Studies on Puccinia polysora Underw. II. Consideration of the method of introduction of Puccinia polysora into Africa


Cammack, R.H.

Transactions of the British Mycological Society 42(1): 27-32

1959


Examination of herbarium specimens of world collections of maize rust has shown that, before 1949, Puccinia polysora, the American corn rust, was confined to the Caribbean area. The appearance of this rust in West Africa in 1949 raised the question of the method of introduction, the only source of infection at that time being over 3000 miles distant and separated by the Atlantic Ocean. A consideration of available evidence indicates that the pathogen was introduced as viable uredospores carried by air transport. The subsequent spread and severity of the epiphytotic emphasize the importance of this introduction in relation to plant quarantine.

[
27
1
Trans.
Brit.
mycol.
Soc.
42
(I),
27-
3
2
(1959).
STUDIES
ON
PUCCINIA
POLYSORA
UNDERW.
II.
A
CONSIDERATION
OF
THE
METHOD
OF
INTRODUCTION
OF
P.
POLYSORA
INTO
AFRICA
BY
R.
H.
CAMMACK
West
African
Maize
Research
Unit,
Moor
Plantation,
Ibadan,
Nigeria
Examination
of
herbarium
specimens
of
world
collections
of
maize
rust
has
shown
that,
before
1949,
Puccinia
polysora,
the
American
corn
rust,
was
confined
to
the
Caribbean
area.
The
appearance
of
this
rust
in
West
Africa
in
1949
raised
the
question
of
the
method
of
introduction,
the
only
source
of
infection
at
that
time
being
over
3000
miles
distant
and
separated
by
the
Atlantic
Ocean.
A
consideration
of
available
evidence
indicates
that
the
pathogen
was
introduced
as
viable
uredospores
carried
by
air
transport.
The
subsequent
spread
and
severity
of
the
epiphytotic
emphasize
the
importance
of
this
intro-
duction
in
relation
to
plant
quarantine.
INTRODUCTION
Whenever
a
new
epiphytotic
breaks
out
the
following
questions
arise.
First,
is
it
in
fact
caused
by
a
new
pathogen
or
by
one
which
has
been
present
previously
and
overlooked,
and
secondly,
if
it
is
a
disease
new
to
the
affected
area,
how
was
it
introduced?
Several
workers
have
suggested
that
P.
polysora
was
not
introduced
into
West
Africa
in
1949,
but
was,
in
fact,
overlooked
and
at
that
time
suddenly
assumed
a
more
virulent
form.
P.
polysora
bears
strong
superficial
resem-
blance
to
a
second
rust
of
Zea
mays,
P.
sorghi
(Cammack,
1955),
and
before
1949
this
latter
rust
had
been
reported
in
twelve
territories
in
the
African
Continent.
After
Cummins
(1941)
had
identified
P.
polysora
on
Z.
mays
in
America,
the
collections
of
maize
rusts
at
the
Commonwealth
Mycological
Institute
were
re-examined.
These
included
all
available
specimens
from
Africa
and
in
1944
it
was
established
that
only
P.
sorghi
was
present
in
the
African
Continent.
Immediately
after
the
severe
outbreak
of
rust
in
Sierra
Leone
in
1949
specimens
collected
in
the
Colony
were
identified
as
P.
polysora
(Deighton,
1951).
Specimens
collected
the
same
year
in
Ghana
were,
without
exception,
P.
sorghi,
and
P.
polysora
did
not
appear
in
that
territory
until
the
following
year,
195o.
All
available
evidence
based
on
herbarium
collections
and
fi
eld
observations
points
to
the
introduction
of
the
rust
in
or
about
1949,
and
none
supports
the
theory
that
the
fungus
was
present
previously
as
a
weak
parasite
of
maize.
Some
possible
methods
of
entry
of
the
rust
into
the
African
Continent
are
discussed.
INTRODUCTION
BY
INOCULUM
CARRIED
IN
WIND
CURRENTS
Wind
is
the
principal
means
of
dissemination
of
rusts
both
locally
and
over
great
distances.
Schmidt
(1925)
estimated
the
absolute
distance
a
uredo-
spore
could
be
carried
in
air
currents
to
be
I
loo
km.
This
calculation
was
28
Transactions
British
Mycological
Society
based
on
physical
reasoning
and
had
no
supporting
experimental
work.
A
better
