Details of DPV and References

DPV NO: 147 October 1975

Family: Potyviridae

Genus: Potyvirus

Species: Iris severe mosaic virus | Acronym: ISMV

There is a more recent description of this virus: DPV 338

Bearded iris mosaic virus

O. W. Barnett Clemson University, Clemson, South Carolina, USA

A. A. Brunt Glasshouse Crops Research Institute, Littlehampton, Sussex, England

Contents

Introduction

Disease described by Brierley & McWhorter (1936) and virus partially characterized by Barnett, de Zoeten & Gaard (1971).

A virus with filamentous particles c. 12 x 750 nm, prevalent in many species of rhizomatous iris. It is transmissible to iris and one other iridaceous species by sap-inoculation, and by aphids after brief acquisition feeds. A typical but apparently distinct member of the potyvirus group. Distribution probably world-wide.

Main Diseases

Causes conspicuous chlorotic markings on the leaves and flower ‘breaking’ in intolerant cultivars of bearded (Eupogon and Oncocyclus groups), crested (Evansia group) and beardless (Apogon group) rhizomatous iris, but inconspicuous leaf chlorosis or even symptomless infection in other cultivars. Symptoms are usually more severe in plants grown at low temperatures (16°C), or in those also containing cucumber mosaic virus (Brunt, 1968; Brunt, 1975; Barnett, de Zoeten & Gaard, 1971).

Geographical Distribution

Reported from the USA (Brierley & McWhorter, 1936), Japan (Fukushi, 1932) and Europe (Moore, 1949). Distribution probably world-wide, but the identity of the virus concerned in early records is uncertain because, until 1971, mosaic diseases in bulbous and rhizomatous irises were generally assumed to be caused by the same virus (Barnett, de Zoeten & Gaard, 1971).

Host Range and Symptomatology

The virus is only known to occur naturally in rhizomatous irises. It is transmissible by aphids or by sap-inoculation to one of three other iridaceous species (Belamcanda chinensis although not Gladiolus or Freesia spp.), but it infects none of 30 species from 7 dicotyledonous families (Barnett, de Zoeten & Gaard, 1971; Brunt, 1975).

Diagnostic species

Belamcanda chinensis. Commonly-occurring isolates of the type strain induce a few spreading necrotic lesions in inoculated leaves after c. 14 days and severe chlorosis in the first few systemically-infected leaves (Fig. 5) but symptomless infection in leaves produced later; avirulent isolates induce inconspicuous or symptomless systemic infection, but virulent isolates cause conspicuous leaf chlorosis, severe stunting, leaf necrosis and death of infected plants (Brierley & Smith, 1948; Barnett, de Zoeten & Gaard, 1971).

Iris spp. Intolerant cultivars of rhizomatous iris produce conspicuous chlorotic mosaic on the leaves (Fig. 1), and short-lived flowers which are often ‘broken’ (Fig. 4) or have ‘tear-drop’ markings (Fig. 7); some tolerant cultivars remain almost symptomless, others have faintly chlorotic leaves but normal flowers. The leaves of I. tectorum (crested iris) often develop chlorotic or water-soaked ringspot and line patterns (Fig. 2).

Propagation species

Cultures are best maintained in Belamcanda chinensis which is also a good source of inoculum and of virus for purification. This species is especially convenient because it germinates and grows quickly, and seedlings are readily infected.

Assay species

Belamcanda chinensis: by recording the proportion of inoculated plants that become infected.

Strains

Three strains of bearded iris mosaic virus have been recognized by the severity of the symptoms they induce in Belamcanda chinensis (O. W. Barnett, unpublished information).

Type strain. In B. chinensis this strain induces a few local necrotic lesions followed initially by conspicuous systemic leaf chlorosis but leaves produced later are often symptomless. Common in bearded Eupogon irises (e.g. Iris pumila, I. ricardi).

Avirulent strain. In B. chinensis this strain induces distinct necrotic or chlorotic stripes and/or ringspots in inoculated leaves (Fig. 3) but almost symptomless infection in systemically-infected leaves. Isolated from I. susiana (bearded Oncocyclus iris) with difficulty.

Virulent strain. In B. chinensis, isolates of this strain from I. spuria (beardless iris) induce severe systemic leaf chlorosis (Fig. 6) and death. Symptoms in I. spuria and B. chinensis are indistinguishable from those previously attributed to ‘beardless iris mosaic virus’ (Brierley & Smith, 1948).

Transmission by Vectors

Transmitted efficiently by some races of the aphids Macrosiphum euphorbiae and Myzus persicae after acquisition feeds of 3-5 min (Brierley & Smith, 1948; Barnett, de Zoeten & Gaard, 1971; O. W. Barnett, unpublished data).

Transmission through Seed

Not seed-borne in Belamcanda chinensis (Barnett, de Zoeten & Gaard, 1971).

Transmission by Dodder

No report.

Serology

The virus is a good immunogen; antiserum collected 6 weeks after injecting a rabbit intra-muscularly with c. 0.45 mg of virus emulsified in Freund’s complete adjuvant reacted in micro-precipitin tests to a homologous titre of 1/2048.

Relationships

The virus has many properties in common with potyviruses, but is serologically unrelated to similar viruses commonly infecting iridaceous species (iris mild mosaic, iris severe mosaic, I. fulva mosaic, bean yellow mosaic, turnip mosaic, freesia mosaic) and to four other viruses of the same group (Barnett, de Zoeten & Gaard, 1971; Brunt, 1975).

Stability in Sap

In Belamcanda chinensis sap, the thermal inactivation point is 50°C, dilution end-point 10^-3, and infectivity is lost at 27°C after 24-36 h (Barnett, de Zoeten & Gaard, 1971).

