ between themselves. The intrinsic rate of increase of S. avenae was smaller in leaves of Embrapa 22 than in cultivars BRS254 and BRS Timbaúva. The smallest net fecundity rates of S. avenae in wheat ears were observed in the cultivars BRS254 and Embrapa 22. The intrinsic rate of increase of the aphid in the ear of cultivar Embrapa 22 was smaller than in BRS Timbaúva and BRS264, but did not differ from BRS254. The organ of the wheat plant in which the aphid was reared influences antibiosis resistance, but the cultivar BRS Timbaúva was considered susceptible and Embrapa 22 resistant to S. avenae in both plant organs tested. Ears of wheat cultivars tested did not show differences in the mechanism of resistance by non-preference to S. avenae.     

Mechanisms of resistance to sulfur dioxide in the Cucurbitaceae

The relative resistance of four cultivars of the Cucurbitaceae (Cucumis sativus L. cv. National Pickling, and inbred line SC 25; Cucurbita pepo L. cv. Prolific Straightneck Squash, and cv. Small Sugar Pumpkin) to SO₂ was determined. According to plots of the degree of exposure to SO₂ (which depends on the SO₂ concentration and the duration of the exposure), there is an 8-fold difference in resistance to this toxic gas among these cultivars. However, if the degree of injury is plotted as a function of the amount of SO₂ absorbed, all four cultivars appear similarly sensitive to the gas. We conclude that the principal reason for special and varietal differences in resistance among these cultivars is the relative rate of absorption of the gas. The densities of stomata on the upper and lower surfaces of leaves did not differ sufficiently between cultivars to account for the differences in absorption rates. It remains to be determined whether the differences in rate of SO₂ absorption reflect differences in stomatal activity.

Resistance of individual leaves changes with position on the plant axis (age of the leaf). There exists a gradient of decreasing resistance from the apex downward. This resistance gradient cannot be accounted for by differences in rates of SO₂ absorption. We infer the existence of a biochemically based, developmentally controlled resistance mechanism which functions after SO₂ has entered the leaf. Biochemical comparisons of old and young leaves with such differences in resistance should be helpful in determining the biochemistry of SO₂ toxicity.

     

Defeating the Warrior: genetic architecture of triticale resistance against a novel aggressive yellow rust race

Key message

Genome-wide association mapping of resistance against the novel, aggressive ‘Warrior’ race of yellow rust in triticale revealed a genetic architecture with some medium-effect QTL and a quantitative component, which in combination confer high levels of resistance on both leaves and ears.

Abstract

Yellow rust is an important destructive fungal disease in small grain cereals and the exotic ‘Warrior’ race has recently conquered Europe. The aim of this study was to investigate the genetic architecture of yellow rust resistance in hexaploid winter triticale as the basis for a successful resistance breeding. To this end, a diverse panel of 919 genotypes was evaluated for yellow rust infection on leaves and ears in multi-location field trials and genotyped by genotyping-by-sequencing as well as for known Yr resistance loci. Genome-wide association mapping identified ten quantitative trait loci (QTL) for yellow rust resistance on the leaves and seven of these also for ear resistance. The total genotypic variance explained by the QTL amounted to 44.0% for leaf and 26.0% for ear resistance. The same three medium-effect QTL were identified for both traits on chromosomes 1B, 2B, and 7B. Interestingly, plants pyramiding the resistance allele of all three medium-effect QTL were generally most resistant, but constitute less than 5% of the investigated triticale breeding material. Nevertheless, a genome-wide prediction yielded a higher predictive ability than prediction based on these three QTL. Taken together, our results show that yellow rust resistance in winter triticale is genetically complex, including both medium-effect QTL as well as a quantitative resistance component. Resistance to the novel ‘Warrior’ race of this fungal pathogen is consequently best achieved by recurrent selection in the field based on identified resistant lines and can potentially be assisted by genomic approaches.

