Resistance of Pepper Lines to the Movement of Cucumber Mosaic Virus

The multiplication and the migration of cucumber mosaic virus (CMV) were studied in greenhouse conditions in one susceptible ‘Yolo wonder’ and two resistant ‘Milord’ and ‘Vania’ pepper varieties. DAS-ELISA tests have revealed that the virus is replicated in inoculated leaves of the resistant varieties as high as in the susceptible variety. In the susceptible variety ‘Yolo wonder’, CMV migrated from the leaf lamina to the petiole two days after inoculation and it became systemic three days later regardless the season. In ‘Milord’ the virus migrated from the leaf lamina to the petiole five days after inoculation and it became systemic during the winter 16 days after inoculation. Whereas plants of the same genotype were not infected systemically during the summer. In ‘Vania’, during the two seasons, CMV spread from the blade to the petiole five days after inoculation, but the virus was not detected beyond the inoculated leaf. These results show that ‘Milord’ and ‘Vania’ are resistant to CMV migration. Therefore, the resistance to CMV migration is affected by plant genotype and temperature. The study of effect of pepper plant phenology on infection has revealed that resistance to CMV migration is also affected by the development stage of the plants.     

The preconditioning effect of day-length and light intensity on adult plant resistance to powdery mildew in oats

Oat genotypes with partial resistance to mildew exhibited a higher level of adult plant resistance (a.p.r.) when grown in 16 h compared with 8 h day-length. This increased resistance was expressed in the upper leaves, and to some extent in the lower, earlier-developed leaves. The expression of increased resistance coincided with floral initiation in Maldwyn, but preceded it in CC4761. The development of a.p.r. and floral initiation were thus not inherently linked, but conditions which accelerated the plants' ontogeny also caused earlier expression of resistance. For the most efficient testing of breeding material, plants should be grown at long day-length and high light intensity. When plants are grown at 16 h day-length and constant temperature of 15 ^oC, the leaf segments should be inoculated at the 6 to 8 leaf stage because normally susceptible genotypes showed marked resistance when inoculated later.     

Age-related Resistance in Bell Pepper to Cucumber mosaic virus

We demonstrated the occurrence of mature plant resistance in Capsicum annuum‘Early Calwonder’ to Cucumber mosaic virus (CMV) under greenhouse conditions. When Early Calwonder plants were sown at 10 day intervals and transplanted to 10‐cm square pots, three distinct plant sizes were identified that were designated small, medium and large. Trials conducted during each season showed that CMV accumulated in inoculated leaves of all plants of each size category. All small plants (with the exception of the winter trial) developed a systemic infection that included accumulation of CMV in uninoculated leaves and severe systemic symptoms. Medium plants had a range of responses that included no systemic infection to detection of CMV in uninoculated leaves with the systemically infected plants being either symptomless or expressing only mild symptoms. None of the large plants contained detectable amounts of CMV in uninoculated leaves or developed symptoms. When plants were challenged by inoculation of leaves positioned at different locations along the stem or different numbers of leaves were inoculated, large plants continued to accumulate CMV in inoculated leaves but no systemic infection was observed. When systemic infection of large plants did occur, e.g. when CMV‐infected pepper was used as a source of inoculum, virus accumulation in uninoculated leaves was relatively low and plants remained symptomless. A time‐course study of CMV accumulation in inoculated leaves revealed no difference between small and large plants. Analyses to examine movement of CMV into the petiole of inoculated leaves and throughout the stem showed a range in the extent of infection. While all large plants contained CMV in inoculated leaves, some had no detectable amounts of virus beyond the leaf blade, whereas others contained virus throughout the length of the stem but with limited accumulation relative to controls.     

