Poplar mosaic virus

P. G. Biddle
Commonwealth Forestry Institute, Oxford, England

T. W. Tinsley
Commonwealth Forestry Institute, Oxford, England

Contents

Introduction Main Diseases Geographical Distribution Host Range and Symptomatology Strains Transmission by Vectors Transmission through Seed Transmission by Grafting Transmission by Dodder Serology Nucleic Acid Hybridization Relationships Stability in Sap

Purification Properties of Particles Particle Structure Particle Composition Properties of Infective Nucleic Acid Molecular Structure Genome Properties Satellites Relations with Cells and Tissues Ecology and Control Notes References Acknowledgements Figures

Introduction

Described by Atanasoff (1935) and Berg (1964).

Synonym

Canadian poplar mosaic (Rev. appl. Mycol. 14: 462)

A virus with flexuous filamentous particles c. 675 nm long. Only known natural host is poplar, in which it is common and can cause severe growth losses. Many other plant species can be infected by inoculation of sap. There is no known vector, but it is widely disseminated in infected cuttings.

Main Diseases

Causes a mosaic (Fig.1) or diffuse spotting (Fig.2) in mature leaves of most poplar clones, but some clones react more severely, with necrosis of veins and leaf stalks, swellings round the bases of the petioles, and small lesions and splits on the stem. Growth of infected trees, particularly of those showing more severe leaf symptoms, is reduced, and there are also effects on the specific gravity and strength of wood from infected trees (Biddle & Tinsley, 1971b).

Geographical Distribution

Throughout the geographical range of susceptible poplar species and clones.

Host Range and Symptomatology

Natural hosts are poplars in the Sections Aigeiros and Tacamahaca, but species in 20 dicotyledonous families can be infected by inoculation of sap (Schmelzer, 1966).

Diagnostic species

Nicotiana megalosiphon (Fig.3) and N. glutinosa. Faint chlorotic local lesions after 10 days, followed by systemic vein-clearing and leaf-curling. Veins occasionally become necrotic.

Lavatera trimestris. Small necrotic local lesions, followed by systemic vein-clearing and vein-necrosis.

Vigna sinensis (cowpea). Red local lesions develop after 7 days, tending to spread along the veins they contact (Fig.4); systemic vein-clearing after about 20 days followed by vein-reddening and leaf deformation.

Propagation species

Nicotiana megalosiphon is the most suitable species for maintaining the virus and providing material for purification.

Assay species

Vigna sinensis sometimes produces countable lesions, but is not a reliable assay host.

Strains

Most isolates seem identical, although Blattný (1965) distinguished a more severe strain in Populus x euramericana cv. Robusta.

Transmission by Vectors

A low rate of natural transmission has been noted in the field, but no vector has been satisfactorily implicated. Boyer (1962) claimed that the aphid Neothomasia populicola transmits the virus, but other workers have been unable to confirm this. Circumstantial evidence suggests that the virus may spread through root grafts, but from experiments with radioactive tracers, the number of root grafts seems too few to account for the rate of virus spread (P. G. Biddle, unpublished).

Transmission through Seed

Not reported.

Transmission by Dodder

Cuscuta californica and C. subinclusa failed to transmit the virus from Nicotiana glutinosa to N. megalosiphon (Schmelzer, 1966).

Serology

The virus is moderately immunogenic. Antisera with titres of 1/2048 are commonly obtained. Gel-diffusion tests are unsatisfactory, but microprecipitation tests give good results (Berg, 1964).

Relationships

The morphology of its particles and their stability and sedimentation coefficient suggest that poplar mosaic virus is a member of the potato virus S group of viruses (Brandes & Wetter, 1959). However, no interaction was detected between poplar mosaic virus and antisera against viruses of this group such as pea streak, red clover vein mosaic, carnation latent, potato S, potato M, chrysanthemum B and freesia mosaic viruses, or against other viruses with filamentous particles such as potato X, potato Y, bean yellow mosaic, iris mosaic and tulip breaking viruses (Berg, 1964).

Stability in Sap

In Nicotiana megalosiphon sap, the thermal inactivation point is about 74°C, dilution end-point about 10^-5, and infectivity persists in vitro for 2 days at room temperature and for 6 days at 4°C (Schmelzer, 1966). An isolate studied by Biddle & Tinsley (1971a) was somewhat less stable and less concentrated in sap. Infectivity in poplar extracts is greatly increased by adding 0.1 g of insoluble polyvinyl pyrrolidone to each 1 ml of extraction medium.

Purification

Freeze infected Nicotiana megalosiphon leaves and triturate in buffer (0.05 M Na₂HPO₄, 1 mM di-sodium ethylene diaminetetraacetate, 0.2 M Na₂SO₃ ; pH 7.5). Heat the extract to 50°C, centrifuge at low speed, precipitate the virus by adding ammonium sulphate to 30-45% saturation, and resuspend in buffer. Further purify by centrifuging in sucrose density gradients or by clarifying with chloroform followed by molecular exclusion chromatography in columns of 2% agarose beads. The virus is liable to aggregate, and virus sedimented in the ultracentrifuge is difficult to resuspend (Biddle & Tinsley, 1971a).

Properties of Particles

Sedimentation coefficient (s_20,w) at infinite dilution: 165 S (Berg, 1964).

Particle Structure

Slightly flexuous filamentous particles c. 675 nm long (Fig.5).

Particle Composition

No reports.

Relations with Cells and Tissues

All tissues in poplar are infected, including cambium, phloem and xylem (Berg, 1964). Metabolic disturbances to the differentiating xylem result in reduced growth and abnormalities of the wood structure (Biddle & Tinsley, 1971b).

Notes

This is the only known virus of poplar. Its widespread occurrence undoubtedly arises from dissemination of infected poplar cuttings. The use of healthy plants for propagation, and rogueing of infected plants. effectively controls the disease, because natural transmission appears to be rare.

References

1. Atanasoff, Phytopath. Z. 8: 197, 1935.
2. Berg, Meded. LandbHoogesch. Wageningen 64: 11, 1964.
3. Biddle & Tinsley, New Phytol. 70: 61, 1971a.
4. Biddle & Tinsley, New Phytol. 70: 67,1971b.
5. Blattný, Lesn. Cas. 11: 637, 1965.
6. Boyer, Can. J. Bot. 40: 1237, 1962.
7. Brandes & Wetter, Virology 8: 99, 1959.
8. Schmelzer, Phytopath. Z. 55: 317, 1966.

Figure 1

Leaf of Populus x euramericana cv. Robusta showing mosaic.

Figure 2

Chlorotic spots in Populus x euramericana cv. Regenerata.

Figure 3

Systemic symptoms in Nicotania megalosiphon.

Figure 4

Local lesions in Vigna sinensis.

Figure 5

Virus particles from clarified sap in uranyl acetate. Bar represents 100 nm.