The effect of inoculum density on uredospore germination and infection of corn by Puccinia polysora, the cause of Southern corn rust


Melching, J.S.

Phytopathology 71(7): 769

1981


Uredospores of Puccinia paysora Underw. were deposited by gravitational settling on the upper surface of 2 × 2-cm leaf pieces of field corn (Pioneer 3369A), which were immediately given a dark 16-hr dew period at 25 C and then were floated on 21/1 kinetic in sterile tap water at 27 C. At spore densities of 530, 3,800, 11,200, and 16,000/cm2, germination percentages on leaves were 51, 43, 7, and 4 (respectively) and pustules/cm2 were 23, 57, 82, and 65 (respectively). Single spore inoculations (one spore/leaf piece) resulted in 13-19 pustules/100 inoculations. Appressorium and substomatal vesicle formation, calculated as percentages of germinated spores, did not differ significantly at the various inoculum densities. These data indicate that inhibition of germination is the primary factor that reduces infection efficiency of inoculum at higher densities.

A
STRAIN
OF
TOMATO
ASPERMY
ISOLATED
FROM
TOMATO
IN
MARYLAND.
Joseph
Kuti
and
Harold
Moline,
Dept.
of
Botany,
Howard
Univer-
sity,
Washington,
D.C.
20059
and
Horticultural
Crops
Quality
Lab.,
HSI,
USDA,
SEA
-AR,
Beltsville,
MD
20705
A
mechanically
transmissible
agent
was
recovered
from
tomato
plants
(Lycopersicon
esculentum)
in
Prince
Georges
Co.
,
Mary-
land.
A
virus,
30
nm
in
diameter,
and
serologically
related
to
the
tomato
aspermy
virus
(TAV)
type
strain
was
isolated.
Spinacia
oleracea
('Bounty'
spinach)
is
a
suitable
propagative
host
yielding
1-5
mg
of
virus
per
100
g
leaf
tissue
12-14
days
after
inoculation.
The
virus
is
most
stable
in
phosphate
buffer
(0.1M)
0.17.6
amended
with
0.1%
thyoglycolate.
Chenopo-
dium
quinoa
is
a
reliable
assay
host
producing
necrotic
local
lesions
6-9
days
after
inoculation.
Its
longevity
in
vitro
is
between
30
and
48
hours,
thermal
inactivation
point
between
60-65°C
and
dilution
end
point
between
/0
4
and
10
3
in
Bounty
spinach.
The
virus
does
not
produce
enations
typical
of
TAV
or
fruit
aspermy
in
any
of
more
than
30
tomato
lines
tested
in
the
greenhouse
but
causes
dwarfing
and
leaf
mottling
of
inocu-
lated
tomato
seedlings.
ASSOCIATION
OF
ROOT
DISEASE
AND
INSECT
INFESTATIONS
WITH
EASTERN
WHITE
PINES
EXPRESSING
SENSITIVITY
TO
AIR
POLLUTION.
A.L.
Lackner
and
S.A.
Alexander.
Dept
of
Plant
Pathology
and
Physiology,
VPI
&
SO,
Blacksburg,
VA
24061.
A
survey
was
conducted
along
the
Blue
Ridge
Parkway
in
Virginia
to
determine
if
there
is
an
association
between
eastern
white
pines
expressing
symptoms
of
air
pollution
a.nsitivity
and
the
presence
of
root
disease
and
insect
infestations.
The
25
sen-
sitive
trees
sampled
possessed
only
current-
year
needles
which
were
short
and
ch/orotic
to
brown.
Two
lateral
roots
were
ex-
cavated
from
each
tree
and
brought
to
the
laboratory
for
Fungal
isolation.
Two
pathogenic
fungi,
Verticicladiella
procera
and
Heterobasidion
annosum,
were
isolated
from
28%
of
the
trees
and
two
blue-
stain
fungi,
V.
serpens
and
a
Graphium
sp.,
were
iso-
lated
from
12%
of
the
trees.
Infected
roots
exhibited
either
resin-
soaking,
dark
staining
or
a
stringy,
rotted
appearance.
Two
species
of
weevils,
Pissoides
approximates