Genetic variation and covariation for resistance and tolerance to Cucumber mosaic virus in Mimulus guttatus (Phrymaceae): a test for costs and constraints

Genetic variation for resistance and tolerance to pathogens may be maintained by costs represented as genetic tradeoffs between these traits and fitness. The evolution of resistance and tolerance also may be constrained by negative genetic correlations between these defense systems. Using a complete diallel, we measured genetic variation and covariation for and among performance, resistance, and tolerance traits in Mimulus guttatus challenged with a generalist pathogen, Cucumber mosaic virus (CMV). Viral coat protein was detected by enzyme-linked immunosorbent assay (ELISA) in all inoculated plants, indicating that all plants were susceptible to infection, although the ELISA absorbance varied quantitatively across plants. Plants inoculated with CMV had significantly reduced aboveground biomass and flower production relative to controls, although date of first flower was unaffected by infection. All three of these performance traits showed moderate to high narrow-sense heritability (h2 = 0.32-0.62) in both inoculated and control plants. We found phenotypic variation for both tolerance of and resistance to our strain of CMV, but both displayed very low narrow-sense heritability (h2 < 0.03). We found no evidence of a trade-off between resistance and tolerance. We also found no evidence for a cost of resistance or tolerance. In fact, a significant genetic correlation suggested that plants that were large when healthy had the greatest tolerance when infected. Significant, positive genetic correlations found between performance of uninfected and infected plants suggested that selection would likely favor the same M. guttatus genotypes whether CMV is present or not.     

Effect of cucumber mosaic virus infection on morphology,yield and phenolic contents of tomato

Ten tomato genotypes were screened for their resistance against cucumber mosaic virus (CMV) and its vector Myzus persicae under natural infection in field, using aphids M. persicae under net-house and mechanical inoculation under greenhouse. Large differences were observed among genotypes for infection percentage (IP) and severity index (SI) among the testing methods used. All genotypes showing tolerance to CMV in the field or through aphid inoculation, however, become susceptible and highly susceptible after mechanical inoculation. All the test genotypes also showed susceptibility to the aphid M. persicae population. Plants inoculated with CMV showed substantial decrease in yield and yield-contributing parameters which varied with cultivars that probably depended upon its genetic make up. All the test genotypes exhibited 0.97–30.19% decrease in plant height, 11.47–52.65% decrease in root length, 46.56–95.56% decrease in fresh plant weight, 65.78–92.84% decrease in root fresh weight, 19.97–87.65% decrease in the dry weight of plants, 75.63–95.43% decrease in dry root weight, 69.51–95.65% reduction in the number of fruits and 89.04–99.89% decrease in yield per plants. After 15 days of inoculation, the quantitative analysis using double beam spectrophotometer showed an increase in total phenolics in CMV-inoculated plants as compared to un-inoculated plants among genotypes. Similarly the thin layer chromatography (TLC) on silica gel G indicated that the number of phenolic compounds was increased in most of the inoculated genotypes while in others they were either decreased or remained same.     

Interactions between a seed-borne strain of cucumber mosaic cucumovirus and its lupin host

The relationship between time of inoculation with cucumber mosaic cucumovirus (CMV) and the growth, seed production and rate of seed transmission of virus in lupin (Lupinus angustifolius cv. Illyarrie) was studied in field-grown plants. Plants inoculated at the seedling stage (2 days post-emergence) showed 45% mortality. Plants infected through the seed were more stunted than plants inoculated at the seedling stage. Plants inoculated up to the mid-vegetative growth stage (58 days post-emergence) yielded ≤ 27% of the dry matter and ≤ 9% of the seed of healthy plants. Late inoculation (114 days post-emergence) did not affect dry matter yield, but reduced seed yield to 75% of that of healthy plants. Rate of seed transmission depended on the time of inoculation of plants. The maximum rate was 24.5% for plants that were inoculated at the mid-vegetative growth stage (58 days post-emergence). However, early inoculation caused a large reduction in seed yield, and it was shown that plants inoculated at the beginning of flowering (94 days post-emergence) produced greater numbers of infected progeny than plants inoculated at earlier or later times. No relationship was observed between seed weight and transmission of CMV. Infectious CMV was recovered from the embryo, but not from the testa. A simple seed transmission model was used to evaluate several hypothetical epidemics and to determine the time of inoculation which results in greatest rates of seed transmission of CMV. For example, when fewer than 73% of plants in a crop become infected with CMV, then the rate of transmission of virus in crop seeds will be greatest when inoculations are at the beginning of flowering.     

