Compendium of Potato Diseases - (PDF, 101 mb) - USAID

Virus diseases, although seldom lethal. reduce plant vigorand
yield potential of seed tubers. Identification is complicated by
wide symptom variation attributable to virus strain differences,
plant response in relation to maturity and duration of infection,
potato cultivar. and environmental influences. l)ifferentiation
of viruses may involve purification, electron microscopy,
comparison of physical properties, clectrophoresis. serology,
and possibly other techniques. Recent developments in serology
using the ELISA (eniyme-linked immunosorbent assay)
,echnique provide an extremely sensitive test for virus presence
and identity and have for the first time permitted serology with
certain viruses difficult to detect in potato.
General References
BODE. 0. 1958, Die Virosen der Kartoffel und des'Tahaks. Pages 1-30
in: M. Klinkowski, ed. i'lianliche Virologie. Vol. II. Akademie-
Verlag. Berlin. 393 pp.
CI.ARK. M. F., and A. N. ADAMS. 1977. Characteristics of the
microplate method of eniime-linked immunosorbent assay for the
detection of plant viruses. .1.Gen. Virol. 34:475-483.
de BOKX, J. A.. ed. 1972. Viruses of Potatoes and Seed-Potato
Production. Pudoc, Wageningen, The Netherlands. 233 pp.
FERNANDEZ VAI.IEI.A, M. V. 1969. Introducci6n a la
Fitopatologiii. Vol. I. Virus, 3rd ed. Pages 764-845. Inst. Nac. de
lecnol. Agropecuaria. Buenos Aires. 1,011 pp.
GIBBS. A.. and B. HARRISON. 1976. Plant Virology: The Principles.
.Joh n Wiley and S o ns, New Y o rk. 292 pp .
KI.INKOWSKI. M.. and 1H.KEGI.ER. 1962. Viruskrankheiten der
Kartoffel. pages 1025-1138 in: R. Schick and M. Klinkowski. eds.
Die Kartoffel, Vol. II.2.112 pp.
MATItEWVS, R. E. F. 1970. Plant Vir,',,gy. Academic Press, New
York. 778 pp.
SMITH. K. M. 1972. A texlbook of .< virus diseases, 3rd ed.
Longman, Iondon. 684 pp.
Potato Leafroll Virus
Potato leafroll, an aphid-transmitted disease is one of the
most serious in potato and is responsible for high yield losses
throughout the world wherever potatoes are grown. In
Colombia and the Andes, the potato leafroll virus (PLVR)
occurs in Solanum andigena potatoes: the disease has been
known as "enanismo amarillo" for many years.
Symptoms
Primary symptoms (Plate 55) follow transmission by aphids
Symptoms appear mainly in the young leaves, which usually
stand uprigh:, roll, and turn slightly pale. In some cultivars,
young leaves are pink to reddish starting at tile margins. Rolling
sometimes affects only the base of the leaflet rather than the
whole leaflet. These symptoms may spread later to the lower
leaves. Primary symptoms may be lacking with late season
infection, making diagnosis in seed stocks a major problem.
Secondary symptoms (Plate 56) become evident when an
infected tuber produces a plant. Lower leaflets are rolled and
higher leaves are slightly pale. l.eaves are stiff,dry, and leathery
and make a crisp, somewhat paperlike sound when touched.
Older leaves of some cultivars are i..ak and or severely necrotic,
especially at the margins. Plants are often noticeably stunted
and rigid. Symptoms are less pronounced in the top of a plant
with secondary infection than in one with primary irfection, and
secondary infection is more damaging to the plant. Severity of
symptoms depends on the isolate of the virus, the potato
cultivar, and the growing conditions.
In S. tuherosumssp. antdigena cultivars. secondary symptoms
(enanismo amarillo) tend to be somewhat different from those
in S. tuherosum ssp. tuherosutm, consisting of a marked upright
habit of growth. stunting, and a marginal and interveinal
68
Viruses
chlorosis of leaflets (Plate 57), especially of' dwarfed upper
leaves. Rolling of lower leaves is often lacking. Secondary
symptoms in hybrids of S. tuherosum ssp. andigena X S.
tuherosum ssp. tuherosum consist of the rolling of lower leaves
typical of S. tuherosuim ssp. tuherostni cultivars. In some
cultivars, however, distinct interveinal and marginal chlorosis
occurs as in S. tuherosumn ssp. antdigena cultivars.
Internal net necrosis (Figs. 28B and 73A), visible to the
unaided eye when the tuber is cut, is particularly marked in
certain American cultivars such as Russet Burbank, Green
Mountain, and Norgold Russet. This net necrosis is found in
tubers from plants with primary, secondary, or tertiary
infection. The color may vary from light translucent to dark. A
definite trend toward a less severe type of infection from
primary to tertiary has been found in tubers of Russet Burbank.
Causal Agent
PI.VR has icosahedral particles 24 nm in diameter(Figs. 73D
and E).
'ransmission is successful only by aphids and grafting;
artificial inoculation by mechanical means has never been
accomplished. The virus is transmitted by aphids in a persistent
(circulative) manner (i.e.. after an aphid has fed on an infected
plant it normally transmits the virus for the rest of its life).
Isolates have not been differentiated by serological, physical,
or chemical properties nor by vector specificity or transmission
e mical No per ti ano r no All tra ns i ssi on
efficiency. No differential plants are known. All strains give the
same type of symptoms. Isolates vary in severity of symptoms
on potato cultivars and on Ph'salisfloridana, but existence of
strains has not been well defined. Some workers refer to three
strains called severe, moderate, and mild; others refer to
four strains called No. 1,2, 3, and 4. A fifth isolate, differing in
symptom severity from No. I on P.floridana but not on other
hosts, has been isolated.
Histopathology
From the moment of symptom appearance, infections are
always accompanied by phloem necrosis, which consists of
thickening of the walls of the primary phloem cells in the stem
and petioles (Fig. 73B). Accumulation of callose is often
pronounced around the sieve plates in the phloem oftubersand
stems (Fig. 73C). Stiffness of leaves in primary and secondary
infections isaconsequence of starch accumulation in leaf cells.
Epidemiology
SThe virus istuberborne and isalso efficiently
persistent
transmitted
manner
in
by
a
aphids that colonize potatoes, My'zus
persicae being the most efficient. Infection efficiency increases
with the length of feeding. The virus is spread over long
distances by windborne winged aphids and over short distances
by nonwinged aphids moving from plant to plant. Aphid
transmission occurs from tuber t(. tuber during seed storage,
especially in tropical countries. Plants from infected tubers and
diseased volunteer potato plants also serve as virus sources.
Moderate temperatures and dry weather favor spread. Plants
become resistant to infection with age, and occasionally some
tubers of plants infected late in the season escape infection.
Other Hists
Several plant species, mostly in the Solanaceac, are known as
hosts.
P.floridana is asuitable indicator, test, and propagation host
that reacts with interveinal chlorosis, darkening of the veinal
areas, and a slight cupping at the first two or three true leaves.
Infections at a later stage cause plants to become somewhat
pale.
Datura siraionium is favored by some workers, especially in
the more tropical regions, as a test and propagation host in
which chlorosis develops. In Brazil, alternative hosts to PI.VR