and
a
Hylobius
sp.
,
were
Found
in
20%
of
the
trees
sampled.
FUNGICIDE
TESTING
IN
RELATION
TO
DISEASE
PROGRESS
OF
APPLE
POWDERY
MILDEW.
Lalancette,
N.
and
N.D.
Hickey,
The
Pennsylvania
State
University
Fruit
Research
Lab,
Biglerville,
Pennsylvania
17307
Bitertanol(Baycor
50W)
was
applied
to
semi
-dwarf
Rome
Beauty,
Stayman
Winesap,
and
Delicious
trees
at
varying
rates
and
spray
intervals.
The
proportion
of
leaves
per
terminal
infected
by
Podosphaera
leucotricha
was
determined
seven
times
during
1980.
The
number
of
treatment
means
that
were
different
at
a
given
time
varied
considerably
during
the
8
weeks
that
the
seven
assessments
were
made.
On
Rome
the
number
of
significant
differences
were
13,14,12,24,
31,
19,
and
L4
for
counts
I
through
7,
respectively.
The
number
of
differences
on
Stayman
and
Delicious
varied
from
15
to
28(2nd
and
1st
counts)
and
5
to
25
(7th
and
1st
counts),
respectively.
These
data
indicate
that
results
of
single
time
assessments
may
be
time
dependent.
SCREENING
SOYBEANS
FOR
RESISTANCE
TO
FUSARIUM
OXYSPORUM.
S.
Leath
and
R.
B.
Carroll,
Univ.
of
DTV
vare,
Newark,
DE
19711
Fusarium
blight
of
soybean
has
increased
in
the
Delmarva
area.
Tests
were
initiated
in
the
field,
greenhouse
and
growth
chamber
to
screen
varieties
for
resistance
to
Fusarium
oxysporum
and
to
develop
a
rapid,
reliable
screening
method.
Varieties
included
Bedford,
Emerald,
Essex,
Forrest,
Miles,
Union,
Verde,
Ware,
Williams
and
York.
Field
trials
indicate
useful
levels
of
re-
sistance
occurs
in
Miles,
Ware,
Williams
and
York.
Miles
and
York
also
had
the
highest
degree
of
resistance
in
greenhouse
tests.
Resistant
varieties
had
the
highest
yield
in
paired
field
trials.
Rapid
screening
was
accomplished
by
growing
soy-
beans
in
test
tubes
in
a
29
C
growth
chamber.
Seedlings
were
wound
-inoculated
when
6
da
old
and
rated
for
disease
10
da
later.
Results,
compared
to
greenhouse
and
field
tests
at
2
locations,
indicate
this
method
can
be
used
to
predict
resist-
ance
under
field
conditions.
Resistant
seedlings
thus
identi-
fied
can
be
transplanted
to
the
greenhouse
for
use
in
a
breeding
program.
THE
EFFECT
OF
INOCULUM
DENSITY
ON
UREDOSPORE
GERMINATION
AND
INFECTION
OF
CORN
BY
PUCCINIA
POLYSORA,
TEE
CAUSE
OF
SOUTHERN
CORN
RUST.
J.
S.
Melching,
Plant
Disease
Research
Laboratory,
AR,
SEA,
USDA,
P.O.
Box
1209,
Frederick,
MO
21701.
Uredospores
of
Puccinia
paysora
Underw.
were
deposited
by
grav-
itational
settling
on
the
upper
surface
of
2
X
2
-cm
leaf
pieces
of
field
corn
(Pioneer
3369A),
which
were
immediately
given
a
dark
16
-hr
dew
period
at
25
C
and
then
were
floated
on
21
/1
kinetic
in
sterile
tap
water
at
27
C.
At
spore
densities
of
530,
3,800,
11,200,
and
16,000/cm
2
,
germination
percentages
on
leaves
were
51,
43,
7,
and
4
(respectively)
and
pustuIes/cm
2
were
23,
57,
82,
and
65
(respectively).
Single
spore
inoculations
(one
spore/leaf
piece)
resulted
in
13-19
pustules/100
inoculations.
Appressorium
and
substomatal
vesicle
formation,
calculated
as
percentages
of
germinated
spores,
did
not
differ
significantly
at
the
various
inoculum
densities.
These
data
indicate
that
inhibition
of
germination
is
the
primary
factor
that
reduces
infection
efficiency
of
inoculum
at
higher
densities.
THE
USE
OF
CYCLOHEXIMIDE
TO
