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

infects certain comnie,-cial stocks, ith yield reductions
estimated to range tp to more than l5('.
Symptoms
FVX Ina\ be atent. without foliage s)nptois or apparent
effect on plant igor except when closely conpared tcPVX-frce
stocks, or it may s h % mild uotte {Ilate 61 ) to sc ere or rugose
iosaic (Fig. 77A),.sith (hkarling of the plant and reduced
leaflet site. 'ertain combinations ol PVX strain and host
genotype cause extensive top necrosis (Fig. 771B). shich may
kill part or all of' the plant and cIuse tuber necrosis. In
comhination \ ithpotto \ iruses A or Y. crinkling !,:gosity. or
necrosis may :c\elop.
Causal Agent
PVX particlc~s a-e Ilexuous filaments. 515 < 13 mil. with
helical (pitch, 3.4 ni) substructure (Fig. 781)). Single-stranded
ribonucleic acid, ith molecular %%eightof 2.1 X 10' , compries
6:, of the particle \seight. Ihermal inctikation is 68-76,
depending on tie s"ain: dilutioniend point is 16'--10':
longe\i.tv in \ttr i-sc\eral e .ks.
PVX is strongly imutnogenica ind s sulTficientl ,yho moge,-ous
to permit se\ eral serolotal methods to be used inidentification
of naturallv inlec:ed plants.
I'VX isolatcs ha.c been grouped into itrains by cross
absorption scrolog\ (four groups), tcoperatur' of inactivatiot
(three groups). and necrotic reaction within S. tzluro.oum spp.
tuhero.mum (11r grotips). Strains related by allty of, the ;e
groupings tna\ \iary considerably in the irtensity of, tlt
synptonis the cause.
Within potatto leaf cells. IPVX forms large ar orphou:i;
inclusions readily o.serva ble .vith light microscopy. These
inclusions. exaittined itt thin sectiot by electron microscopy,
contain \irUs particles itttersper,;ed between altertivting layers
0I curved or rolled laminate inclusion components.
Epidemiology
k)\ X is with few exceptions. transo, .ttcd by tubers in
susceptible cutltivars. lransmission through sap inoculation is
readilv aeccnip!isltid by centact of plant pars in tilefield due to
wind, atnimal.. or machinery: by root contact: by sprout to
sprout contact; by the cutting knife before planting; and by
biting insects (grasstoppers). Zoospores of' Sv'ipchvriwm
elohiofictm ate reported to transmit the virus. Anhids are not
knovtn to transr.i. PVX. nor is the virus known to be
transmitted through true seed.
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Symptoms inmost plants are enhanced by low temperatures
oi 16-20'C and are mild or may be masked at temperatures
above 28' C.
Culi /ars may be freed from PVX by meristem culture of
sprout tip:; grown at 32-36'( ".
Other Hosts
PVX isolates cause widely different symptom severity ranging
from rild to severe. Mild isolates may spontaneously become
severe or vice versa. PVX is chiefly systemic in tite Solanaceae,
usually produces local lesions in the Chenopodiaceae or
Amaranthaceae, and also infects certain Leguminusae.
In Nicoia:a Iahacuin (White Burley 01 Samsun types),
infection i,systemic, with mottles or rin, spots (Fig. 78B and C,
Plate 62). 'I he plant is useful for virus prolpagation.
laturastramonium and I). taiula show systernic infection,
latent to severe mottle, and some leaf necrosis.
Gomphrenaghhosa is a local lesion host (Fig. 78A) useful for
quantitz'tive infectivity assay.
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Resistance
Compreltensixe or extreme resistance, earlier called
"immunity,"is determined by a singl dominant gene(Rx). This
gene or one similar confers resistance to all strains of PVX
except one recently isolated in the Andes. It is present in
selections of Solanuim acaule and its derivatives, including the
zultivar Saphir, and incertain selections of S. tuherosum ssp.
atdfiget'a, which include C. P. C. 1673, tite cultivar Villaroela,
and its Irobable derivative S.41956.
Field immunity itt S. tutierosu.m ssp. tuoerosui.,determined
by dominant genes, is a strong necrotic, often top necrotic,
reaction to infection. The Nx:nb genetic type confers resistance
to Cockerham's (1954) PVX ,'roips I and 3,nx:Nb to groups I
and 2, -.nd Nx:Nb to groups 1-3. Genes conferring similar
rt'iistdnce are present in -ertain other tuber-bearing Solanum
sp}p.
Certain cultivars, although susceptible, apparently prevent
completely free PVX movement within the plant, permitting
certain plant parts to escape infection. By contrast, other
cultivars are so thoroughly invaded that obtaining virus-free
parts for propagation is difficuli.
A strait, of PVX (XI 1 i) recently isolated in the Andean
highlands differs markedly from previously descr;bed PVX
strains by infecting I) plants with the Rx gene (USDA S.41956,
Saco, Saphir, and resistant selections of S. acaule) and 2) G.
glohosa without symptoms in inoculated leaves. This strain does
not become systemic in G. glohosa.
Fig. 77. Potato virus X: A, tatent strain in branch at right a id severe rugose strain at left; B, top necrosis 'n fie!d-resistant Epicure following
graft inoculation with potato virus X. (A, Courtesy J. Munro)
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