     

Inheritance of resistance to yellow rust (Phragmidium rubi-idaei) in red raspberry

The inheritance of resistance in red raspberry (Rubus idaeus) to yellow rust (Phragmidium rubi-idaei) was studied in a diallel cross among the cultivars Boyne, Meeker, Mailing Jewel, Glen Prosen and Glen Clova. The progenies and clonally propagated parents were exposed initially in a rust-infected plantation where the incidence and severity of infection was assessed at the telial stage. The following spring leaves on new canes of the same plants were inoculated with urediniospores in a glasshouse at 18 ± 2 °C. The latent period and number of uredinia per cm² of abaxial leaf surface were determined 13 and 18 days after inoculation. Cv. Boyne developed only chlorotic flecking on inoculated leaves in the glasshouse and no sporulation was observed. It is postulated that this reaction was determined by a major gene which is designated Yr. Some of the other parents showed incomplete resistance of the slow rusting type, and segregants of Boyne selfed that were susceptible also showed low levels of rust, which suggests that Boyne carries both major- and minor-gene resistance. Diallel analyses of both the field and glasshouse data from the susceptible segregates indicated that the degree of incomplete resistance present was determined predominantly by additive gene action, though small but significant non-additive effects also occurred: cv. Meeker was the most resistant parent and contributed the most resistance to the progenies. The Meeker progenies also segregated for gene H, which determines cane hairiness and is known to be associated with resistance to other diseases. Segregants without hairs (genotypes hh) were found to be the more resistant to rust.     

Genetic variation in chrysanthemum for resistance toFrankliniella occidentalis

In a choice-experiment, 42 chrysanthemum cultivars were screened for resistance toFrankliniella occidentalis (Pergande). Oviposition preference, two types of feeding damage and thrips numbers per flower were recorded as measures of resistance. A large genetic variation in thrips resistance was found among the cultivars screened. The amount of feeding damage was strongly determined by oviposition preference. Besides, a positive correlation was found between the oviposition preference in non-flowering chrysanthemums (number of eggs) and flowering chrysanthemums (number of thrips per flower). Thrips feeding on young, developing tissues, causes growth damage because affected cells are unable to expand and leaves become distorted. Thrips feeding on older, expanded leaves causes cells to become filled with air, resulting in ‘silver’ damage. The amounts of growth-and ‘silver’ damage were negatively correlated suggesting that thrips chose either young or older leaves to feed on. The order of resistance among cultivars did not change during the experiment. In order to get more insight in resistance mechanisms the influence of some plant- and flower characters on resistance was examined. The plant characters height, number of leaves, flower production and flower weight were all negatively correlated with resistance. It is suggested that tall chrysanthemum cultivars with many and large flowers may invest less in defence than smaller cultivars, and therefore are more damaged by thrips.     

Assessing cultivar resistance to Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae) using a phenotyping method under semi‐field conditions

The orange wheat blossom midge, Sitodiplosis mosellana (Géhin), can significantly reduce wheat yield. Growing resistant wheat cultivars is an effective way of managing this pest. The assessment of cultivar resistance in field trials is difficult because of unequal pressure of S. mosellana caused by differences in cultivar heading dates relative to the flight period of S. mosellana adult females and huge variations of egg laying conditions from 1 day to another. To overcome these hurdles and to expose all cultivars homogeneously to the pest, an assessment method of cultivar resistance was developed under semi‐field conditions. In 2015, the resistance of 64 winter wheat cultivars to S. mosellana was assessed. Few or no larvae developed in the ears of resistant cultivars, but in susceptible cultivars, large numbers of larvae developed. Seventeen cultivars proved to be resistant, whereas 47 were susceptible. The identification of new resistant cultivars offers more opportunities to manage S. mosellana. The phenotyping method is easy, cheap, efficient and reliable. It can be used to guide the breeding of new resistant wheat cultivars. Using specific midge populations, this method could also be used in research on new resistance mechanisms in winter wheat or in other cereal species.     

