Transfer of polygenic resistance to ryegrass mosaic virus from perennial to Italian ryegrass by backcrossing

Polygenic resistance to ryegrass mosaic virus (RMV) was transferred from perennial to Italian ryegrass cv. Tribune by repeated cycles of backcrossing, polycrossing and selection. The resulting synthetic variety (Bb 2113) was highly resistant to infection by RMV whereas the Italian ryegrass cultivars Tribune, RvP, Bartolini, Augusta, Wilo and Dalita all were highly susceptible. Leaf width and length of Bb 2113 was similar to that of cv. Tribune and, like Tribune, Bb 2113 had awned florets. In a field trial, Bb 2113 had a similar persistency rating and number of inflorescences in the regrowth to cv. Tribune but produced 11% less dry matter. The results emphasise the need for a more efficient means of transferring small numbers of genes from one species of Lolium to another than backcrossing provides.     

Decreased severity of ryegrass mosaic virus after passage through resistant perennial ryegrass

After exposure to infection in the field, the proportion of plants showing distinct symptoms of ryegrass mosaic virus (RMV) was less in perennial than in Italian ryegrass. The perennial ryegrass cv. Mascot had a smaller proportion of plants with symptoms than cv. S.23. Far milder symptoms were induced in test plants by RMV from naturally infected perennial ryegrass plots than from Italian ryegrass plots. Within perennial ryegrass, RMV from cv. Mascot caused milder symptoms than that from cv. S.23. Severe RMV isolated from Italian ryegrass cv. Trident (RMVT) became milder after one passage through cv. Mascot, although not as mild as RMV obtained from field plots of cv. Mascot (RMVM). Families from two highly resistant perennial ryegrass clones and two randomly selected clones of cv. S.23 crossed in all possible combinations varied in symptom severity when inoculated with RMVT but not when inoculated with RMVM. Families inoculated with RMVT also yielded virus which varied in the severity of symptoms induced in test plants, families with severe symptoms yielding severer virus. Thus, much of the variation in the resistance of these clones could be attributed to infection with RMV of differing severity. Resistance was controlled by several genes which were mainly additive in their effect.     

The resistance of the perennial ryegrass cultivar Endura to ryegrass mosaic virus

The perennial ryegrass cv. Endura is particularly resistant to infection with two ryegrass mosaic virus isolates, RMV-Roth and RMV-Sax. A few plants appeared immune to RMV-Roth but were infected by RMV-Sax. Two plants that developed only mild symptoms after infection with RMV-Roth contained few virus particles, but passage through these resistant plants caused no detectable change in the virulence of RMV-Roth towards S 22 Italian ryegrass. In offsets derived from one of these resistant plants, RMV-Sax caused severe symptoms and attained a high virus concentration but it was unable to infect if the plant was already infected with RMV-Roth.     

Effect of ryegrass mosaic virus on the quality of perennial ryegrass

In 1976 fields of perennial ryegrass located on two farms in the west of Scotland were found to be far more severely and extensively infected by ryegrass mosaic virus (RMV) than had been seen in earlier surveys of ryegrass fields in 1972 and 1973. The chemical analysis of samples of ryegrass from 10 of the fields examined in 1976 showed that RMV significantly reduced the organic matter content, organic matter digestibility, water soluble carbohydrate content and D value of the grass. Furthermore, the losses suffered by grass with the severe necrotic symptom of RMV were significantly greater than those incurred by the grass with the mild mottle only. A correlation analysis between water soluble carbohydrate level and D value showed that whilst the loss in water soluble carbohydrates in mottled grass accounted for much of the loss in digestibility this was not the case in the necrotic grass where loss of green tissue appeared to be a more important contributory factor.     

The effect of ryegrass mosaic virus on the yield of perennial ryegrass swards

The influence of ryegrass mosaic virus (RMV) on yield of perennial ryegrass swards was examined under two harvesting frequencies and at two levels of nitrogen fertilizer, in a field trial planted with a clone of ryegrass, cv. S 321, using small plots established with proportions of healthy and infected plants. RMV reduced the height and the number of tillers of plants grown in pots and reduced the height also of plants in the field. The swards were planted in August and at a harvest 8 wk later RMV reduced the yield from 2–12 to 1–52 t dry matter (d.m.)/ha. In the first full harvest year, RMV caused only a small reduction in yield where no N fertilizer was applied and the application of a very small amount of fertilizer would have recouped the loss. On the other hand RMV severely restricted the increase in yield resulting from heavy dressings of the fertilizer. Thus where 400 kg N/ha was applied, RMV reduced the total annual yield from 18-6 to 13-8 t d.m./ha. The effect of the virus was especially great both in the Spring, the period of maximum herbage production, and when the level of fertilizer was high.     