theoretical
approach
with
a
considerable
amount
of
experimental
evidence
is
that
of
Sutton
(1932)
and
this
is
fully
discussed
by
Gregory
(1945).
Stepanov
(1935)
calculated
the
absolute
limit
of
dissemination
of
uredospores
of
P.
triticina
as
1282
km.
Naumov
(1939)
calculated
the
theoretical
limit
of
P.
triticina
to
be
around
1200
km.
Moulton
(1942)
has
illustrated
the
large
overland
distances
travelled
by
rust
spores
in
his
study
of
black
stem
rust
of
wheat,
which
is
carried
northwards
each
year
from
Texas
to
Canada.
On
release
from
the
source
uredospores
may
be
raised
high
in
the
atmosphere
by
thermal
convection
and
frictional
turbulence.
Stakman,
Henry,
Curran
&
Christopher
(1923)
trapped
wheat
leaf
rust
uredospores
at
a
height
of
16,00o
-16,50o
ft.
Height
attained
is
of
importance
in
the
distance
travelled
from
the
source
by
uredospores
in
wind
currents.
Ukkelberg
(1933)
has
shown
theoretically
that
a
wheat
leaf
rust
uredo-
spore
at
5000
ft.
in
a
3o
m.p.h.
wind
could
travel
a
horizontal
distance
of
000
miles.
Little
is
known
of
the
dispersal
of
fungus
spores
over
large
areas
of
water.
Bisby
(1935)
and
McCubbin
(1944)
have
found
spores
present
in
small
quantities
over
mid
-Atlantic.
Pady
&
Kapica
(1955)
exposed
nutrient
agar
plates
and
silicone
-coated
slides
at
heights
of
8000
and
9000
ft.
respectively
on
two
trans
-Atlantic
fl
ights
and
found
uredospores
to
be
comparatively
rare,
occurring
as
single
uredospores
on
four
slides
exposed
over
eastern
Canada
and
one
on
a
slide
exposed
in
a
tropical
air
mass
over
Iceland.
It
is
known
that
prior
to
1949
P.
polysora
was
confined
to
Central
America
and
the
West
Indies.
On
the
appearance
of
the
rust
in
West
Africa
that
year
the
source
of
infection
was
3500
miles
distant
and
separated
by
the
Atlantic
Ocean.
INTRODUCTION
BY
SEED
-BORNE
INOCULUM
The
possibility
of
seed
-borne
infection,
with
special
reference
to
wheat
rust,
received
much
attention
from
early
workers
and
their
theories
have
been
reviewed
by
Levine
(1919).
Chester
(1946)
states
that
in
severely
rusted
wheat
crops
the
grain
has
been
observed
to
be
orange
-tinted
by
the
heavy
covering
of
uredospores.
Naumov
(1939)
maintains
that
it
is
un-
likely
that
uredospores
carried
on
grains
would
be
capable
of
initiating
infection
of
the
young
seedling
since
the
exposed
tissues
(coleoptiles,
roots,
etc.)
are
not
susceptible
and,
by
the
time
parts
of
the
seedling
prone
to
infection
become
available,
the
spores
have
lost
their
viability
or
germinated
in
the
soil.
Weber
(1922)
germinated
maize
and
allowed
the
seedlings
to
grow
until
the
radicle
and
coleoptile
were
approximately
o•5
in.
long
and
then
immersed
them
in
an
aqueous
suspension
of
uredo-
spores
of
P.
sorghi
for
5
min.
The
seedlings
were
then
planted
2
in.
deep
and
4
weeks
later
mature
uredosori
were
found
1.5
in.
below
the
surface
of
the
soil.
The
mechanical
effect
of
rain
splash
could
cause
these
sori
to
become
exposed
and
afford
a
source
of
inoculum.
There
have
been
no
reports
of
possible
seed
-borne
transmission
of
P.
polysora,
but
Rhind
(1952)
Puccinia
polysora.
II.
R.
H.
Cammack
29
has
suggested
the
possibility
of
uredospores
becoming
lodged
in
the
rudimentary
glumes.
Several
experiments
were
designed
to
investigate
the
possibility
of
seed
-
borne
infection.
These
were
carried
out
in
a
greenhouse
at
Ibadan
using
the
rust
susceptible
cultivar
of
maize,
Lagos
White.
Since
all
experiments
gave
negative
results,
only
a
brief
description
of
the
methods
is
given.
I.
The
immersion
of
maize
grains
in
a
uredospore
suspension
(a)
Maize
grains
were
washed
in
running
water
for
15
min.
and
then
immersed
in
a
thick
suspension
of
uredospores
in
distilled
water
for
5,
15
and
6o
min.,
and
12
hr.
respectively.
The
batches
of
grain
were
removed
