Oat necrotic mottle virus
C. C. Gill
Agriculture Canada Research Station, 195 Dafoe Road, Winnipeg, Manitoba, Canada
Contents
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Gill, 1967 and unpublished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
- Gill, Phytopathology 57: 302, 1967.
- Gill, J. gen. Virol. 12: 259, 1971.
- Gill, Can. J. Bot. 52: 621, 1974.
- Gill, Phytopathology 66: 415, 1976.
- Gill & Westdal, Can. Pl. Dis. Surv. 46: 18, 1966.
Oat leaves; (left) showing the early chlorotic stage and
(right) the late necrotic stage of the disease.
Virus particles from a purified preparation stained with potassium
phosphotungstate. Bar represents 200 nm.
Electron micrograph of sections of cells from systemically infected
oat leaves: cylindrical inclusions in cross-section. Bar represents 400 nm.
Electron micrograph of sections of cells from systemically infected
oat leaves: cylindrical inclusions in longitudinal section. Bar represents 300 nm.
Electron micrograph of sections of cells from systemically infected
oat leaves: localized wall deposits (arrows) at plasmodesmata. Bar represents 500 nm.
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.
Electron micrograph of sections of cells from systemically infected
oat leaves: fibrillar wall deposit. Bar represents 500 nm.