Identification of resistance to Peanut bud necrosis virus (PBNV) in wild Arachis germplasm

Eighty three wild Arachis germplasm accessions, belonging to 24 species of five sections and one natural hybrid derivative of a cross between the cultivated and a wild Arachis species, were evaluated along with a susceptible groundnut cultivar for resistance to Peanut bud necrosis virus (PBNV) in a replicated field trial at ICRISAT, Patancheru, India. Thirty days after sowing, the percentage of infected plants were recorded for all the accessions and subsequently young leaflets from all these accessions were tested for the presence of the virus by enzyme linked immunosorbent assay (ELISA). One accession each of A. benensis and A. cardenasii, and two accessions of A. villosa, in the section Arachis, two accessions of A. appressipila in the section Procumbentes, and one accession of A. triseminata under section Triseminatae were not infected by PBNV. These seven field‐resistant accessions were tested under glasshouse conditions for virus resistance by mechanical sap inoculations. One accession of A. cardenasii and two accessions of A. villosa did not show systemic infection. Similarly, in another glasshouse test, where 13 A. cardenasii accessions of section Arachis were evaluated, two accessions did not show systemic infection. In all these resistant accessions, the inoculated leaves showed infection, but the systemic leaves did not show the presence of virus in spite of repeated mechanical sap inoculations. So, the resistance in these accessions appears to be due to a block in systemic movement of the virus. To our knowledge this is the first report on the identification of resistance to PBNV in wild Arachis species. Since both A. cardenasii and A. villosa are the progenitors of cultivated groundnut and can be hybridised with the latter, the resistant accessions are being utilised in conventional breeding programmes to transfer PBNV resistance to widely adapted groundnut cultivars.     

Protocol for assessing soybean antixenosis to Heliothis virescens

Larvae of Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae) often infest soybean crops, Glycine max (L.) (Fabaceae), causing significant yield losses in important soybean-producing regions. The use of soybean varieties resistant to lepidopteran larvae is a major approach in soybean integrated pest management. However, standardization and optimization of bioassays that are used to screen genotypes for insect resistance are essential for high-throughput phenotyping. Methodologies for screening were assessed to determine the most effective method for discriminating levels of antixenosis to H. virescens in soybean plants. Feeding and oviposition preference assays were performed to determine optimal densities of larvae and adults, and optimal plant structures and growth stages for conducting assays. In addition, trichome densities, and fiber and lignin contents were quantified in plant structures of soybean cultivars differing in resistance. Resistance levels of cultivars were best differentiated using nine neonate larvae and two 6-day-old larvae, and by using young leaves of plants at the vegetative stage. This was likely due to the more pronounced differences in lignin and fiber contents in young leaves of vegetative-stage plants. Density of adult pairs, plant structure, and growth stage did not affect ability to distinguish differences in oviposition preference by H. virescens. Higher numbers of eggs were found on the leaves, which were the plant structures that exhibited the lowest trichome densities. The protocol developed in this work will benefit future evaluations of soybean genotypes for antixenosis against H. virescens.     

Components of partial resistance to Septoria nodorum in winter wheat

Components of partial resistance to Septoria nodorum were investigated in 10 cultivars of winter wheat having similar field resistances. The components measured were infection frequency, latent period, size, shape and rate of growth of lesions, spore production and its rate of increase. Latent period was found to be lognormally distributed. Some of the components of resistance were found to be significantly different between cultivars. Cluster analysis also showed that cultivars could be distinguished on the basis of their components of resistance. Principal components analysis indicated that resistance could be broken down into four underlying factors, three of which could be readily interpreted. The measurements of the components of resistance were combined in a model, the r-index, based on Van der Plank's r. The amount of variation between cultivars cast some doubt on the predictive value of the index but all the cultivar values were well within the range bounded by two ‘synthetic’ cultivars made up of combinations of either the most resistant or the most susceptible components. It is considered that the r-index has potential in screening for field resistance. The possibility of incorporating the most resistant-type components into one cultivar is discussed. The use of cultivar mixtures containing cultivars having similar field resistances is also discussed in the light of the variability found in this study.     

