A field study of the pattern of local spread of ryegrass mosaic virus in mown grassland

In a field experiment at Hurley, small areas of old ryegrass sward, known to be infected with ryegrass mosaic virus (RMV) were partially surrounded by newly-sown Italian ryegrass. The experiment was harvested by mowing on five occasions. Samples of Italian ryegrass taken when it was 11 months old showed a very marked increase in the levels of RMV close to the areas of old sward. Thus, 84% of the Italian ryegrass plants bordering the old sward to the north contained RMV, compared with 63% of plants only 2.7 m away. The corresponding values to the south were 68% and 39%. The much higher values encountered to the north are presumed to be caused by the movement of the mite vector of RMV (Abacarus hystrix) in relation to the prevailing wind. Two different mowers were used, a rotary mower and a reciprocating knife mower. We could demonstrate no increase in the level of RMV as a result of mowing from the infected sward and on to the new sowing and could demonstrate no difference in the infection levels as a result of using the different mowers. We conclude that in mown swards migration by the mite is responsible for all or nearly all of very local spread of RMV.     

The effect of ryegrass mosaic virus on carbon assimilation and growth of ryegrasses

The growth of simulated swards of Italian and perennial ryegrass, artificially infected with ryegrass mosaic virus (RMV) or RMV-free, was investigated. During the build up of RMV infection, dry-matter yield was usually decreased in infected swards, and the effect was confirmed when single vegetative regrowths of swards were investigated in detail. The primary cause of lower yields in RMV-infected swards appears to be a decrease in net canopy photosynthesis (maximum decrease about 50%) and an associated increase in dark respiration (maximum increase about 50%). The decrease in net canopy photosynthesis is due primarily to a lowered rate of net photosynthesis of constituent leaves within the canopy. A secondary effect of RMV infection is to decrease tillering (maximum decrease about 30%) which results in a change in canopy structure and in particular a lower leaf area index. In the present experiments, lower light utilization by the swards is less important than the decrease in leaf photo-synthetic efficiency in lowering yield.     

The effects of mixing Italian ryegrass (Lolium multiflorum) with perennial ryegrass (L. perenne) or red clover (Trifolium pratense) on the incidence of viruses

Mixing the ryegrass mosaic virus (RMV) resistant perennial ryegrass (Lolium perenne) cv. Endura with the susceptible Italian ryegrass (L. multiflorum) cv. RvP decreased infection of RvP wth RMV from 37% when grown alone to 22% when mixed. However, Endura yielded less than RvP and there was no yield benefit from mixing the two cultivars. Mixing red clover (Trifolium pratense) cv. Hungaropoly with RvP had no detectable effect on RMV incidence in RvP but did decrease the incidence of red clover necrotic mosaic virus in Hungaropoly from 9% to 1% and of white clover mosaic virus from 53-5% to 41%. The yield of the mixture was equal to that of RvP grown alone but given nitrogen fertiliser. The numbers of eriophyid mites, including Abacarus hystrix the vector of RMV, on ryegrass leaves were similar in pure and mixed swards. It is concluded that with herbage crops, the common practice of sowing mixtures of species may help control virus diseases.     

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.     

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.     

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.     

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.     

Selection of perennial and Italian ryegrass plants resistant to ryegrass mosaic virus

Perennial ryegrass plants collected from fields and Italian ryegrass plants grown from seed were selected for resistance to infection by ryegrass mosaic virus (RMV) by repeated manual inoculation. Two of 108 perennial ryegrass plants and one of 150 Italian ryegrass plants were symptomless after seven and nine inoculations respectively. These three plants were propagated vegetatively. Plants of the two perennial ryegrass clones showed no symptoms after further manual inoculations with the initial isolate of RMV, or with an inoculum from infected plants collected from several fields, or after inoculation by viruliferous mites. Electron microscopy and back tests indicated that the plants were virus free. Some plants of the selected Italian ryegrass clone became infected after a further inoculation with mites or sap, but fewer than similarly inoculated unselected plants.     