Hybrids between tetraploid Italian and perennial ryegrass

Summary Hybridization frequency was investigated between tetraploid perennial and Italian ryegrass (Lolium perenne X multiflorum) without emasculation by using genetic markers. The Italian phenotypes, fluorescentroots and awned florets, were dominant. About 82% of the plants in perennial X Italian and nearly 93 % of the plants in the reciprocal crosses were hybrids. The hybrids had a high multivalent frequency and involved homoeologous chromosome pairing. Aneuploids with 2n = 26, 27 and 29 chromosomes were present. The hybrids were highly fertile. The cytogenetic behaviour of these allopolyploids suggested that the genomes of the parental species have undergone little repatterning and have free genetic exchanges. The species maintained their self-incompatibility and cross-compatibility at the tetraploid level.     

Specialisation of crown rust on highly and moderately resistant plants of perennial ryegrass

From two field samples of crown rust, isolates were selected for increased aggressiveness on four of 10 resistant perennial ryegrass clonal lines. Two lines which were formerly highly resistant became highly susceptible to the selected isolates but two which were formerly moderately resistant became only moderately susceptible. Thus, moderate resistance, although no less likely to be race-specific, was more stable than a high level of resistance. Cultures from highly resistant lines did not show increased aggressiveness on the varieties from which the resistant lines had been selected; hence individual genes of major effect were unimportant in determining varietal resistance. In contrast, cultures from moderately resistant lines were substantially more aggressive on their corresponding varieties. Three varieties contained many genes which tended to buffer changes in virulence of the rust.     

The adaptation of tomato mosaic virus to resistant tomato plants

Tomato mosaic virus derived from susceptible tomato plants (the standard virus) was cultured in resistant plants. Sap from non-inoculated leaves of resistant tomato plants infected with virus from the resistant host was more infective and contained more virus particles than leaf sap of resistant plants infected with the standard virus. Leaves of resistant tomatoes infected with virus from the resistant host also showed more obvious symptoms. Susceptible plants infected with virus from resistant plants not only showed fewer symptoms than when infected with standard virus, but samples were less infective and contained less virus up to 26 weeks, when values for infectivity were similar. This modification in activity was not reversible and was obtained with two lines of tomato having different types of resistance. Passage of virus from resistant plants through susceptible plants did not impair its ability to infect resistant plants.     

The resistance of two clones of perennial ryegrass and their progeny to ryegrass mosaic virus from different parts of Britain

Cavity spot is initiated anywhere on the carrot storage organ, when cells of the outermost layers of the secondary phloem collapse progressively, leading eventually to rupture of the periderm. A wound periderm forms around the lesion and suberin and lignin are deposited in the cell walls. Polyphenols accumulate in tissue around the lesion. Neither Ca or K concentrations in the soil or their ratios were related to cavity spot in surveys of crops in E. Scotland. Of the many characteristics recorded, only soil bulk density was related to the incidence of the disorder. No consistent responses to high concentrations of Ca or K were obtained in field and pot experiments. A response to Ca as gypsum was possibly due to its effect on soil structure.     

Infection of ryegrass plants with ryegrass mosaic virus decreases numbers of the mite vector

A single spray of tridemorph on 1 June almost doubled leaf area index of Zephyr barley during much of the second half of June. Increases in leaf area duration before ear emergence seemed important in these plots, because they yielded much better than plots sprayed on 11 June although differing little from them in leaf area duration after ear emergence.     

Effect of cocksfoot mottle virus on resistant and susceptible cocksfoot cultivars grown alone or in combination with healthy or ryegrass mosaic virus-infected Italian ryegrass

Cocksfoot mottle virus (CfMV) was readily transmitted on cutting implements. However, after six harvests, only a quarter as many plants of the resistant cv. Cambria as of the susceptible cv. S.37 became infected. In addition to resistance to becoming infected, cv. Cambria also possessed some resistance once infected, and suffered less mortality than S.37. More CfMV-infected plants died when competing with healthy plants than when competing with other infected plants. Both CfMV and ryegrass mosaic virus (RMV) caused greater reductions in the yield of their respective hosts in mixed stands of cocksfoot and Italian ryegrass than in single species stands. CfMV reduced the proportion of cocksfoot in the mixture and RMV the proportion of ryegrass. However, CfMV-infected cv. Cambria competed better than CfMV-infected cv. S.37 with healthy or with RMV-infected ryegrass. It is concluded that grass cultivars selected for resistance to virus infection will slow down the rate of disease spread within the sward, and selecting for postinfection resistance will help maintain the aggressiveness and competitive ability of plants once they become infected.     

