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

Fungi
Powdery Scab liberating powdery masses of cystosori into the soil. Galls
superficially resemble those of wart, except that S.
Although powdery scab develops best under cool, moist endohioticumn does not attack roots.
conditions, it is found in practically every potato-producing
area in the world from latitudes 65" N to 530 S and at higher Causal
altitudes
Organism
in the tropics.
Spongospora subuerranea (Wallr.) lagerh. f. sp.suhierranlea
Tomlinson is a member
Symptoms
of the Plasmodiophorales. Cystosori
are ovoid, irregular,
Tuber
or elongate,
infection
19-85
in lenticels,
Mm in diam.ter,
wounds, and
and
(less frequently) in consist of an aggregate of closely associated resting spores
(cysts). Each spore
diameter,
is polyhedral,
extending
3.5-4.5
laterally
pm in
under
diameter,
the periderin
with
and forming a smooth, thin, yellow-brown
raised or
walls.
pimplelike
Primary
lesion.
and
Enlargement
secondary
and division of host zoospores are uninucleate,
cells force
ovoid
the periderm
to spherical,
to rupture,
2.5-4.6 Pim
resulting
in
in white, wartlike diameter, with two flagella of unequal length (e.g., 13.7
outgrowths
and 4.35
(Plate 22). p m).
Wound periderm forms beneath the lesion, which gradually Disease Cycle
darkens and decays, leaving a shallow depression filled with
powdery
a
mass
The
of
f yc
dark
e
brown spore balls (cystosori) (Fig. 38, The fungus survives in soil
Plate
in the
22).
form
The
ofcystosori
lesion is usually
made up
surrounded
of
hv the raised, torn resting spores. Stimulated by the presence of roots
edges
from
ofthe
suscep­
burst periderm. If, in very wet soil,'wound periderm tible plants, resting spores germinate
does
to produce
not
primary
develop, the lesion expands in depth and width, zoospores. These penetrate epidermal cells of roots and
forming
stolons
hollowed-out areas or very large warts. This is the
cankerous
or root hairs,
form
ultimately
of powdery
producing multinucleate
scab.
fungus masses
(sporangial plasmodia),
In
which
storage.
yield secondary
powdery scab
zoospores
may lead
that
warts or cankers. If infected
to a dry rot
tissue
or to more
has not
further
burst
spread
through
infection to
the
roots and tubers. Invasion by
pcrid errs. infeettion
secondary
ad necrosis
zoospores
may spread
stimulates
latera lly,
the
and
host
more
cells
numerous,
to become
and gallsare
larger
periern,ndinecton n.er.osisma
produced. Within
sprad
these
lterlly
galls,
producing one or two necrotic
balls
rings surrounding
of resting
the
spores
original
are ultimately formed (Fig. 39).
infection. Under humid conditions, after the periderm has
ruptured, warts may become somewhat larger and secondary
warts
Epidemiology
may develop beside the primary warts with little or no Inoculum is spread
necrosis
by
beneath
soil and
the
by
skin.
tuberborne resting spores.
Tuber and root
lPowderv
infection
scab lesions
is favored
may
by
serve
cool,
as infection
moist soil
courts for late conditions in the
blight
earlier
and
stages
a number
of infection
of wound
and
pathogens.
later by gradual
drying of the soil. Cysts may persist
Infection
in the soil
on roots
for up
and
to six
stolons
years.
parallels that on tubers, with The
small
time
necrotic
from tuber
spots
and
developing
root infection
into milky
to gall
white
formation
galls varying
;s
in less than three weeks at a
diameter
temperature
from
of
I to 10
16-20'
mm
C.
or
Powdery
more. Galls on roots may become scab occurs
so
in field
severe
soils
that
ranging
young
from
plants wilt
pH 4.7
and
to
die.
7.6.
As galls mature, they Fertilization experiments
turn dark
with
brown
N, P, K,
(Plate
ammonium
23) and
sulfate,
gradually break down, calcium nitrate, and minor elements have shown generally that
nutrition of the soil has little or no effect on incidence of
powdery scab. However, sulphur added to the soil can decrease
the intensity of scabbing.
incorporated Recent studies indicate
intp
that zinc
soil
oxide
reduces the amount of scab.
-The effect of liming is not clear. In some areas, liming of the
soil has resulted in an increase of powdery scab, whereas in
others, liming has decreased or had no effect on incidence of the
disease.
Spores survive passage through the digestive tracts of
animals.
S. subterranea is a vector for potato mop-top virus.
'"
-infect
root
7opofZZosyo
Zooreoo
([I)Resting
4.I/
hosmodim Zoosticcongi -,
porongium~m
4 Zygote
smoe Ro atRo
( Restng sOOwes
Root l RoofJqX~ infection /
"re T" infection
Fig. 38. Powdery scab: A, lesions breaking tuber periderm; B, 0 l Scab -"o" i cell
spore balls of Spongospora subterranea within lesion. (A, ..In scobs on tuber
Courtesy R. Salzmann and E. R. Keller; B, courtesy C. H.
Lawrence)
Fig. 39. Disease cycle of powdery scab caused by Spongospora
subterranea. (Courtesy G. N.Agrios)
35