Natural resistance to cucumber mosaic virus in lupin species

Ten species of lupins (Lupinus spp.) were tested for resistance to cucumber mosaic cucumovirus (CMV) in field experiments where inoculation was by naturally-occurring aphid vectors, and in the glasshouse by sap or graft-inoculation. L. albus and six species of ‘rough-seeded’ lupins did not become infected with CMV either under intense inoculum pressure in the field or when graft-inoculated. Two L. hispanicus, 17 L. luteus and four L. mutabilis genotypes became infected with CMV in the field, but no infection was detected in L. hispanicus P26858 or seven L. luteus genotypes. CMV was detected at seed transmission rates of 0.2–16% in seedlings of infected L. luteus, differences in levels of seed transmission between genotypes being significant and relatively stable from year to year. Graft-inoculation of CMV to plants of six genotypes of L. luteus in which no infection was found in the field induced a systemic necrotic reaction suggesting that the resistance they carry is due to hypersensitivity. In L. hispanicus accessions P26849, P26853 and P26858, CMV sub-group II isolate SN caused necrotic spots in inoculated leaves without systemic movement, while sub-group I isolate SL infected them systemically without necrosis. Another sub-group I and two other sub-group II isolates behaved like SL in P26849 and P26853 but infected only inoculated leaves of P26858. This suggests that two strain specific hypersensitive resistance specificities are operating against CMV in L. hispanicus. When plants of L. luteus genotypes that gave hypersensitive reactions on graft-inoculation were inoculated with infective sap containing two sub-group I and seven sub-group II isolates, they all responded like L. hispanicus P26858. A strain group concept is proposed for CMV in lupins based on the two hypersensitive specificities found: strain group 1 represented by isolate SN which induces hypersensitivity with both specificities, strain group 2 represented by the three isolates which induced hypersensitivity only with the specificity present in L. luteus and L. hispanicus P26858, strain group 3 by as yet hypothetical isolates that induce hypersensitivity only in presence of the specificity in L. hispanicus P26849 and P26853 that responded just to isolate SN, and strain group 4 by isolate SL which overcomes both specificities. When F₂ progeny plants from crosses between hypersensitive and susceptible L. luteus parents were inoculated with isolate SN, the resistance segregated with a 3:1 ratio (hypersensitive:susceptible), suggesting that a single dominant hypersensitivity gene, Ncm-1, is responsible. As gene Ncm-1 had broad specificity and was not overcome by any of the five CMV isolates from lupins tested, it is valuable for use in breeding CMV resistant L. luteus cultivars.     

Natural resistance to cucumber mosaic virus in lupin species

Ten species of lupins (Lupinus spp.) were tested for resistance to cucumber mosaic cucumovirus (CMV) in field experiments where inoculation was by naturally-occurring aphid vectors, and in the glasshouse by sap or graft-inoculation. L. albus and six species of ‘rough-seeded’ lupins did not become infected with CMV either under intense inoculum pressure in the field or when graft-inoculated. Two L. hispanicus, 17 L. luteus and four L. mutabilis genotypes became infected with CMV in the field, but no infection was detected in L. hispanicus P26858 or seven L. luteus genotypes. CMV was detected at seed transmission rates of 0.2–16% in seedlings of infected L. luteus, differences in levels of seed transmission between genotypes being significant and relatively stable from year to year. Graft-inoculation of CMV to plants of six genotypes of L. luteus in which no infection was found in the field induced a systemic necrotic reaction suggesting that the resistance they carry is due to hypersensitivity. In L. hispanicus accessions P26849, P26853 and P26858, CMV sub-group II isolate SN caused necrotic spots in inoculated leaves without systemic movement, while sub-group I isolate SL infected them systemically without necrosis. Another sub-group I and two other sub-group II isolates behaved like SL in P26849 and P26853 but infected only inoculated leaves of P26858. This suggests that two strain specific hypersensitive resistance specificities are operating against CMV in L. hispanicus. When plants of L. luteus genotypes that gave hypersensitive reactions on graft-inoculation were inoculated with infective sap containing two sub-group I and seven sub-group II isolates, they all responded like L. hispanicus P26858. A strain group concept is proposed for CMV in lupins based on the two hypersensitive specificities found: strain group 1 represented by isolate SN which induces hypersensitivity with both specificities, strain group 2 represented by the three isolates which induced hypersensitivity only with the specificity present in L. luteus and L. hispanicus P26858, strain group 3 by as yet hypothetical isolates that induce hypersensitivity only in presence of the specificity in L. hispanicus P26849 and P26853 that responded just to isolate SN, and strain group 4 by isolate SL which overcomes both specificities. When F₂ progeny plants from crosses between hypersensitive and susceptible L. luteus parents were inoculated with isolate SN, the resistance segregated with a 3:1 ratio (hypersensitive:susceptible), suggesting that a single dominant hypersensitivity gene, Ncm-1, is responsible. As gene Ncm-1 had broad specificity and was not overcome by any of the five CMV isolates from lupins tested, it is valuable for use in breeding CMV resistant L. luteus cultivars.     