Serological detection of ryegrass mosaic virus and ryegrass seed-borne virus

Ryegrass mosaic virus (RMV) was reliably detected in both perennial (S24) and Italian (S22) ryegrass, by enzyme-linked immunosorbent assay (ELISA) when plants had been infected for 8 wk. ELISA detected more infections in field-grown perennial ryegrass cv. Premo than either visual assessment or electron microscopy. However, with plants of Italian ryegrass cultivars only recently infected with RMV, positive reactions were more difficult to separate from the reactions of RMV-free plants, which varied considerably with cultivar, some giving high absorbance values. Immunosorbent electron microscopy showed that the RMV antiserum also contained antibodies to ryegrass seed-borne virus (RGSV), suggesting that these high values were caused by RGSV infection in the material tested.     

Tolerance to ryegrass mosaic virus and its inheritance

Plant yield within and between four cultivars of perennial ryegrass infected with ryegrass mosaic virus (RMV) was closely related to symptom severity. Distribution of symptom severity was continuous in four perennial ryegrass and four Italian ryegrass cultivars infected with a severe RMV isolate, and also in another perennial ryegrass cultivar infected with a severe isolate of the virus, a mild one and one of intermediate severity. Symptom expression was polygenically inherited in both Italian (cv. RvP) and perennial (cv. S.24) ryegrass. Both additive and non-additive genetic variation was present in RvP, but the variation in S.24 was additive only. No significant maternal inheritance was present in either species.     

Electron microscopy of cocksfoot streak virus and its differentiation from ryegrass mosaic virus in naturally infected Dactylis glomerata plants

The length and rigidity of some cocksfoot streak virus (CSV) particles were greater (1) in ammonium molybdate negative stain at pH 5 than at pH 8 and (2) when magnesium was added either directly to extracted plant sap or indirectly via increased plant uptake. Most CSV particles were flexuous and 750–760 run long in low concentrations of magnesium but many were rigid and 800–810 nm long in higher concentrations. In sodium phosphotungstate and ammonium molybdate at pH 5, frequency distributions of particle lengths from cocksfoot plants infected with both CSV and ryegrass mosaic virus (RMV) produced two modal peaks, one at less than 700 nm (RMV) and the other at more than 750 nm (CSV); in ammonium molybdate at pH 8, however, only one peak (at 725 nm) was resolved. In ultrathin sections of CSV-infected cocksfoot leaves, virus-like particles were scattered or in bundles within the cytoplasm, or aligned in aggregates near the tonoplast. Some cells contained pinwheel inclusions, often with attached or associated laminar plates and occasionally with scroll-like or circular tubes. These CSV-induced inclusions were easily distinguished from those induced by RMV, even when both types occurred in the same cell.     

Tolerance to ryegrass mosaic virus, its assessment and effect on yield

Symptom severity of eighteen populations of Italian ryegrass infected artificially in the glasshouse and naturally in the field with ryegrass mosaic virus (RMV) was strongly correlated. A smaller proportion of plants of the more tolerant populations showed symptoms in the field, but this was probably due to an association of tolerance with increasing incidence of symptomless infection rather than with resistance to infection. Under sward conditions, the yield of a sensitive genotype was reduced by 27% and that of two more tolerant ones by 15 and 13 %. The percentage dry matter yield loss of the most sensitive genotype was similar in all cuts, despite the appearance of extensive necrosis at the time of one cut. With the more tolerant genotype the yield loss varied from 7 to 25 % according to cut. Over the period of the experiment RMV infection did not increase plant mortality.     

Co-transformed, diploid Lolium perenne (perennial ryegrass), Lolium multiflorum (Italian ryegrass) and Lolium temulentum (darnel) plants produced by microprojectile bombardment

A total of 37 plants (30 Lolium multiflorum Lam., 6 L. perenne L., 1 L. temulentum L.) were regenerated from cell suspension colonies bombarded with plasmid DNAs encoding a hygromycin resistance gene (HYG) expressed under a CaMV35S promoter and a β-glucuronidase (GUS) gene expressed under a truncated rice actin1 promoter and first intron, or a maize ubiquitin promoter and first intron. Resistant plants were regenerated under hygromycin selection and transferred to soil. PCR analysis showed that the co-transformation frequency of the GUS gene varied from 33% to 78% of transformants, while histochemical staining of leaf tissue from soil-grown plants showed that the co-expression frequency varied from 37% to 50%. The transgenic nature of the plants was demonstrated by Southern hybridisation analysis, which also showed that the non-selected (GUS) gene was generally present at a higher copy number than the selected (HYG) gene. Received: 10 October 1997 / Revision received: 18 March 1998 / Accepted: 29 November 1998