Interaction between crown rust (Puccinia coronata) and two viruses of ryegrass

In Lemtal Italian and S.24 perennial ryegrass plants, two isolates of ryegrass mosaic virus (RMV) suppressed the amount of crown rust emerging on leaves inoculated with Puccinia coronata uredospores by up to 75% compared with the amount on virus-free plants. Severity of rust infection on barley yellow dwarf virus (BYDV) infected plants generally did not differ significantly from that on virus-free plants. When both RMV and BYDV were present, rust was restricted in Lemtal plants to a level intermediate between those occurring on plants infected by either virus alone, and in S.24 plants to a level below that obtained with either virus alone. The mean water soluble carbohydrate (WSC) content of Lemtal plants was reduced more than 20% by RMV, but was not significantly altered by BYDV. In S.24 plants the WSC content was increased by 10% by RMV and by 60% by BYDV. Rust reduced the WSC content of healthy and virus-infected plants, the reduction being positively correlated with the level of rust on the sampled leaves. In plants of Lemtal, but not of S.24, the degree of rust infection was positively correlated with the WSC content of leaves from rust-free control plants.     

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.     

Effect of nitrogen on interactions between ryegrass mosaic virus (RMV) and Lolium species in mixed swards

The effect of rate of nitrogen applied to three Lolium perenne-based field swards on the development of RMV symptoms and on the impact of the virus on the growth and composition of the swards was examined over two growing seasons. Glasshouse experiments investigated the competition between Lolium spp. in relation to RMV infection and rate of nitrogen. In general, there was a higher incidence of RMV symptoms at the higher rates of nitrogen but the relationship was not consistent in all experiments. RMV infection only significantly reduced the cumulative total yield of the sward that contained the very susceptible Lolium multiflorum cv. Barverdi and was treated with the higher rate of nitrogen. However, yield losses were consistently greater at the higher rates of nitrogen. RMV altered the relative yields of sward components and the effect was greatest after an extended period of severe frost at the end of 1995. There is evidence that the frost severely restricted the yield of RMV-infected L. multiflorum tillers in 1996. Where RMV reduced the yield of susceptible species, there was significant compensatory growth by less susceptible companion species, i.e. L. perenne and Phleum pratense, but the compensation was generally incomplete, resulting in reduced yields for virus-infected swards. RMV had no significant effect on the overall quality of the three field swards but for cv. Barverdi grown in the glasshouse it significantly reduced the percentage organic matter and water soluble carbohydrate, and D value. The reductions were greater at the higher rates of nitrogen.     

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.     

The elimination of ryegrass mosaic virus from Lolium multiflorum by meristem tip culture

Meristem tips were cultured from Lolium multiflorum plants infected with ryegrass mosaic virus (RMV). Meristem tips within the size range O'2-fi mm long were cultured on media with or without 2 ,4-D at 1 mg/1. The plants that regenerated in culture showed no symptoms over a 10 month period and no RMV particles were observed by electron microscopy. It was concluded that RMV had been eliminated. The method should prove useful in eliminating the virus from desirable genotypes used as parent material for seed production.     

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.     

Effect of winter cover crops on soil nitrogen availability,corn yield,and nitrate leaching

Biculture of nonlegumes and legumes could serve as cover crops for increasing main crop yield, while reducing NO3 leaching. This study, conducted from 1994 to 1999, determined the effect of monocultured cereal rye (Secale cereale L.), annual ryegrass (Lolium multiflorum), and hairy vetch (Vicia villosa), and bicultured rye/vetch and ryegrass/vetch on N availability in soil, corn (Zea mays L.) yield, and NO3-N leaching in a silt loam soil. The field had been in corn and cover crop rotation since 1987. In addition to the cover crop treatments, there were four N fertilizer rates (0, 67, 134, and 201 kg N ha(-1), referred to as N0, N1, N2, and N3, respectively) applied to corn. The experiment was a randomized split-block design with three replications for each treatment. Lysimeters were installed in 1987 at 0.75 m below the soil surface for leachate collection for the N 0, N 2, and N 3 treatments. The result showed that vetch monoculture had the most influence on soil N availability and corn yield, followed by the bicultures. Rye or ryegrass monoculture had either no effect or an adverse effect on corn yield and soil N availability. Leachate NO3-N concentration was highest where vetch cover crop was planted regardless of N rates, which suggests that N mineralization of vetch N continued well into the fall and winter. Leachate NO3-N concentration increased with increasing N fertilizer rates and exceeded the U.S. Environmental Protection Agency's drinking water standard of 10 mg N l(-1) even at recommended N rate for corn in this region (coastal Pacific Northwest). In comparisons of the average NO3-N concentration during the period of high N leaching, monocultured rye and ryegrass or bicultured rye/vetch and ryegrass/vetch very effectively decreased N leaching in 1998 with dry fall weather. The amount of N available for leaching (determined based on the presidedress nitrate test, the amount of N fertilizer applied, and N uptake) correlated well with average NO3-N during the high N leaching period for vetch cover crop treatment and for the control without the cover crops. The correlation, however, failed for other cover crops largely because of variable effectiveness of the cover crops in reducing NO3 leaching during the 5 years of this study. Further research is needed to determine if relay cover crops planted into standing summer crops is a more appropriate approach than fall seeding in this region to gain sufficient growth of the cover crop by fall. Testing with other main crops that have earlier harvest dates than corn is also needed to further validate the effectiveness of the bicultures to increase soil N availability while protecting the water quality.     

