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SUMMARY Five spring barleys, grown either in pots out of doors or in the field, were inoculated with barley yellow dwarf virus (BYDV) using 5, 10, 20 or 50 infective aphids (Rhopalosiphum padi) per plant. Control plants of each variety received no aphids. Infection with all aphid numbers had highly significant adverse effects on all varieties except Cb 1029, an early maturing BYDV-resistant barley of Ethiopian origin. 12583 Co, a locally bred, late maturing barley possessing the same resistance gene as Cb 1029 suffered more in a pot experiment, but less than three susceptible varieties all of which were severely damaged even when few infective aphids were used. Progressive effects with increasing aphid numbers, indicative of dosage response, occurred in some varieties. These effects included delay in heading and increased stunting, but not less yield. In Cb 1029, BYDV infection caused a reduction in the number of heads per plant, but this was partly compensated for by an increase in the number of grains per head. Conversely, BYDV infection in 12583 Co caused an increase in the number of heads, partly offset by a decrease in the number of Brains tier head.

SUMMARY 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.

Infection of Dactyls glomerata with cocksfoot mottle virus (Cf MV) and of Cynosurus cristatus with cynosurus mottle virus (CyMV) is frequently lethal. However, varieties of each species differ in the proportion of genotypes which survive, and in the damage suffered by these surviving plains. Infected plants of the CfMV-resistant cocksfoot, cv. Cambria, were generally much more aggressive in the competitive environment of the sward than were infected plants of the susceptible cv. S37. Thus, Cambria populations outyielded comparable populations of S37 by only 7% when healthy, but by 40% when infected with CfMV. A few genotypes in the infected Cambria population apparently recovered from the disease, became virus-free and were immune to further infection Resistance to CyMV in C. cristatus takes the form of resistance to systemic spread. A few symptomless and apparently virus-free tillers grow up from the bases of infected tillers. However, these tillers are susceptible to further infection. It is suggested that, in resistant genotypes, CfMV and CyMV trigger the production of antiviral agent(s) which block or restrict virus replication. There is some evidence that CfMV resistance is virus isolate specific, and this could present problems when breeding for durable resistance.

SUMMARY A total of 100 varieties and advanced lines of autumn-sown oats were assessed for reaction to soil-borne oat mosaic virus. Two apparently distinct types of resistance, resistance to becoming infected and post-infection resistance, were detected. Of the named varieties tested, cv. Lustre was outstanding, possessing the highest level of resistance to infection. Several breeding lines, but no named variety, possessed outstanding post-infection resistance. A few varieties such as cv. Solva, which lacked either type of resistance, were tolerant of the virus.

SUMMARY 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.

Summary A virus from Agrostis stolonifera (tribe: Aveneae) with filamentous particles about 685 nm long was serologically closely related to, and considered to be a strain of, ryegrass mosaic virus (RMV). This strain caused symptoms in Polypogon monspeliensis (tribe: Aveneae) more readily and in Lolium multiflorum (Italian ryegrass) less readily than did RMV from Lolium and Festuca spp. (tribe: Festuceae). In L. multiflorum and P. monspeliensis the Agrostis isolate induced pinwheel inclusions with associated laminar aggregates and tubes, but Lolium isolates induced pinwheels with laminar aggregates only. The intracellular distribution of the pinwheels differed with the severity of host response. Thus, in plants with mild symptoms most pinwheels were contiguous with the plasmalemma close to plasmodesmata, but in plants with severe symptoms the pinwheels were free in the cytoplasm. Virus particles occurred either randomly or in bundles in the cytoplasm of mesophyll and phloem companion cells. In P. monspeliensis infected with the Agrostis isolate, fibrous inclusions, possibly virus particles, occurred in some nuclei. When symptoms were severe, mitochondria and chloroplasts were amorphous and the latter had many marginal vesicles.

SUMMARY 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.

Summary Cells of five grass and cereal species infected with cocksfoot mottle virus, phleum mottle virus or cocksfoot mild mosaic virus were examined in ultra-thin sections. The intracellular distribution of the virus particles and their effects on cell ultrastructure appeared dependent more on the severity of the host/virus interaction than on the particular virus or host species involved. Particles of all three viruses occurred in the cytoplasm of mesophyll and phloem companion cells. Those of cocksfoot mottle virus were observed occasionally also in the nucleus. Relatively few particles, randomly distributed in the cytoplasm, were present in cells of plants with mild symptoms. In plants with severe symptoms the particles were numerous and often in near- or true-crystalline aggregates. In partially disrupted cells, virus particles occurred occasionally in membrane-bound, sometimes vesiculated, packets in the cell vacuole. These seemed to originate from the tonoplast. In more severely disrupted cells the tonoplast was lost and virus particles were scattered randomly throughout the vacuole. Ultrastructural effects ranged from an increase in endoplasmic reticulum and vacuolation of the cytoplasm in the cells of plants with mild symptoms, to the total disintegration of all cell organelles in plants with the severest symptoms. Extrusions, which often encircled a volume of cytoplasm developed from the chloroplasts and mitochrondia; membrane-bound vesicles, containing virus particles, appeared in the stroma of chloroplasts and in the mitochondria.

