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

may form.
When the causal agents survive storage in seed tubers. plants
in the second year develop symptoms. In the more usual
situation, in which causal agents failto survive in seed tubers.
plants in the second year may he normal, lack diagnostic
symptoms, be smaller than normal, or failto emerge. Hair
sprouts frequently develop on tubers from infected plants (Plate
79). Plants with stolbur may bear simplified "round" leaves, but
this symptom disappears in subsequent years.
Causal Organisms
Mycoplasmalike organisms (MLO) occur in phloem sieve
cells and occasionally in phloem parenchyma cells of infected
plants (Fig. 97C). They ,are pleomorphic, lack a cell wall. and are
bounded by aunit membrane. M I.O vary in diameter from 50 to
1,000 nm. The larger, more prevalent forms are roughly
spherical and contain a central fibrillar network of strands,
presumably DNA. and a peripheral area of ribosomelike
granules. The presence of elongate forms and small, dense
"elementary bodies" suggests propagation by binary fission.
budding, or fragmentation.
Epidemiology
The principal leatfhopper vector ol aster yellows. .iacrosteh's
fascifrons,overwinters on weeds, grasses, and small grains. The
favorite host of tile main vector of stolbur, Iiraesthe.s
obsohtus, is perennial hindweed, (on volvulu arven.sis, which
simultaneously acts as the main inoculum reservoir plant.
All pathogens are transmitted by and propagative in
lea fh oppers. Potato is not a fhavored host and usualIv escapes
extensive disease incidence. Stolbur is an exception that, in
certain European regions, mav become ,cry destructive.
Leafhopper vectors do not complete their life cycles on potato.
Neither nymphs nor adults can acquire the pathogen from
potato. Transmission to potato occurs only when the leafhopper
has fed on some other infected host. two to three weeks before
feeding on potato.
Other Hosts
Many vegetable. ornamental, field crop plants, and weeds tire
among the 350 species froni at least 54 families susceptible to
aster yellows.
Witches' Broom
Witches' broom. also called northern stolbur and dwarf shrub
virosis, has been reported from North America, Europe, and
Asia but is of minor economic importance. Several distinct
pathogens or strains of a single pathogen may be involved in
different geographical areas.
Symptoms
Symptoms are generally similar. Plants have many axillary
and basal branches with simple "rotnd" leaves, which may he
chlorotic (Fig. 98A). This disease is distinct from wilding, a
somatic aberration. Current symptoms appear late. if at all. and
consist of an upright habit of growth. rolling of leaflets, and
some chlorotic or anthocyanin discoloration in apical leaves.
Most strains cause no flower symptoms. but witches' broom
strains of central and eastern Europe are characterized by
virescence, phyllody, and associated flower symptoms (Fig.
98B).
Mycoplasmas offwitches' broom are tuber-perpetuated, in
contrast to those of aster yellows, which usually are not.
Affected hills usually produce tubers that appear normal during
the year of infection hut gIive rise to plants with advanced
symptoms the subsequent 'ear. Tubers from infected plants
frequently produce hair sprouts. Such plants have many small
tubers and an abnormally high number of sprouts with
simplified, small leaves. Occasionally these sympt(ms follow
92
early infections during the current season. Infected tubers
usually lack dormancy.
Causal Organism
Pathogens are mycoplasmalike organisms, such as those that
cause aster yellows.
Epidemiology
The pathogens are transmitted by leaf hopper vectors,
Ophiola (Scleroracus)lavopitu.s, S. dasidus (Fig. 98C),and S.
halli, in which they are propagative. In most areas, the vectors
are not yet known, nor are the natural host ranges of the
pathogens.
lcafhuoppers are unable to acquire the pathogen from potato.
Spread to potato occurs when a vector feeds on oth-r infected
hosts several days before feeding on potato. Witches' broom in
perennial legumes isusually more serious than in potato because
the former are preferred hosts by the vector and inoculum is
usuall, available within the crop.
Other Hosts
L.r,'oliervicon escuhe'ttm, C, 'vphomandra betacea, and
Nicotiana tahactum have been used to distinguish between
strains.
ifedicago sativa, Jelilotus alba, or l.otus corniculatusserve
as natural reservoirs and may be used as indicator plants.
Trifolium r 'ens isalso susceptible.
Control
I) Generally these diseases are of minor importance and do
not justify elaborate control measures.
2) Control leafhopper migrations into potato fields by
,, ..
C
Z
Fig. 98. Witches' broom: A, in potato plant; B, current season,
medium intensity flour phyllody of Czechoslovak type II; C,
Scleroracus dasidus female, one of the leafhopper vectors. (A,
Courtesy N. S. Wright; B, courtesy V. Valenta: C, courtesy J.
Raine)
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