169
September 1976
Family: Potyviridae
Genus: Tritimovirus
Species: Oat necrotic mottle virus
Acronym: ONMV


Oat necrotic mottle virus

C. C. Gill
Agriculture Canada Research Station, 195 Dafoe Road, Winnipeg, Manitoba, Canada

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 Gill & Westdal (1966).

A virus with filamentous particles c. 720 nm long and 11 nm in diameter infecting oat and other grass species. Readily sap-transmissible. Vector unknown. Found in Canada.

Main Diseases

Causes a mosaic of oat and a mild or symptomless disease in Poa compressa (Canada bluegrass), P. pratensis (Kentucky bluegrass) and other grasses. In spring-sown oat the disease causes losses in seed, blasting of florets and stunting (Gill, 1967).

Geographical Distribution

Manitoba, Canada, but may be more widely spread.

Host Range and Symptomatology

Infects many cultivars of oat (Avena sativa), other species of Avena and some wild and cultivated grasses including species of Poa, Bromus and Lolium. No dicotyledonous plants have been infected (Gill, 1967 and unpublished work). Readily sap-transmissible using inoculum made by grinding infected oat leaves in water.

Diagnostic species

Avena sativa (oat). Systemic infection (Fig.1) first shows as chlorotic lines on emerging leaves. This chlorosis develops into an irregular light and dark green mottle which in turn is followed by the appearance of necrotic lines and irregularly shaped necrotic areas on the mottled leaves. Leaf sheaths are also mottled and necrotic. Stunting is moderate. Mosaic symptoms are pronounced at 15-18°C. Necrotic symptoms appear sooner and are more severe with increase in temperature.

Other susceptible grasses are Bromus mollis, B. racemosus, B. secalinus, B. tectorum, Lolium multiflorum, L. temulentum, Poa annua, P. compressa, P. pratensis and P. trivialis.

Hordeum vulgare (barley), Triticum aestivum (wheat) and Secale cereale (rye) are immune.

Other immune grass species are Agropyron elongatum, A. intermedium, A. repens, Agrostis alba, A. palustris, A. tenuis, Bromus inermis, Dactylis glomerata, Elymus junceus, Festuca elatior, F. rubra, F. rubra var. commutata, Lolium perenne, Phalaris arundinacea, Phleum pratense, Zea mays.

Propagation species

Oat cv. Clintland-64.

Assay species

No local lesion host is known. Clintland-64 oat is used for systemic assays.

Strains

None have been characterized, though variation in symptom severity has been noted. The type strain is deposited in the American Type Culture Collection as No. PV107.

Transmission by Vectors

No vector has been found. Several species of aphids and two species of leafhoppers did not transmit the virus. There was no evidence of transmission through the soil (Gill, 1967). The distant serological relationship to wheat streak mosaic virus (Gill, 1976) suggests that oat necrotic mottle virus may be transmitted by a mite.

Transmission through Seed

None detected.

Serology

The virus is a good immunogen. Antiserum with a microprecipitin titre of 1/4096 was prepared by injecting a rabbit with a total of 4.9 mg of virus in three injections. In microprecipitin tests, virus could be detected in clarified sap from leaves of oats grown and inoculated in the greenhouse. Gel-diffusion tests were less satisfactory than microprecipitin tests and virus was only detectable in preparations by gel-diffusion with unabsorbed antiserum rather than with absorbed and reconcentrated antiserum, and when methods for disrupting the virus were used (Gill, 1976).

Relationships

Particles resemble those of agropyron mosaic, hordeum mosaic, ryegrass mosaic and wheat streak mosaic viruses. A distant serological relationship was found when the virus was tested against antiserum to wheat streak mosaic virus. The virus did not react with antisera to agropyron mosaic, hordeum mosaic, maize dwarf mosaic, ryegrass mosaic, or sugarcane mosaic viruses (Gill, 1976).

Stability in Sap

Thermal inactivation point in oat sap (10 min) is 50°C; dilution end-point is 10-3. Survives from 3 to 7 days in sap at 23°C and up to 6 or 8 weeks at 4°C. Infectivity was undiminished after 17 weeks at -15°C (Gill, 1967).