The transmission and effect on yield of ryegrass mosaic virus in a filtered air environment

The eriophyid mite vectors of ryegrass mosaic virus (RMV) were excluded from ryegrass by covering plots with a polyethylene film house ventilated by filtered air. Initially plots were manually inoculated to give different levels of RMV. During two seasons no mites were found within the house so that all transmission of virus was by sap. Rolling and frequent cutting of uninfected plots decreased yields, but surprisingly, increasing virus infection did not decrease yields significantly. Cutting transmitted virus within infected plots but not from infected to healthy plots, whereas combined rolling and cutting did transfer virus to healthy plots.     

Influence of an organic mulching on fertilizer nitrogen use efficiency and herb and essential oil yields in geranium (Pelargonium graveolens)

In a field study, conducted at Lucknow ( 26.5 degrees N, 80.5 degrees E and 120 m altitude), India for two years (1996-1997 and 1997-1998), eight treatment combinations of two variables of organic mulch (paddy straw at 7 t/ha and no mulch) and four levels of fertilizer nitrogen (0, 80, 160 and 240 kg/ha) were examined to observe the effect of organic mulching on N-use efficiency and essential oil yield in a multi-harvested geranium crop. Results revealed that application of paddy straw mulch increased the herb and essential oil yields in geranium by 23% and 27%, respectively, over the unmulched control at planted crop harvest. Corresponding values at regenerated crop harvest were 18.7% and 19.2%. A significant response to N was observed with 160 kgN/ha in mulched plot over the same level of N in the unmulched plot. Using paddy straw mulch, nitrogen uptake by plants of planted and regenerated crops was increased by 33% and 28.4%, respectively, over the unmulched control. Apparent N recoveries by planted and regenerated crops were estimated to be 33.7% and 22.7% for the unmulched control, as against 40% and 29.2% with paddy straw mulch at 160 kgN/ha. The quality of essential oil of geranium in terms of its major constituents, citronellol and geraniol, was not affected by the use of organic mulching and nitrogen fertilization and these constituents were found to be of a standard acceptable in international trade.It was concluded from this study that use of an organic mulch with 160 kgN/ha proved better in terms of economising 80 kgN/ha to produce an economic yield of 96.1 kg geranium oil from two harvests under subtropical conditions of the north Indian plains. At 160 kgN/ha, paddy straw mulch application permitted the geranium crop to produce 18.4 kg/ha more oil which gave an additional return of Rs. 53,600/ha than that of unmulched control. Paddy straw mulch and nitrogen fertilization had no adverse effect on the quality of essential oil of geranium.     

Measured and simulated nitrous oxide emissions from ryegrass- and ryegrass/white clover-based grasslands in a moist temperate climate