Virus infection improves drought tolerance

Viruses are obligate intracellular symbionts. Plant viruses are often discovered and studied as pathogenic parasites that cause diseases in agricultural plants. However, here it is shown that viruses can extend survival of their hosts under conditions of abiotic stress that could benefit hosts if they subsequently recover and reproduce. Various plant species were inoculated with four different RNA viruses, Brome mosaic virus (BMV), Cucumber mosaic virus (CMV), Tobacco mosaic virus and Tobacco rattle virus. The inoculated plants were stressed by withholding water. The onset of drought symptoms in virus-infected plants was compared with that in the plants that were inoculated with buffer (mock-inoculated plants). Metabolite profiling analysis was conducted and compared between mock-inoculated and virus-infected plants before and after being subjected to drought stress. In all cases, virus infection delayed the appearance of drought symptoms. Beet plants infected with CMV also exhibited significantly improved tolerance to freezing. Metabolite profiling analysis showed an increase in several osmoprotectants and antioxidants in BMV-infected rice and CMV-infected beet plants before and after drought stress. These results indicate that virus infection improves plant tolerance to abiotic stress, which correlates with increased osmoprotectant and antioxidant levels in infected plants.     

Factors underpinning the responsiveness and higher levels of virus resistance realised in potato genotypes carrying virus-specific R genes

Responses to Potato virus A (PVA, genus Potyvirus) segregate to three phenotypic groups in a diploid cross between Solanum tuberosum subsp. andigena and a highly interspecific potato hybrid. The aim of this study was to compare gene expression between the progeny genotypes which react with hypersensitive response (HR) to PVA, allow PVA accumulation in inoculated leaves but restrict PVA infection to the inoculated leaf by blocking systemic movement [non-necrotic resistance (nnr)], or are susceptible (S) and systemically infected with PVA. Expression levels of ca 10 000 genes were compared using probes arranged in a microarray format, and real-time RT-PCR was applied for quantitative comparison of the expression of selected defense-related genes (DRGs). Results showed that a few DRGs were autoactivated in HR genotypes at an early stage of plant growth in the absence of PVA infection, which was not observed in the two other phenotypic groups (nnr and S). More detailed studies on the DRGs encoding a beta-1,3-glucanase, a chitinase and a basic PR-1b protein showed that autoactivation of the genes was not evident in vitro and up to 2 weeks of growth in soil in a controlled growth cabinet but was apparent 2 weeks later. Hence, autoinduction of these DRGs in the HR genotypes could be associated with growth stage, environmental factors or both. Furthermore, a number of other DRGs were induced in the inoculated leaves of HR genotypes as a response to infection with PVA, which was not observed in nnr and S genotypes. These results provide some novel information about factors underpinning the higher levels of virus resistance realised in potato genotypes carrying virus-specific R genes and suggest that part of the resistance is attributable to additional ‘minor’ genes functioning simultaneously, hence adding to the overall responsiveness and level of resistance against infection. These results also imply that some genotypes might be more responsive to chemical induction of pathogen and pest resistance, which could be considered in screening of progenies in plant-breeding programs.     

