EPIDEMIOLOGY OF LETTUCE NECROTIC YELLOWS VIRUS
IN SOUTH AUSTRALIA
11." DISTRIBUTION OF VIRUS, HOST PLANTS, AND VECTORS
By J. W. RANDLES?
and MARYCARVER?
[Manuscript received September 30, 19701
Abstract
A survey has shown that both lettuce necrotic yellows virus (LNYV) and its
vector Hyperomyzus lactucae are commonly associated with the widely distributed
host plant, Sonchus oleraceus. Three newly discovered naturally infected host plant
species, Reichardia tingitana, Sonchus hydrophilus, and Embergeria megalocarpa, have a
restricted distribution, yet their ability to support colonies of the vector shows that they
are potential sources of LNYV. Both S . hydrophilus and E. megalocarpa appear to be
endemic to Australasia, and the possibility that either could be the original source of
LNYV is discussed.
Serological relationships have been demonstrated between some isolates of
LNYV. A simple and rapid method for concentrating LNYV from sap extracts for
serological testing is described.
Hyperomyzus carduellinus has been shown to transmit LNYV, and the distinguishing morphological characteristics of this species are described. Its geographical distri
bution is limited, and it was found most commonly on R. tingitana.
Demonstrations that the incidence of lettuce necrotic yellows virus (LNYV) in
lettuce crops may be reduced by eradicating the weed host Sonchus oleraceus L. within
and around crops (Stubbs, Guy, and Stubbs 1963), and that high incidence of the
disease is associated with increased flight activity of the aphid Hyperomyzus lactucae
(L.) (Randles and Crowley 1970) suggest that S. oleraceus is the major source of LNYV,
and H. lactucae the major vector in south-eastern Australia. But although both weed
host and vector have a world-wide distribution, LNYV has been identified only in
Australia (Stubbs and Grogan 1963) and New Zealand (Close, personal communication). This implies that LNYV may have originated from plant species endemic to
Australia and perhaps New Zealand. A survey was therefore carried out to examine
the natural distribution of LNYV-infected S. oleraceus in South Australia, and to
determine whether any native species allied to S. oleraceus are naturally infected with
the virus, and support populations of the vector.
* Part I, Aust.
J. agvic. Res., 1970, 21, 447.
7 Waite Agricultural Research Institute, University of Adelaide, Glen Osmond, S.A. 5064.
Aust. J. agric. Res., 1971, 22, 231-7
J. W. RANDLES AND MARY CARVER
LOCATION OF SITES SURVEYED
INFECTED WITH
Site
Ceduna
Venus Bay
Wilmington
Willochra Creek
Carrieton
Orrorroo
Booleroo Centre
Whyte Yarcowie
Crystal Brook
Burra
Merildin
Wallaroo
Balaklava
14. Semaphore
15. Grange
16. West Beach
17. Marino
18. Port Noarlunga
19. Moana
20. Maslin Beach
21. Aldinga Beach
22. Norlnanville
23. Gool\va
24. Meningie
25. Younghusband Peninsula
26. Salt Creek
27. Cantara
28. Coolatoo
29. Kingston, SE.
30. Cape Jaffa
31. Robe
32. Southend
33. Millicent
34. Carpenters Rocks
Mt. Gambier
Naracoorte
Keith
Coonalpyn
Parndana
* Number
(SEEALSO FIG. 1)
SHOWING THE PLANT SPECIES
AND THE INCIDENCE OF APHID VECTORS AT THESE SITES
Plant Species
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
35.
36.
37.
38.
39.
LNYV,
Number
Infected
Presence of
H. lacfucae
H. carduellinus
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Sonclzus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Soizchus olevaceus
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Reichardia tingifana
Reichardia tingifana
Reichardia fingitana
Cryptosreinnza calendula
Picris hieracioides
Senecio laufus
Sonchus oleraceus
Picris echioides
Einlgrgeria niegalocarpa
Sonchus oleraceus
Reichardia tingifana
Sonchus oleraceus
Sonchus oleraceus
Picvis echioides
Sonchus oleraceus
Embergeria megalocarpa
Embergeria megalocarpa
Sonchus oleraceus
Sonchus oleraceus
Sonchus hydropkilus
Embergeria megalocarpa
Sonchus hydrophilus
Sonchus hj~drophilus
Sonchus oleraceus
Embergeria nzegalocarpa
Sonchus oleraceus
Sonchus oleraceus
Sonchus hydrophilus
Embergeria megalocarpa
Sonchus hj'drophilus
Embergeria nzegalocarpa
S o i d a n oleraceus
Sonchus oleraceus
Embergeria megalocarpa
Sonchus oleraceus
Sonchus oleraceus
Sonchus oleraceus
Sonchus aleraceus
Sonchus oleraceus
of infected plants colonized/number of plants colonized. -Aphids
not observed.
LETTUCE NECROTIC YELLOWS VIRUS IN SOUTH AUSTRALIA. I1
233
11. DISTRIBUTION
OF NATURALLY
INFECTED
HOSTSOF LNYV
In the period May 1969 to May 1970 Sonchus oleraceus and related plant species
viere collected at 39 sites, in areas free of commercial horticulture. The oldest specimens at each site were selected to increase the probability that plants had grown through
a period of vector activity. Plants were indexed separately on Nicotiana glutinosa L.,
Datura stramonium L., and Gonzphrena globosa L. Aphids found on plants were
preserved and identified. S. oleraceus is widely distributed in South Australia, and infected plants were found in 19 of the 31 sites where specimens were collected (Fig. 1,
Fig. 1.-The distribution of infected and uninfected host plants of LNYV in South
Australia. Open symbols, no infection detected; solid symbols, infected plants detected.