SUMMARY Cocksfoot mild mosaic and two hitherto unreported viruses, one from bromegrasses and one from Holcus lanatus, were isolated from grasses collected in north and mid Wales. All had isometric particles c.28 nm in diameter and were transmissible by sap inoculation, but they differed in their host-range and symptomatology. Serologically, none were identical, but all were sufficiently closely related to each other and to phleum mottle virus to be considered as strains of a single virus.

Moderately severe strains of barley yellow dwarf virus (BYDV) and ryegrass mosaic virus (RMV) significantly reduced the heights and yields of perennial (Lolium perenne) and Italian (L. multifiorum) ryegrasses. BYDV caused greater reductions in perennial ryegrass than in Italian, whereas the converse was true of RMV. Both viruses together rarely caused greater damage than the most damaging virus on its own. BYDV reduced the aggressiveness of perennial ryegrass more than of Italian, whereas with RMV the converse was true. In mixtures of perennial and Italian ryegrasses, the perennial was suppressed by the Italian when only BYDV was present, the Italian was suppressed by the perennial when only RMV was present, and the status quo was maintained when both viruses were present, even though they reduced the yield of the mixture by over 16%.

SUMMARY In four out of five seasons in Wales, during 1970-76, the winter oat cvs Maris Osprey and Peniarth were very susceptible to oat mosaic virus, cv. Padarn was moderately susceptible and cvs Pendrwm and Century were resistant. However, plants of resistant cultivars were sometimes severely damaged and some symptomless plants contained virus particles. It is suggested that a minimum threshold of virus in the plant is necessary to induce symptoms and that this threshold is lower in susceptible than in resistant cultivars. Cvs Pendrwm and Century were also the most resistant to oat tubular virus. Sap from cv. Padarn induced most local lesions in the assay host Nicotiana clevelandii, indicating that oat tubular virus reached its highest concentration in this cultivar.

SUMMARY Eleven isolates of phleum mottle virus, each from a different grass species, were grouped into five distinct strains: phleum mottle (PM), holcus transitory mottle (HTM), festuca mottle (FM), cocksfoot mild mosaic (CMM) and brome stem-leaf mottle (BSM). Their interrelationship was expressed three-dimensionally using serological differentiation indices. When 105 species of Gramineae were inoculated with each strain, eighty-one in forty genera became infected. These included festucoid, chloridoid, panicoid and andropogonoid species. Electron microscopy was more reliable than back-inoculation for detecting infection in plants that remained symptomless. Annual species were usually more severely affected than perennials. PM, HTM, FM, CMM and BSM did not appear to differ in host range, but sometimes differed in the symptoms induced in a given host. Each was symptomless in some hosts, caused a mild or severe mottle in others and mottling plus necrosis in a few. HTM was symptomless in most, and BSM in fewest species: BSM caused necrosis in most and HTM in fewest hosts.

SUMMARY 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.

Ryegrass plants (Lolium spp.) rarely developed obvious foliar symptoms when infected with barley yellow dwarf virus, but their yield was often substantially reduced. Perhaps of even greater importance, the virus modified their morphological development and seasonal growth rhythm. Consequently, the disease introduces considerable non-heritable variation into populations of grass plants so its possible presence may introduce difficulties into selecting varietal mother plants for specific growth characteristics and when assessing bred varieties for uniformity. Virus infection caused the greatest deviation in growth morphology and the smallest reduction in yield in early-flowering hay type varieties: the converse was true of late-flowering pasture type varieties. Unless a source of immunity or extreme resistance to the virus can be found therefore, it may be difficult to combine minimum deviation in growth morphology and minimum yield loss in a single variety.

SUMMARY Small experimental swards of three types-healthy, 50% and 100% streak-virus-infected-were established from single tillers of randomly selected cocksfoot genotypes. In these swards 44% of infected plants died within 2 years compared with 21% of healthy plants. Mortality of infected plants was not increased by the presence of healthy plants in the same sward. In swards containing both healthy and infected plants, increased growth of healthy plants compensated for the low yield of infected ones only when the swards were frequently defoliated. Infrequent cutting apparently allowed infected plants to check the growth of healthy ones, and 50% infected swards yielded little more than 100% infected swards. This difference in response was attributed to differences in the growth habits of healthy and infected plants. Few initially healthy plants became infected during 2 years of the experiment.