Effects of host plant development and genetic determinants on the long-distance movement of cauliflower mosaic virus in Arabidopsis.

During systemic infections, viruses move long distances through the plant vascular system. The long-distance movement of cauliflower mosaic virus (CaMV) in Arabidopsis has been examined using a whole plant in situ hybridization technique called plant skeleton hybridization. CaMV moves long distance through the phloem largely following the flow of photoassimilates from source to sink leaves. During the course of plant development, sink-source relationships change and the region of the plant that CaMV can invade is progressively reduced. In Arabidopsis, we have found that conditions that influence the rate of plant development dramatically impact the long-distance movement of CaMV, because under normal conditions the rate of plant development is closely matched to the kinetics of virus movement. Ecotypes and mutants of Arabidopsis that flower early show a form of resistance to systemic CaMV infection, which we call "developmental resistance." Developmental resistance results from the fact that the rosette leaves mature early in the life of an early flowering plant and become inaccessible to virus. On the other hand, if the development of early flowering plants is retarded by suboptimal growth conditions, inoculated plants appear more susceptible to the virus and systemic infections become more widespread. We have found that other Arabidopsis ecotypes, such as Enkheim-2 (En-2), show another form of resistance to virus movement that is not based on developmental or growth conditions. The virus resistance in ecotype En-2 is largely conditioned by a dominant trait at a single locus.     

Screening for Barley Yellow Dwarf Luteovirus Resistance in Barley on the Basis of Virus Movement

The movement of barley yellow dwarf luteovirus (BYDV) was evaluated in susceptible and resistant barley and bread wheat genotypes. After leaf inoculation, the virus infected the root system and the growing point of susceptible earlier than resistant, barley genotypes. No difference in virus movement occurred in resistant and susceptible wheat genotypes. It was possible to reliably differentiate susceptible from resistant genotypes when root extracts of 41 barley genotypes were tested by DAS-ELISA 3 or 4 days after inoculation at the oneleaf stage. When barley plants inoculated at the two- or three-leaf stage were assayed by tissue-blot ELISA on nitrocellulose membrane, virus was detected in the phloem vessels of the growing points of the susceptible, but not of the resistant genotype, 4–6 days after inoculation. Our results thus suggest that screening for BYDV resistance in barley could be done quickly and cheaply especially when assays are made by the tissue-blot test.     

Extreme resistance to cucumber mosaic virus (CMV) in transgenic tomato expressing one or two viral coat proteins

For the production of broad commercial resistance to cucumber mosaic virus (CMV) infection, tomato plants were transformed with a combination of two coat protein (CP) genes, representing both subgroups of CMV. The CP genes were cloned from the CMV-D strain and Italian CMV isolates (CMV-22 of subgroup I and CMV-PG of subgroup II) which have been shown to produce severe disease symptoms. Four plant transformation vectors were constructed: pMON18774 and pMON18775 (CMV-D CP), pMON18831 (CMV-PG CP) and pMON18833 (CMV-22 CP and CMV-PG CP). Transformed R0 plants were produced and lines were selected based on the combination of three traits: CMV CP expression at the R0 stage, resistance to CMV (subgroup I and/or II) infection in growth chamber tests in R1 expressing plants, and single transgene copy, based on R1 segregation. The results indicate that all four vector constructs generated plants with extremely high resistant to CMV infection. The single and double gene vector construct produced plants with broad resistance against strains of CMV from both subgroups I and II at high frequency. The engineered resistance is of practical value and will be applied for major Italian tomato varieties. This revised version was published online in June 2006 with corrections to the Cover Date.     

