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

Selected References
KASSANIS, B. 1970. TFbacc ecrosis virus. No. 14 in: Dt'scriptions
of Plant Viruses. Common". Mycol. Inst.. Assoc. Appl. Biol.. Ke .
Surrey. England.
NOOPDAM. 1). 1957. Tahaksnecrosevirus in samenhang met een
opperslakkigc aantasting Nan aardanpelknollen. Tijdschr.
Planteickten 63:237-241.
(Prepared by I). Peters)
Deforrming Mosaic
Relatively little is known about this disease, although it is
economically important in Argentina.
Secondary symptoms appearing soon after plant emergence
are severe mosaic and irregular occurrence of' yellowish green
patterns, leaves are distorted, and leaf surfaces are ;ough, with
interN eiaial tissue extending above the veins. Later in the season,
severity of svmptonis lessen, and infection may be masked,
Tubers are symptomiess, internally and externally. Tuber
transmission of the virus is not consistent, and up to a third of
the tubers from infected plants may produce healthhy plants.
The disease isgra ft-t ran.,mitted to Solaniont demissum and S.
Itahtoen.se and then readily transmitted to potato. It has not yet
bc .i transmitted mechanically nor by aphids. The potato
cultivar Huinkul is rapidly infected in the field. with losses of
20' or more (Fig. 92). Others, Kennebec and Pontiac, usually
do not show%synipioios under similar exposure.
Early roguing of diseased plants in seed fields is
recommended.
Selected Reference
CAL.I)tFRONI. A. 1965. An unidentified virus of deforming mosaic
typc in potato variclies in Argentina. Am. I'otato.l. 42:257 (Abstr.).
(Prepared hy W. .1.Hooker)
Tomato Spotted Wilt Virus
The tomato spotted wilt virus (TSWV) occurs worldwide but
is more frequent in subtropical and temperate region,, of the
African, American, and Australian continents than in Europe
and Asia. Losses in potato can be extensive.
Symptoms
Primary symptoms are necrotic spotting of leaves, stem
necrosis, death of the top of one or more stems, and occasionally
death of the whole plant (Plate 76). More frequently, however,
only the upper parts die.
Local chlorotic or necrotic lesions may appear at points of
thrip feeding (Fig. 93A and I)). Pale green upper growth may
precede the onset of systemic necrotic spotting. Leaf lesions
appear as single necrotic rings around a central green area with
or without a central dot, as concentric necrotic ri tg, interspaced
with green tissues, or as solid necrotic spots with concentric
zonation soeewhat resembling Alternaria ,cal spot. Necrotic
lesion.i appear also in the petioles and veins and in the stem
(cortex and or pith). A:illarv buJs may sprout in necrotic
plants with chlorotic concentric ringspots.
Tubers produced by infected plants may appear normal or
they may be malformed, with cracks and internal rusty or dark
necrotic spots. Spots are visible when the tuber is cut or may be
visible through the skin. sometimes as concentric patterns (Fig.
93B and C).
Secondary syriptotns on shoots from infected tubers may
include necrosis, early death, varying degrees of stunting, or a
rosette type of growth with coarse, dark green leaves. Leaves
may show necrotic spotting or chlorotic concentric ringspots.
Many diseased plants survive with minimal yield of tubers,
which are generally small and malformed.
Causal Agent
TSWV consists of isometric particles 70-100 nm in diameter
(Fig. 93F). [hey contain ribonucleic acid and 19% lipids.
Particles occur singly or in clusters inside membrane-bound
structures that correspond to cisternae in the endoplasmic
reticulum or nuclear envelope. The virus membrane may be
partly of ccllular origin. Virus particles occur in leaf, stem, and
root tissues and are present in all types of leaf cells except
tracheids.
TSWV is unstable in plant extracts but mare stable with
buffers near pH 7containing a reducing system such as sodium
sulfite, thioglycollate, or cvsteine. Inactivation isat 500 C for 10
min; longevity in vitro is 2-5 hr: infectivity is lost at relatively
low dilutions. -ISWV is serologically active.
The virus occurs in nature as a series of strain complexes with
certain strains predominating in potato and thus differing, in
part, from those affecting other crops. Some strains tend to
become localized in potat.o leaf tissues when inoculated
individuall,' but may invade the plant systemically in the
presence of others that act in an auxiliary capacity.
Epidemiology
The spotted wilt virus is vectored by thrips: Thrips tabaci,
Frankliniella schulizei, ' fusca, and F. occidentalis. The first
two are involved in spread in potato. The virus is acquired only
bv immature stages of thrips and transmitted only by adults.
Therefore, symptoms of a new infection transmitted from other
potato plants by thrips may not appear for several wecks while
the vector is developing.
Transmission is unlikely through true seed of potato,
although some transmission through seed of other plants has
been reported.
Virus perpetuation in either normal or malformed tubers may
reach 30-40j but generally does not exceed 5C. Some tubers
from a diseased hill may be virus-free, and some buds from a
diseased tuber carry the virus whereas others do not.
The spotted wilt virus can be transmitted mechanically with
relative ease if infected leaves are triturated with a reducing
substance.
Mechanical transmission in potato apparently does not occur
Fig. 92. Deforming mosaic in cultivar Huinkul. (Courtesy A. V.
Calderoni)
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