Purification

Yields of c. 5 mg virus/kg of Belamcanda chinensis leaf tissue are obtainable by the following procedure (Barnett, de Zoeten & Gaard, 1971). Homogenize each 1 g infected leaves in 2 ml of extractant at pH 8.9 containing 0.1 M borate buffer, 0.01 M sodium ethylenediamine-tetraacetate and 0.02 M 2-mercaptoethanol. Remove coarse debris, shake the fluid with 0.5 vol chloroform and centrifuge for 10 min at 8000 g. Sediment virus from the aqueous phase by centrifugation for 90-120 min at 100,000 g, and allow pellets to resuspend for 18-24 h in 0.1 M borate buffer at pH 8.9 containing 0.01 M sodium ethylenediamine-tetraacetate. Adjust the solution to pH 4.8 with 0.05 M citric acid, collect pellets by centrifugation and shake gently for 5-6 h in 0.1 M borate buffer at pH 8.9. Purify the virus further by exclusion chromatography in 2% agarose beads and concentrate by ultrafiltration.

Similar or higher yields of mostly unaggregated virus are obtained as described by Huttinga (1973). Homogenize each 1 g infected leaves in 3 ml 0.1 M Tris-thioglycollate buffer (pH 8.9) and 1.6 ml of a 1:1 mixture of carbon tetrachloride and chloroform, subject the aqueous phase to one or two cycles of differential centrifugation (1.5 h at 44,000 g; 10 min at 5000 g), and further purify the virus by rate zonal sucrose density gradient centrifugation.

Properties of Particles

Sedimentation coefficient (s°_{20, w}): c. 150 S; some preparations may also contain faster sedimenting virus aggregates (A. A. Brunt, unpublished information).

A₂₆₀/A₂₈₀: 1.12; A_260(max)/A_248(min): 1.15 (both values after correction for light-scattering).

Particle Structure

Particles (Fig. 10) are slightly flexuous filaments mostly c. 750 x 12 nm. Particles mounted in uranyl formate often have a central canal and show the helical arrangement of the protein subunits.

Particle Composition

Nucleic acid: 5% of particle weight (estimated spectrophotometrically).

Protein: c. 95% of particle weight; one type of polypeptide of M. Wt 33,000 (A. A. Brunt, unpublished information).

Relations with Cells and Tissues

In ultrathin sections of infected iris leaves and petals, the virus particles are often scattered throughout the cytoplasm, but in Belamcanda chinensis cells they are also sometimes found in membrane-enclosed ovoid masses c. 8 µm in diameter (Fig. 9). Infected cells of Belamcanda chinensis leaves, but not iris petals, also contain inclusions (Fig. 8) seen in sections as pinwheels, bundles and laminated aggregates (Barnett, de Zoeten & Gaard, 1971).

Notes

In particle morphology, bearded iris mosaic virus is indistinguishable from iris mild mosaic and iris severe mosaic viruses, which commonly infect bulbous irises, and from Iris fulva mosaic virus, which is reported to infect some species of rhizomatous iris. The two viruses infecting bulbous irises are serologically distinct from bearded iris mosaic virus (Brunt, 1968; Brunt, 1973; Barnett, de Zoeten & Gaard, 1971). Similarly, Iris fulva mosaic virus (Brierley & Smith, 1948; Travis, 1957) is serologically distinct from bearded iris mosaic virus and differs also in infecting Amaranthus caudatus and Chenopodium quinoa, and having cytoplasmic inclusions which can be seen in leaf sections as scrolls and tubes as well as pinwheels, bundles and laminated aggregates (O. W. Barnett, unpublished information).

Recent tests suggest that ‘beardless iris mosaic virus’ (Brierley & Smith, 1948; Travis, 1957) is a virulent strain of bearded iris mosaic virus (O. W. Barnett, unpublished), but further tests are needed to establish whether the two are synonymous.

Rhizomatous irises infected with bearded iris mosaic virus in Britain commonly also contain cucumber mosaic virus (Brunt, 1968; Barnett, de Zoeten & Gaard, 1971) or, probably rarely, broad bean wilt virus (Bailiss, Brunt & Dale, 1975); tobacco ringspot virus also occurs occasionally in rhizomatous iris in the USA (Travis, 1957; Travis & Brierley, 1957) and Britain (Brunt, 1972). Unlike bearded iris mosaic virus, these three viruses have isometric particles and infect a wide range of plant species; moreover, all three are readily purified and identified by serological tests and by their physico-chemical properties.

Figures

References list for DPV: Bearded iris mosaic virus (147)

1. Bailiss, Brunt & Dale, Pl. Path. 24: 60, 1975.
2. Barnett, de Zoeten & Gaard, Phytopathology 61: 926, 1971.
3. Brierley & McWhorter, J. agric. Res. 53: 621, 1936.
4. Brierley & Smith, Phytopathology 38: 574, 1948.
5. Brunt, Ann. appl. Biol. 61: 187, 1968.
6. Brunt, Rep. Glasshouse Crops Res. Inst. 1971: 116, 1972.
7. Brunt, CMI/AAB Descriptions of Plant Viruses 116, 4 pp., 1973.
8. Brunt, Acta hort. 47: 45, 1975.
9. Fukushi, Trans. Sapporo Nat. Hist. Soc. 12: 130, 1932.
10. Huttinga, Neth. J. Pl. Path. 79: 125, 1973.
11. Moore, Bull. Minist. Agric. Fish Fd, Lond. 117: 140, 1949.
12. Travis, Phytopathology 47: 454, 1957.
13. Travis & Brierley, Pl. Dis. Reptr 41: 524, 1957.