CMV抑制PVYN CP基因沉默及其介导的抗病性

以前曾报道用RNA介导的抗病毒策略,获得了高度抗病的表达马铃薯Y病毒坏死株系外壳蛋白基因(PVY^N CP)的转基因烟草,并对T1、T2代转基因植株进行了遗传和抗病性分析。此次以T,代转基因植株为试验材料,在筛选高度抗病植株并证明其抗病性是基于转基因沉默的基础上,采用Northern杂交的方法,证明CMV侵染抑制了转基因植株中PVY^N CP基因的沉默,而且CMV对PVY^N CP基因沉默的抑制部位是发生在接种后的新生叶上,接种叶及其下部叶片中PVY^N CP基因沉默则未受到影响。采用ELISA方法对CMV PVY^N复合接种的转基因植株进行PVY^N检测,结果表明,接种叶及下部叶没有检测到PVY^N,植株叶片对PVY^N表现为抗病。而在CMV接种后植株新生叶中则检测出了高滴度的PVY^N,植株叶片对PVY^N表现为感病。该文报道了在表达PVY^N CP基因的RNA介导抗性转基因植株中,异源病毒侵染抑制了转基因的沉默,并导致转基因植株的抗病性丧失。     

Comparison of Methods for Assessing Resistance to Meloidogyne arenaria in Peanut

Use of resistant cultivars is a desirable approach to manage the peanut root-knot nematode (Meloidogyne arenaria). To incorporate resistance into commercially acceptable cultivars requires reliable, efficient screening methods. To optimize the resistance screening protocol, a series of greenhouse tests were done using seven genotypes with three levels of resistance to M. arenaria. The three resistance levels could be separated based on gall indices as early as two weeks after inoculation (WAI) using 8,000 eggs of M. arenaria per plant, while four or more weeks were needed when 1,000–6,000 eggs/plant were used. High inoculum densities (over 8,000 eggs/plant) were needed to separate the three resistance levels based on eggs per gram of root within eight WAI. A gall index based on percentage of galled roots could separate the three resistance levels at lower inoculum levels and earlier harvest dates than other assessment methods. The use of eggs vs. second-stage juveniles (J2) as inoculum provided similar results; however, it took three to five more days to collect J2 than to collect eggs from roots. Plant age affected gall index and nematode reproduction on peanut, especially on the susceptible genotypes AT201 and D098. The genotypes were separated into their correct resistance classes when inoculated 10 to 30 days after planting, but were not separated correctly when inoculated on day 40.     

Functionally Homologous Host Components Recognize Potato Virus X in Gomphrena globosa and Potato

All known isolates of potato virus X (PVX), with the exception of a South American isolate PVXHB, induce an extreme resistance response on potato carrying the Rx gene and elicit the production of necrotic lesions on Gomphrena globosa: PVXHB establishes systemic infection on Rx genotypes of potato and infects the inoculated leaf of G. globosa without lesion formation. Previously, we have shown that the Rx-mediated resistance is affected by a feature of the coat protein that depends on the presence of a threonine residue at position 121 in the coat protein of PVXCP4 and that the resistance is an induced response expressed in protoplasts of potato with the Rx genotype. In this study, we provide evidence, based on the analysis of PVXCP4/PVXHB hybrids, that the elicitation of lesions on G. globosa also requires the presence of a threonine residue at position 121 of the viral coat protein. The lesion-forming phenotype was not associated with the ability of the viral isolate to accumulate in the infected plant. We therefore propose that there is a homologous component of both potato carrying Rx and G. globosa that interacts with a feature of the PVX coat protein and, following the interaction, activates an induced response in the plant cell.     

Stress and asymmetry during arrested development of the Australian sheep blowfly.

The dieldrin and diazinon resistance systems of the Australian sheep blowfly (Lucilia cuprina) have been used previously to relate stress, departures from bilateral symmetry, developmental stability and relative fitness. These systems are now used to consider stress and asymmetry in a developmental context. Larval to adult development is shown to be significantly impaired after arrested development at 8 degrees C, however the asymmetry score of adults of a given genotype is similar after arrested or continuous development. Selection against dieldrin-resistant and unmodified diazinon-resistant genotypes occurs during arrested development because greater proportions of these genotypes pupae at 8 degrees C than do susceptible or modified diazinon-resistant genotypes. Pre-pupae of all genotypes complete development equally successfully when transferred from 8 degrees C to 27 degrees C. Adults fail to emerge when pupae formed at 8 degrees C undergo this temperature transition. Temperature-shift experiments show the asymmetry score is determined between pre-pupal and pupal stages of the life cycle. This stage occurs at 27 degrees C in arrested and continuously developing cultures providing an explanation for the independence of stress, selective mortality during developmental arrest and asymmetry score. The results emphasize the need for genetic, environmental and developmental data before an asymmetry phenotype can be directly related to developmental stability and relative fitness.     

