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.     

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.     

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.     

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 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 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.     

The effect of some isolates of ryegrass mosaic virus on different genotypes of Lolium multiflorum

Some isolates of ryegrass mosaic virus produced necrosis in Lolium multiflorum much more frequently than others. This frequency could be increased or decreased by selecting plants with or without necrosis, respectively, as sources of inoculum. Infection with the milder, mosaic-inducing isolates which rarely caused necrosis did not protect the plants against subsequent infection with the more severe necrosis-inducing isolates. Oat plants were infected much more readily by isolates which frequently caused necrosis than by the less necrotic isolates, but passage through oat appeared to have no effect on the frequency with which an isolate produced necrosis in ryegrass.
Genotypes of L. multiflorum varied greatly in their tolerance of infection. Their level of tolerance was closely correlated with symptom expression. All genotypes tested were tolerant to a mild, mosaic-inducing, isolate. Two genotypes were tolerant to a severe, necrosis-inducing, isolate and to an original Rothamsted isolate; three genotypes were equally susceptible to both isolates, and two were more susceptible to the necrotic isolate than to the Rothamsted isolate.     

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.     

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 effect of cassava mosaic geminiviruses on symptom severity, growth and root yield of a cassava mosaic virus disease-susceptible cultivar in Uganda

A study was carried out to assess the effect of different cassava mosaic geminiviruses (CMGs) occurring in Uganda on the growth and yield of the susceptible local cultivar ‘Ebwanateraka’. Plants infected with African cassava mosaic virus (ACMV), ‘mild’ and ‘severe’ strains of East African cassava mosaic virus‐Uganda (EACMV‐UG2) and both ACMV and EACMV‐UG2 were grown in two experiments in Kabula, Lyantonde in western Uganda. The most severe disease developed in plants co‐infected with ACMV and EACMV‐UG2 and in those infected with the ‘severe’ form of EACMV‐UG2 alone; disease was least severe in plants infected with the ‘mild’ strain of EACMV‐UG2. ACMV‐infected plants and those infected with the ‘mild’ strain of EACMV‐UG2 were tallest in the 1999–2000 and 2000–2001 trials, respectively; plants dually infected with ACMV and EACMV‐UG2 were shortest in both trials. Plants infected with ‘mild’ EACMV‐UG2 yielded the largest number and the heaviest tuberous roots followed by ACMV and EACMV‐UG2 ‘severe’, respectively, whilst plants dually infected with ACMV and EACMV‐UG2 yielded the least considering the two trials together. Reduction in tuberous root weight was greatest in plants dually infected with ACMV and EACMV‐UG2, averaging 82%. Losses attributed to ACMV alone, EACMV‐UG2 ‘mild’ and EACMV‐UG2 ‘severe’ were 42%, 12% and 68%, respectively. Fifty percent and 48% of the plants infected with both ACMV and EACMV‐UG2 gave no root yield in 1999–2000 and 2000–2001, respectively. These results indicate that CMGs, whether in single or mixed infections, reduce root yield and numbers of tuberous roots produced and that losses are substantially increased following mixed infection.     

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.     

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.     

The effect of oil on the transmission of pea enation mosaic virus during short inoculation probes

The transmission efficiency of pea enation mosaic virus by Acyrthosiphon pisum (Harris) in short inoculation periods to pea plants sprayed with oil prior to the test, was compared with that to plants which were not coated with an oil film. The transmission of virus to the former group was lower (approximately 11%). However, this difference was not significant. The probing behaviour measured by parameters such as test-probe frequency prior to phloemseeking probes, duration of the probes and the cumulative duration of the short inoculation periods, did not differ for aphids probing on plants with and without oil film. The results suggest that the mechanism of pea enation mosaic virus transmission in short inoculation access periods differs from that of the non-persistent viruses.

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Zusammenfassung Die Übertragung nicht-persistenter Viren wird durch eine Behandlung der Pflanzen mit Mineralöl eingeschränkt. Auf die Übertragung persistenter Viren haben Öle keinen Einfluß. Dies ist offensichtlich auf einen anderen Übertragungsmechanismus bei diesen Viren zurückzuführen. Erbsenenationen-Virus — ein persistentes Virus — kann aber auch in kurzfristigen Infektionszeiten auf Pflanzen übertragen werden.In dieser Untersuchung wurde geprüft, ob die Übertragung dieses Virus bei solchen Infektionszeiten von Öl gehemmt werden kann. Es wurde jedoch keine entscheidende Einschränkung der Übertragungsraten gefunden, wenn die Pflanzen mit Öl behandelt worden waren.Auch das Verhalten der Blattläuse auf diesen Pflanzen wurde durch Anwendung verschiedener Parameter analysiert. Verglichen wurden die Häufigkeit der Probestiche, bevor ein Saugstich gemacht wird, die Dauer der Probestiche und die Summe der Dauer kurzfristiger Infektionszeiten auf Pflanzen, die mit Öl behandelt wurden oder nicht. Die Saugstiche wurden nach einer Dauer von 1 min abgebrochen. Jede Blattlaus machte 4 Stiche an ein und derselben Pflanze.Die genannten Parameter zeigen, daß im Verhalten der Blattläuse auf mit Öl bedeckten Planzen keine Änderung eintritt.

     

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.     

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.     

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.     

The effect of the transmission of the beet mosaic virus on the variability of its incubation period

The variability of the incubation period of the beet mosaic virus [Beta virus 2(Lind) Smith] was investigated in a three year experiment. The virus was transmitted by the black bean aphid (Aphis fabae Scop.) or by a mechanical inoculation onAmaranthus caudatus L. Variability characterized by the standard deviation indicates a closer distribution of measurements when the virus was transmitted by mechanical inoculation. The differences in variability of the incubation period between the two methods of virus tramission are statistically significant; dispersion and standard deviation are lower after mechanical inoculation. The average length of the incubation period after virus transmission by aphids and by mechanical inoculation was 10.8 and 7.7 days, respectively. The difference was found statistically significant and may be explained by the higher number of viral particles transmitted by mechanical inoculation which may accelerate the process of infection. A significant correlation between the temperature of the environment and the length of the incubation period was also found in some experiments.     

The effect on yield in courgette and marrow of the mild strain of zucchini yellow mosaic virus used for cross-protection

The effects on yield in courgette and marrow (Cucurbita pepo) crops resulting from inoculation with the mild strain of zucchini yellow mosaic virus (ZYMV:WK), have been determined in polythene-house trials and in three years of outdoor, commercial field trials. In polythene-house trials ZYMV:WK inoculated plants were up to 10 days later in flowering than uninoculated plants and their cumulative yields were between 5% and 26% less than uninoculated plants depending on the cultivar. In most field trials cumulative yields from inoculated plants were between 4% and 38% less than uninoculated plants depending on the site and cultivar, but in one trial the yield was 7% higher from inoculated plants. In all experiments, courgette and marrow fruits harvested from ZYMV:WK inoculated plants were symptomless and indistinguishable from fruit harvested from uninoculated plants. The mild leaf symptoms induced by ZYMV:WK infection did not intensify to severe leaf symptoms and where there were natural outbreaks of severe ZYMV infection, fruits from inoculated plants remained symptomless whilst those from uninoculated plants were severely affected and unmarketable.