SUMMARY Tolerance to barley yellow dwarf virus (BYDV) in five Ethiopian barley genotypes was conditioned by major genes which apparently occurred at, or near, the same chromosome locus in each genotype. The genes or alleles differed in effectiveness, one providing a high, three an intermediate and one a low level of tolerance. In plants having a slow rate of growth, the effectiveness of the tolerance genes tended to be masked, but the effectiveness of the gene or allele which provided the highest level of tolerance was masked to a lesser extent than that of genes or alleles providing low or intermediate levels of tolerance.

SUMMARY Irish perennial rye-grass plants infected with barley yellow dwarf virus (BYDV) showed inconspicuous leaf symptoms but were more stunted and yielded less total herbage, but more early spring herbage, than healthy plants. During a period of 2 years, infection did not decrease plant survival; infected plants produced more tillers and gave a higher ratio of vegetative to fertile tillers than healthy plants. BYDV caused a greater loss of yield in simulated rye-grass swards cut twice a year than in those cut four times. Swards in which half the plants were infected gave yields intermediate between those of healthy and infected swards. Infected plants tended to dominate healthy ones in swards cut four times, but in those cut twice a year the initial status quo was maintained. In swards containing rye-grass and white clover, the clover yielded more for most of the year when with BYDV-infected than with healthy rye-grass, but this trend was reversed in spring. The different effects of barley yellow dwarf virus on rye-grass plants and of cocksfoot streak virus on cocksfoot seem reflected by the different effects of the viruses on the competition between infected grass, healthy grass and white clover plants, when these are grown in different combinations.

SUMMARY A soil-borne virus with rod-shaped particles (700–750 × 12 nm) was isolated from oats. It was transmitted with difficulty by sap inoculation to oats, but not to nine other species of Gramineae nor to Chenopodium amaranticolor. Two distinct strains of the virus were found; one causing chlorotic or ash-grey eyespots with dark green centres to appear on the basal portion of the oldest leaves and the other a general mottling of the upper portion of the youngest leaves. It is concluded that the virus is similar to, and probably identical with, oat mosaic virus in the United States.

SUMMARY Healthy and streak-virus-infected cocksfoot plants of a single genotype were examined over a 2-year period. In the first year infection decreased tillering by about 40%, but in pot-grown plants individual tiller weights were 30% higher so dry-matter yield was decreased by only 10%. In the second year the larger tiller weights compensated completely for decreased tillering. No similar compensation was observed in field-grown plants. The virus had the largest effects when soil fertility was highest, healthy plants producing significantly greater responses to nitrogen effects, but potassium, at certain times of the year, produced larger tiller weights in infected than in healthy plants. Infection decreased dry-matter yield more in frequently than in infrequently cut plants. Infection greatly decreased the number of vegetative but not of fertile tillers. Infected plants tended to flower earlier and produced fewer, slightly smaller, viable seeds.

SUMMARY Oat sterile dwarf virus was found for the first time in Britain. It occurred in a crop of oats in which 15 per cent of the plants were infected. Infected plants were severely stunted, deep blue-green in colour and produced a profusion of juvenile shoots with 2–3 much-shortened mis-shapen leaves. Panicle emergence was completely or almost completely suppressed. The virus was transmitted by the Delphacid, Javesella pellucida to wheat, oats, barley, meadow fescue and to Italian and perennial ryegrasses. Nearly 25 per cent of the J. pellucida population caught in the vicinity of the diseased oat crop were infective. A large proportion of non-infective J. pellucida became infective within 2–4 weeks of feeding on infected plants.

SUMMARY Barley varieties were most tolerant to infection with barley yellow dwarf virus (BYDV) when they grew rapidly, whether the rate of growth was determined by manipulation of the environment or by the innate genetic constitution of the host. A specific, incompletely dominant gene conditioning a high level of tolerance to the virus in certain rapidly growing genotypes in which it occurred naturally, failed to do so when transferred to slower growing genotypes. However, reintroduction into genotypes capable of rapid growth led to full restoration of the gene's effectiveness. Virus-free aphids recovered BYDV less readily from quick- than from slow-growing genotypes, all homozygous for the tolerance gene.

A series of barley varieties of practical interest in Britain and Western Europe were assessed for reaction to the barley yellow-dwarf virus (BYDV). All proved to be highly susceptible. Fourteen barley selections found to be tolerant to BYDV in the United States were also tested. Although all were tolerant to BYDV when tested in the glasshouse, only two were tolerant under British field conditions. One selection was slightly less tolerant of one virus isolate than it was of another isolate.