Tolerance and overcompensation to infection by Phytophthora infestans in the wild perennial climber Solanum dulcamara

Studies of infection by Phytophthora infestans—the causal agent of potato late blight—in wild species can provide novel insights into plant defense responses, and indicate how wild plants might be influenced by recurrent epidemics in agricultural fields. In the present study, our aim was to investigate if different clones of Solanum dulcamara (a relative of potato) collected in the wild differ in resistance and tolerance to infection by a common European isolate of P. infestans. We performed infection experiments with six S. dulcamara genotypes (clones) both in the laboratory and in the field and measured the degree of infection and plant performance traits. In the laboratory, the six evaluated genotypes varied from resistant to susceptible, as measured by degree of infection 20 days post infection. Two of the four genotypes susceptible to infection showed a quadratic (concave downward) relationship between the degree of infection and shoot length, with maximum shoot length at intermediate values of infection. This result suggests overcompensation, that is, an increase in growth in infected individuals. The number of leaves decreased with increasing degree of infection, but at different rates in the four susceptible genotypes, indicating genetic variation for tolerance. In the field, the inoculated genotypes did not show any disease symptoms, but plant biomass at the end of the growing season was higher for inoculated plants than for controls, in‐line with the overcompensation detected in the laboratory. We conclude that in S. dulcamara there are indications of genetic variation for both resistance and tolerance to P. infestans infection. Moreover, some genotypes displayed overcompensation. Learning about plant tolerance and overcompensation to infection by pathogens can help broaden our understanding of plant defense in natural populations and help develop more sustainable plant protection strategies for economically important crop diseases.     

Effect of Temperature and Air Humidity on Multiplication of Xanthomonas campestris pv. undulosa and Symptom Expression in Susceptible and Field-Tolerant Wheat Genotypes

Multiplication of Xanthomonas campestris pv. undulosa in the leaf tissues of susceptible and field-tolerant wheat genotypes was determined at the secdling stage, under different incubation conditions of temperature and air humidity. Dry air conditions did not hamper the multiplication of the population trend. Symptoms were not observed at 15°C. The occurrence of lesions coincided with populations of baeteria that had reached an estimated threshold of 10⁸ colony forming units/leaf. Differences in populations between genotypes Alondra and Pavon were only noticeable at 25°C and dry air conditions, without correlation to differences in symptom expression. Considering these two genotypes, resistance to bacterial leaf streak did not seem to operate at the seedling stage when plants were inoeulated by infiltration. In both genotypes, lesions did not extend significantly. An aetive mechanism stopping bacterial growth which is apparently temperature-dependent or a saturation of the, target sites was possibly involved, as shown in the different experiments by the fit-curves calculated for Alondra and Pavon. This study provided information on the role of climatic factors in relation to epidemics. Temperature appeared to have a major effect on the disease and results confirmed that the risk of bacterial leaf streak prevails in warmer non-traditional wheat growing areas. Several types of resistance are likely to exist in the X. e. pv. undulosa/wheat system.     

The effects of the mating system on the evolution of migration in a spatially heterogeneous population

Summary Verbal explanations for the evolution of migration and dispersal often invoke inbreeding depression as an important force. Experimental work on plant populations indicates that while inbreeding depression may favor increased migration rates, adaptation to local environments may reduce the advantage to migrants. We formalize and test this hypothesis using a two-locus genetic model that incorporates lowered fitness in offspring produced by self-fertilization, and habitat differentiation. We also use the model to address questions about the general theory of genetic modifiers and the modifier reduction principle. We find that even under conditions when migration would increase the mean fitness of a population, migration may not be favored. This result is due to the associations that develop between genotypes at a locus subject to overdominant selection and at a neutral locus controlling the migration rate. Thus, it appears that, in this model, the forces of local adaptation, which favor a reduction in the migration rate, overwhelm those of inbreeding depression, which may favor dispersal.