There is uncertainty about the potential reduction of soil nitrous oxide (N(2)O) emission when fertilizer nitrogen (FN) is partially or completely replaced by biological N fixation (BNF) in temperate grassland. The objectives of this study were to 1) investigate the changes in N(2)O emissions when BNF is used to replace FN in permanent grassland, and 2) evaluate the applicability of the process-based model DNDC to simulate N(2)O emissions from Irish grasslands. Three grazing treatments were: (i) ryegrass (Lolium perenne) grasslands receiving 226 kg FN ha(-1) yr(-1) (GG+FN), (ii) ryegrass/white clover (Trifolium repens) grasslands receiving 58 kg FN ha(-1) yr(-1) (GWC+FN) applied in spring, and (iii) ryegrass/white clover grasslands receiving no FN (GWC-FN). Two background treatments, un-grazed swards with ryegrass only (G-B) or ryegrass/white clover (WC-B), did not receive slurry or FN and the herbage was harvested by mowing. There was no significant difference in annual N(2)O emissions between G-B (2.38±0.12 kg N ha(-1) yr(-1) (mean±SE)) and WC-B (2.45±0.85 kg N ha(-1) yr(-1)), indicating that N(2)O emission due to BNF itself and clover residual decomposition from permanent ryegrass/clover grassland was negligible. N(2)O emissions were 7.82±1.67, 6.35±1.14 and 6.54±1.70 kg N ha(-1) yr(-1), respectively, from GG+FN, GWC+FN and GWC-FN. N(2)O fluxes simulated by DNDC agreed well with the measured values with significant correlation between simulated and measured daily fluxes for the three grazing treatments, but the simulation did not agree very well for the background treatments. DNDC overestimated annual emission by 61% for GG+FN, and underestimated by 45% for GWC-FN, but simulated very well for GWC+FN. Both the measured and simulated results supported that there was a clear reduction of N(2)O emissions when FN was replaced by BNF.     

Interactions between white clover and ryegrass under contrasting nitrogen availability: N2 fixation,N fertilizer recovery,N transfer and water use efficiency

Seasonal variation in N₂ fixation, N transfer from clover to ryegrass, and soil N absorption in white clover–ryegrass swards were investigated under field conditions over three consecutive years. The plots were established with different seeding ratios of clover and ryegrass and contrasting fertilizer N ranging from 3 to 72 kg ha^-1 year^-1.An initially poor clover population needed at least one growing season to reach the same yield output as an initially well established clover population. The clover content of the sward decreased by the annual application of 72 kg N ha^-1 but not by smaller N dressings.The total amount of atmospherically derived N in clover growing in mixture with ryegrass was, on average over the three years equal to 83, 71, 68 and 60 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively. The proportion of atmospherically derived N declined with increasing N application, but never became smaller than 80% of total clover N. The proportion of atmospherically derived N in a pure stand white clover amounted to 60–80% of the total N content, equivalent to 109, 110, 103 and 90 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively.Only small amounts of atmospherically derived N was transferred to the associated ryegrass during the first production year, while in each of the following years up to 21 kg ha^-1 was transferred. The average amount of N transferred from clover to ryegrass was equivalent to 3, 16 and 31% of the N accumulated in ryegrass in the first, second and third production year, respectively. Expressed relative to the total amount of fixed N₂ in the clover–ryegrass mixture, the transfer amounted to 3, 17 and 22% in the first, second and third production year, respectively. Thus transfer of atmospherically derived N from clover contributed significantly to the N economy of the associated ryegrass.The clover–ryegrass mixture absorbed constantly higher amount of soil derived N than the pure stands of the two species. Only 11% of the total accumulated fertilizer N and soil derived N in the mixture was contained within the clover component. Lower water use efficiencies for the plants grown in mixture compared to pure stands were mainly related to the increased N uptake in the mixture, with the subsequent increase in growth compared to the pure stands.It is concluded that positive interactions between clover and ryegrass growing in mixture ensure a more efficient fixation of atmospheric N₂ and absorption of fertilizer N and soil derived N than pure stands of the same species.     

Variation in reaction to barley yellow dwarf virus in ryegrass and its inheritance

Genotypes of Italian and perennial ryegrass differed greatly in their reaction to infection with barley yellow dwarf virus (BYDV). One genotype of perennial ryegrass appeared unaffected, whilst the yield and height of other genotypes were reduced. Progeny of the Italian ryegrass genotypes showed a similar degree of variation. Tolerance, assessed by the effect of the virus on yield, was inherited in a wholly additive manner. Symptoms were rare and their severity was under both additive and non-additive genetic control and strongly correlated with increasing severity of reaction. The utilization of the variation in BYDV reaction in breeding programmes may prove difficult because plants with the highest performance tended to suffer most from BYDV infection, and half-sib progeny ranked in a different order of tolerance in the glasshouse from that of their respective parent genotypes in the field. It is suggested that the observed variation in BYDV response results from a readjustment in the balance of genotypically controlled and environmentally conditioned variation in plant growth.