Sites are listed by number and name in Table 1.
Table 1). The other three species found to be infected with LNYV (Fig. 1, Table 1)
are apparently confined to the coastal regions (Black 1957; Eichler 1965). The dune
thistle, Enzbergeria megalocavpa (Hook. f.) Boulos, was found between Semaphore
and Carpenters Rocks on the seaward dunes within c. 0 . 5 mile of the high water mark.
Infected plants were found in five of the eight sites where they were collected. Reichardia tingitatza (L.) Roth. was found only in the coastal area near Adelaide. Sonchus
hydr.oplziltts Boulos was found along the Younghusband Peninsula and south to
Southend. It is common on the flats behind [he dunes extending north-west of Kingston to Salt Creek. None of these species showed symptoms which could be associated
with LNYV infection. SeveraI other species collected from West Beach were not
infected (Table 1).
234
J. W. RANDLES AND MARY CARVER
On N. glutinosa, symptoms produced by isolates of LNYV were of the following
types :
Mild:
slight stunting of the plant accompanied by a faint mosaic and a
downward cupping of young systemically infected leaves.
Medium severity: marked systemic vein clearing, chlorosis, and leaf distortion.
Severe: chlorotic or necrotic lesions on inoculated leaves followed by marked
systemic vein clearing, curling, and stunting of the terminal leaves.
The symptoms produced on D. stranzonium were interveinal chlorosis, sometimes
w ~ leaf
h distortion. The severity of symptoms on this host uas not as a rule related
to their severity on N. glutinosa. A few isolates produced a faint mottle in G. globosa.
One isolate, from E. nzegalocarpa at Goolwa, was transmitted to and recovered
from Nicotiana tabaczm A faint chlorosis was associated with infection. LNYV-like
particles Rere found in sap of the infected tobacco plant by electron microscopy.
111. SEROLOGICAL
RELATIONSHIPS
BETWEEN ISOLATES
The concentration of virus antigen in sap extracted from LNYV-infected plants
is generally too low for the development of visible precipitin lines in double-diffusion
serological tests. A method was therefore devised to concentrate the antigen. Systemically infected leaves from N. glutinosa indicator plants were homogenized in one volume (w/v) of 0 . 2 Na,HPO,,
~
and two volumes (w/v) of Freon 113. After centrifugation
at 3000g for 15 min, the supernatant was mixed with polyethylene glycol (PEG 4000)
added to a final concentration of 8 %. The precipitate was collected after 10 mill by
centrifugation at IOOOg, and resuspended in water to c. one-tenth of the original supernatant volume. By serological assay in double-diffusion plates, and local lesion
infectivity assay on N. glutinosa, it was shown that the above procedure concentrated
LNYV.
Concentrated antiserum (McLean, Wolanski, and Francki 1971) to the SE3
isolate (Stubbs and Grogan 1963) of LNYV was used for investigations of serological relationships between isolates. All serological tests were carried out by the
double-diffusion technique (Crowle 1961) in 0.75 % agar containing 0 . 0 1 phosphate
~
buffer, p H 7.6, and 0.02 % sodium azide. Antigen was placed in wells 18 hr before the
antiserum wells were filled.
A positive reaction was observed in nearly all tests between antigen concentrated
from field isolates, and the LNYV antiserum. When concentrated homologous and
heterologous antigens were placed in adjacent wells, the lines of precipitation formed
with the antiserum were confluent (Fig. 2). Spur production in some instances indicated that antigenic differences occur between isolates.
IV. APHID SPECIESASSOCIATED
WITH HOST PLANTSON LNYV
Three species of aphid have been found on naturally infected plants described in
the previous section.
H. lactucae has been found in all areas surveyed and infests all the hosts of LNYV
listed in Table 1. Hyperomyzzrs carduellinus (Theob.) was found only in the coastal
LETTUCE NECROTIC YELLOWS VIRUS IN SOUTH AUSTRALIA. I1
235
region within 50 miles of Adelaide (Table I), where it was found most commonly on
R. tingitana but also on S. oleraceus and E. megalocarpa. Macrosiphum euphorbiae
(Thos.) was associated with R. tingitana, but not commonly.
I ig. 2.-Reactions in double
gel-ditfusiun tests betwen antiserum prepared to the S t 3 isolate ot' L Y Y V (centre \vell) and
homologous fS) and heterologouj ( 1-6) antigens.
The ability of H. carduellinus to transmit LNYV was tested to determine whether
it could have a role in the ecology of LNYV. In one experiment H. carduellinus
nymphs and adults found to be feeding on 12 infected R. tingitana plants at West
Beach and Semaphore were transferred to S. oleraceus seedlings. LNYV was transmitted to S. oleraceus by four of the 12 groups of aphids.
In another experiment, 12 apterous H. carduellinus were fed in leaf cages on two
LNYV-infected R. tingitana plants in the glasshouse. After 6 days, 27 nymphs and
three adults were each caged on a small S. oleraceus seedling for 7 days. The seedlings
were indexed after 22 days. Seven of the 30 test seedlings were infected. Transmission
occurred only when nymphs were used.
Fig. 3.-Typical
abdominal dorsum of alata vivipara of Hyperomyzus lactucae (L.) (left), and
Hyperomyzus carduellinus (Theob.) (right).
Hyperomyzus Iactucae (L.) and H. carduellinus (Theob.) are morphologically
very similar. The most reliable distinguishing characteristic appears to be the difference
in hair size. In both apterous and alate morphs, H. carduellinus has, in contrast to H.
lactucae, very short antenna1 and dorsal body hairs (Fig. 3). In addition, H. carduellinus