Genetic analysis and mapping of resistance to lettuce dieback: a soilborne disease caused by tombusviruses

A diverse collection of modern, heirloom and specialty cultivars, plant introduction (PI) accessions, and breeding lines of lettuce were screened for susceptibility to lettuce dieback, which is a disease caused by soilborne viruses of the family Tombusviridae. Susceptibility was evaluated by visual symptom assessment in fields that had been previously shown to be infested with Lettuce necrotic stunt virus. Of the 241 genotypes tested in multiple field experiments, 76 remained symptom-free in infested fields and were therefore classified as resistant to dieback. Overall, resistant genotypes were as prevalent among modern cultivars as in heirloom cultivars or primitive germplasm. Within modern germplasm, however, all crisphead (iceberg) cultivars were resistant, while all romaine cultivars were susceptible. Using enzyme-linked immunosorbent assay, tombusviruses were detected in leaves of some plants of resistant genotypes that were grown in infested fields, suggesting that symptom-free plants are not immune to viral infection. The inheritance of resistance was studied for Salinas, a modern iceberg cultivar, and PI 491224, the progenitor of recently released romaine germplasm with resistance to lettuce dieback. Resistance was conferred by a dominant allele at a single locus in both genotypes. The tombusvirus resistance locus from Salinas, Tvr1, was mapped in an intraspecific Lactuca sativa population to a location that corresponds to linkage group 2 on the consensus map of Lactuca. The largest cluster of resistance genes in lettuce, the Dm1/Dm3 cluster, is found on this linkage group; however, the precise position of Tvr1 relative to this cluster has not yet been determined. To our knowledge, Tvr1 is the first tombusvirus resistance gene identified for any plant host.Electronic Supplementary Material Supplementary material is available for this article at     

Characteristics of resistance to the grain aphid Sitobion avenue in winter wheat

Five stocks of winter wheat were resistant to S. avenae in glasshouse screening tests, where resistance was more evident in older than younger plants but different types of resistance were not separated. Antibiosis was measured from the growth and reproduction of caged individual aphids and antixenosis from the settling of adult aphids on detached leaf portions. Cvs Bounty, Rapier and Virtue were resistant due to antibiosis, whilst cv. Kador and line A4501-4E showed antixenosis. These and other differences indicated that these five wheat stocks probably include at least four distinct sources of resistance to S. avenae. The existence of different types of resistance could account for variability in the results of screening tests.     

Seasonal population fluctuations of the diamondback moth,Plutella xylostella (L.) (Lep.: Plutellidae) on different cauliflower cultivars

The diamondback moth, Plutella xylostella (L.) (Lep.: Plutellidae) is one of the most important pests of cruciferous plants throughout the world. In recent years, it has been identified as a serious pest of the cauliflower fields in Tehran province. Resistance of P. xylostella to all main groups of insecticides has been recorded and it is ranked in the 20 most resistant pest species reported until now. According to many researchers, to solve the problem of pest resistance to chemical pesticides, an integrated pest management programme should be used. Despite this condition, it seems that the use of resistant cauliflower cultivars is an appropriate policy for integrated control of the pest in the field. In order to identify the most resistant cultivar in the field, eight cauliflower cultivars in a completely randomised design with five replicates were planted at the Shahed University research field (south of Tehran). Density of eggs, larvae and pupae of P. xylostella were measured every 10 days in these cultivars. The results showed that there is no significant difference between numbers of eggs per plant on different cultivars. But number of larvae and pupae per plant were significantly different among different cultivars. Smilla and Snow mystique cultivars had the highest number of larvae and pupae. On the other hand, Buris and Snow crown cultivars had the lowest number of pupae and Snow crown and SG cultivars had the lowest number of larvae per plant. According to the results, the Buris and Snow crown cultivars had the lowest infestation and had a kind of resistance to pest.     

Investigations on Adult Plant Resistance of Barley Against Erysiphe graminis f.sp. hordei

Spring barley cultivars and lines were tested for 3 years in field studies for adult plant resistance against Erysiphe graminis f.sp. hordei. The cultivars Osiris and Asse were selected for further detailed cytological studies and compared with the susceptible cultivar Peruvian. Under controlled greenhouse conditions, the percentage of conidia that had formed a functional haustorium and secondary hyphae (infection efficiency) was reduced in fifth leaves of the adult plant resistant cultivars. On fifth and flag leaves of adult plant resistant cultivars, papillae were formed more frequently under primary germ tubes and appressoria, and fungal penetration was prevented more often than on the susceptible cultivar Peruvian. In ultrastructural studies various types of papillae were observed, but could not be strictly correlated with penetration success or failure of the fungus.