Purification

Virus is propagated in oat grown at 16 to 18°C. The following method gives consistently good results with source material grown in winter and spring (Gill, 1971; 1976). Harvest leaves with systemic symptoms 14 days after inoculation and store frozen for at least 2 weeks. Express sap in an electric juicer and homogenize the remaining fibre in the expressed sap with a Waring blender. Squeeze the homogenate through nylon mesh and centrifuge the sap at low speed. Clarify the supernatant fluid by slowly adding 0.01 M aqueous silver nitrate with stirring until flocculation occurs, and remove the precipitate by centrifuging at low speed. To improve recovery of infectivity add sodium citrate to 0.01 M before centrifuging for 2 h at 70,800 g. Resuspend the pellet in 0.1 M sodium citrate buffer, pH 6.3. Purify the virus further by centrifuging twice for 2.25 h at 92,600 g through a column of 5 ml of 30% sucrose in citrate buffer containing 4% polyethylene glycol (M.Wt 6000), then by rate-zonal centrifugation in a linear 10-40% sucrose density gradient in citrate buffer, and finally by exclusion chromatography in a 63 x 1.3 cm column of 4% agarose gel (Bio-Gel, 15 M-100 mesh; BioRad Labs., California). Yields average about 2.5 mg virus per kg of leaves.

Properties of Particles

A260/A280: 1.33; A259(max)/A247(min): 1.06; shoulder at A290 (uncorrected for light-scattering) (Gill, 1976).

Particle Structure

Particles are flexuous filaments (Fig.2). Normal length determined from clarified sap is 720 nm, and mean diameter is 11 nm. Negative stain is taken up for a short distance at the ends of the particles suggesting the presence of a central canal (Gill, 1971).

Particle Composition

No reports.

Relations with Cells and Tissues

Cytoplasm of mesophyll and phloem cells of oat and Canada bluegrass (Poa compressa) contained, in addition to filamentous virus-like particles, cylindrical inclusions (‘pinwheels’) (Fig.3, Fig.4) and aggregates of granular material. Three types of wall deposit were also seen, namely, localized deposits at plasmodesmata (Fig.5), extensive deposits (Fig.6) and fibrillar deposits (Fig.7). In oat leaves with mosaic symptoms, the inclusions and wall deposits were seen in all samples of tissue from the pale green areas. In dark green areas, the inclusions and wall deposits were not seen in samples from immature leaves, and only in some samples from mature leaves (Gill, 1974).

Notes

Infected oat plants in commercial fields occur mainly within 0.5-1 m of the borders, though infected plants may also be found scattered within the field. Samples of symptomless Poa compressa and P. canadensis, collected from alongside cereal crops in several areas of Manitoba, were all infected, suggesting that these species may constitute the main reservoir of the virus. The virus may be distinguished from wheat streak mosaic, hordeum mosaic, ryegrass mosaic and agropyron mosaic viruses by the fact that it infects oat but not wheat or barley.

References

  1. Gill, Phytopathology 57: 302, 1967.
  2. Gill, J. gen. Virol. 12: 259, 1971.
  3. Gill, Can. J. Bot. 52: 621, 1974.
  4. Gill, Phytopathology 66: 415, 1976.
  5. Gill & Westdal, Can. Pl. Dis. Surv. 46: 18, 1966.


Figure 1

Oat leaves; (left) showing the early chlorotic stage and (right) the late necrotic stage of the disease.

Figure 2

Virus particles from a purified preparation stained with potassium phosphotungstate. Bar represents 200 nm.

Figure 3

Electron micrograph of sections of cells from systemically infected oat leaves: cylindrical inclusions in cross-section. Bar represents 400 nm.

Figure 4

Electron micrograph of sections of cells from systemically infected oat leaves: cylindrical inclusions in longitudinal section. Bar represents 300 nm.

Figure 5

Electron micrograph of sections of cells from systemically infected oat leaves: localized wall deposits (arrows) at plasmodesmata. Bar represents 500 nm.

Figure 6

Electron micrograph of sections of cells from systemically infected oat leaves: part of an extensive deposit (arrow) covering the whole cell wall. Bar represents 500 nm.

Figure 7

Electron micrograph of sections of cells from systemically infected oat leaves: fibrillar wall deposit. Bar represents 500 nm.