DIFFERENTIATE
ENDOTHIA
PARASITICA
FROM
E.
GYROSA.
J.A.
Micales,
and
R.J.
Stipes,
Dept.
Plant
Pathol.
&
Physiol.,
Va.
Tech,
Blacksburg,
VA
24061
Selective
sensitivity
to
cycloheximide
(Acti-dione)
was
used to
differentiate
Endothia
parasitica
from
E.
Ayrosa.
Radial
growth
of
ten
specimen-vouchered
isolates
of
each
taxon
was
assayed
on
cycloheximide-amended
glucose
(0.5%)
-yeast
extract
(0.1%)
agar
(1.5%).
At
1
Ug/m1
cycloheximide,
the
growth
of
E.
parasitica
was
slightly
depressed
(6-25%),
while
that
of
E.
gyrosa
was
greatly
inhibited
(80
-95%).
Endothia
gyrosa
did
not
grow
at
concentrations
above
2
pg/m/,
while
E.
parasitica
showed
traces
of
growth
at
50
ug/ml.
A
similar
response
was
observed
on
cycloheximide-amended
potato
-dextrose
agar.
Data
from
studies
using
cycloheximide-amended
glucose
-yeast
extract
broth
culture,
inoculated
with
spores
of
the
two
species,
indicated
a
comparable
trend.
USE
OF
TWO-DIMENSIONAL
POLYACRYLAMIDE
GEL
ELECTROPHORESIS
FOR
THE
IDENTIFICATION
OF
PATHOGENIC
SOFT
ROTTING
BACTERIA.
H.E.
Moline,
K.S.
Johnson,
and
J.D.
Anderson,
Horticultural
Science
Institute,
USDA,
SEA
-AR,
Beltsville,
MD
20705
The
O'Farrell
two-dimensional
electrophoretic
separation
of
proteins
is
a
potential
tool
for
identifying
bacteria
iso-
lated
from
diseased
plant
tissues.
Three
strains
of
Erwinia
carotovora,
2
of
E.
atroseptica,
2
of
E.
chrysanthemi
and
1
of
Pseudomonas
fluorescens
were
used
in
the
study.
A
profile,
which
contains
at
least
100
silver
stained
proteins,
may
be
equivalent
to
50
or
more
conventional
biochemical
tests
used
for
identifying
these
bacteria.
However,
these
soluble
pro-
tein
profiles
are
very
complex,
whereas
ribosome/
protein
profiles,
containing
5-10
major
proteins,
are
less
complex
and
are
easier
to
analyse.
Either
the
soluble
or
the
ribsomr
al
protein
profiles
could
be
used
to
distinguish
the
four
species.
Differences
could
also
be
detected
between
some
strains.
Most
significant
differences
in
the
protein
profile
of
Erwinia
app,
occurred
in
the
20-50
Kd
molecular
weight
range
acidic
proteins.
MICROFLORA
ASSOCIATED
WITH
BACTERIAL
WETWOOD
IN
AMERICAN
ELM.
C.W.
Murdoch
and
R.J.
Campana,
Dept.
of
Botany
and
Plant
Path-
ology,
University
of
Maine,
Orono,
ME
04469.
Enterobacter
cloacae
(=Erwinia
nimipressuralis),
considered
the
single
causal
agent
of
bacterial
wetwood
of
elm,
has
been
asso-
ciated
with
other
microflora
in
elm.
To
elucidate
the
etiology
of
wetwood
in
elm,
an
attempt
was
made
to
isolate
all
microflora
possible
from
capillary
liquid,
wetwood
tissues
and
unaffected
sapwood,
involving
1300
wood
samples
from
62
elms.
Enrichment
broth
cultures,
incubated
under
aerobic
or
anaerobic
conditions
were
followed
by
repeated
streaking
on
agar
plates
to
obtain
pure
cultures.
Cultures
were
identified
by
standard
bacterio-
logical
methods.
A
total
of
16
species
of
microflora
were
obtained;
14
species
of
bacteria
and
2
of
yeasts.
Based
on
frequency,
Enterobacter
and
Klebsiella
species
were
most
often
isolated
from
wetwood
tissues.
L.
cloacae
was
always
a
compon-
ent
of
mixed
bacterial
populations,
including
two
other
species
of
Enterbacter.
The
data
suggest
that
wetwood
may
be
attribu-
ted
to
;nixed
populations
of
bacteria,
rather
than
to
one
species.
Vol.
71,
No.
7,
1981
769