The effect of cocksfoot streak virus (CSV) on Dactylis glomerata L. and barley yellowdwarf virus (BYDV) on Lolium perenne L. was investigated under both single-plant and simulated-sward conditions. The effect of cocksfoot mottle (CFMV) was examined on single plants. CFMV was lethal, and in conditions of inter-plant competition CSV was semi-lethal and BYDV was non-lethal. When the virus was lethal, a considerable lag-phase was observed between mortality and complete compensation in yield. When the virus was non-lethal, plant growth was restricted, either horizontally (CSV) or vertically (BYDV). In mixed, healthy and infected plant populations healthy individuals compensated only for reductions in lateral growth. CSV infection transformed plants of a pasture variety into ‘hay’ types and BYDV infection transformed plants of a hay variety into ‘pasture’ types. CSV increased the plant's aggressiveness in hay conditions, but reduced its ability to withstand frequent defoliation. Conversely, BYDV increased the plant's aggressiveness in pasture conditions, but reduced it in hay conditions. Compensation from healthy plants occurred only in CSV-infected swards subjected to frequent defoliation. Some possible reasons for the absence of compensation in other treatments are discussed.

SUMMARY A barley variety of Ethiopian origin, with a single Mendelian gene con-fering tolerance to barley yellow dwarf virus (BYDV), was equally tolerant to a number of isolates of the virus, whereas a susceptible European barley was more susceptible to isolates transmitted by Rhopalosiphum padi L. than to those transmitted by Macrosiphum (Sitobion) avenae (Fab). However, hybrids between these two varieties homozygous for the Ethiopian tolerance gene were more tolerant to ‘mild’ than to ‘severe’ isolates, irrespective of the vector specificity. The European variety was damaged more severely by all isolates when infected early than when infected late in its development, but the hybrids were damaged more severely by M. awraae-transmitted isolates when infected late. It is suggested that in susceptible plants the concentration, rather than the virulence, of the virus determines disease severity, whereas the reverse is true in plants possessing a gene which reduces virus multiplication. Virus concentration appears to determine the severity of R. padi-transmitted isolates, while virulence determines the severity of M. avenae-transmitted isolates. The latter would also seem to be adapted towards late infection.

SUMMARY Agropyron mosaic virus has been found in Britain for the first time. It was transmitted by the eriophyid mite, Abacarus hystrix to wheat, and by sap-inoculation to 17 other festucoid species of Gramineae. The virus had flexuous, filamentous particles c. 715·15 nm and was serologically related to a Canadian isolate. Many pinwheel and crystalline inclusions occurred in the cells of infected plants.

SUMMARY Macrosiphum (Sitobion) fragariae transmitted cocksfoot mild mosaic and cocksfoot streak viruses two to three times more frequently from plants in which they occurred together than from plants in which they occurred alone. In cells of doubly infected plants, particles of the two viruses commonly occurred in close association.

SUMMARY Cocksfoot mottle virus (CfMV) and cynosurus mottle virus (CyMV) had similar, but not identical, host-ranges within the Gramineae. Each infected fifteen out of forty-one species tested, considerably fewer than phleum mottle virus (PMV), but unlike PMV both infected Triticum aestivum (wheat), causing a lethal mottle. The species most useful for differentiating CfMV, CyMV and PMV are listed. Some species appear especially susceptible to virus infection.

SUMMARY A hitherto unrecorded virus having flexible rod-shaped particles about 740–760 × 13 nm was isolated from Anthoxanthum odoratwn L. It was transmitted by sap inoculation, but not by several species of insect, seed or soil to 18 species of Gramineae including wheat, oats and barley. In susceptible species the virus normally produced a mosaic mottling of the leaves which was sometimes followed by a necrotic streaking or striping.

SUMMARY A hitherto unrecorded virus having almost spherical particles about 30 nm in diameter was isolated from Phleum pratense and P. bertolonii. It was transmitted to five other species of Phleum, Agrostis nebulosa, Apera spica-venti and Polypogon maritimus by sap inoculation. The virus was also transmitted by two species of beetle, Lema melanopa and L. lichenis. Some beetles became infective after short feeds on infected plants; others after long feeds. The virus sometimes persisted in the beetles for several days, but sometimes only for a few hours. Several beetles were already infective when caught in the field.

SUMMARY A virus apparently indistinguishable from the ryegrass.chlorotic streak virus in New Zealand has been found in Britain where it appears to be similar to, if not identical with, isolates of barley yellow dwarf virus transmitted specifically by Rhopalosiphum padi.