from
the
suspension
with
forceps
and
planted
i
in.
deep
in
sterilized
sand
culture.
(b)
Previously
washed
grains
were
soaked
in
sterile
water
for
24
hr.
and
then
immersed
in
a
uredospore
suspension
for
the
same
range
of
times
as
in
Ia,
before
being
planted
in
sand
culture.
II.
The
inoculation
of
the
surface
of
maize
grains
(a)
Dry
grains
were
rapidly
washed
with
7o%
alcohol,
rinsed
with
sterile
water
and
allowed
to
dry
thoroughly
before
being
heavily
dusted
with
uredospores.
Half
the
grains
were
planted
i
in.
deep
and
the
rest
2
in.
deep
in
sand
culture
with
forceps.
(b)
Grains
were
washed
in
running
water
for
15
min.,
rinsed
in
sterile
water
and
then
dipped
in
warm
1%
agar.
The
surface
of
the
agar
was
dusted
with
uredospores
and
the
grains
were
then
planted
as
in
Ila.
III.
A
repeat
of
the
experiment
by
Weber
(1922)
(a)
Maize
grains
were
germinated
until
the
coleoptiles
were
o•5
in.
long.
The
seed-
lings
were
then
immersed
in
a
uredospore
suspension
for
5,
15
and
6o
min.
and
planted
2
in.
deep
in
sand
culture.
(b)
Washed
grains
were
soaked
in
sterile
water
until
the
coleoptiles
began
to
grow.
The
seedlings
were
then
dipped
in
warm
i
%
agar,
dusted
with
uredospores
and
planted
as
previously.
The
potted
seedlings
from
the
above
experiments
were
kept
in
a
green-
house
for
5
weeks
and
examined
every
2
days.
All
experiments
gave
negative
results.
Meijers
(1938)
observed
that
P.
sorghi
was
unknown
in
Holland
until
a
North
American
variety
of
maize
was
imported
for
experimental
pur-
poses.
P.
sorghi
appeared
on
the
fi
rst
generation
of
this
variety
and
sub-
sequently
on
local
varieties.
Humphrey
&
Cromwell
(193o)
suggest
that
the
introduction
of
P.
glumarum
into
Argentina
may
have
been
associated
with
the
importation
of
grain.
Naumov
(1939)
mentions
that
the
possi-
bility
of
inter
-continental
spread
of
rust
becomes
more
likely
when
one
considers
the
many
years
of
grain
commerce
between
the
continents,
and
that
it
only
requires
a
single
successful
spore
in
the
correct
environment
to
establish
the
disease.
In
the
immediate
post-war
years
there
were
large
imports
of
bulk
maize
grain
into
West
Africa
from
the
Americas
and
especially
into
Ghana,
for
use
both
as
fodder
and
seed.
3o
Transactions
British
Mycological
Society
INTRODUCTION
ON
A
LIVING
HOST
Records
of
grass
importations
kept
by
the
Nigerian
Federal
Department
of
Agricultural
Research
reveal
that
clonal
material
of
Tripsacum
laxum,
an
alternative
host
of
P.
polysora,
was
imported
into
Nigeria
from
Trinidad
in
1945
and
found
to
be
infected
with
that
rust.
The
clonal
material
was
intercepted
by
the
Plant
Quarantine
Division
and
all
infected
plants
destroyed,
the
remainder
being
sterilized
and
subsequently
multiplied.
Although
a
most
careful
watch
was
kept
no
further
infection
was
observed.
INTRODUCTION
OF
INOCULUM
BY
AIR
TRANSPORT
Chester
(1946)
states
that
modern
inter
-continental
air
transport
could
be
a
most
effective
means
of
introducing
rusts.
The
rapidity
of
travel
would
ensure
that,
if
an
aeroplane
or
its
contents
were
contaminated,
the
uredo-
spores
would
have
an
excellent
chance
of
being
liberated
at
the
point
of
arrival
in
a
viable
state.
During
the
war
and
immediate
post-war
years
foodstuffs
including
green
vegetables
were
fl
own
into
West
Africa
from
the
Americas
to
supply
forces
stationed
on
the
coast.
Among
these
fresh
vegetables
was
sweet
corn
`on
the
cob'.
On
frequent
occasions
uredosori
of
P.
polysora
have
been
observed
growing
on
the
inner
surfaces
of
the
husks
of
maize
ears.
Samples
of
maize
grain,
for
experimental
purposes,
were
also
frequently
fl
own
in
during
the
post-war
years.
DISCUSSION
Most
of
the
work
on
the
limits
of
spore
dispersal
has
been
based
on
physical
reasoning
with
no
experimental
backing.
The
very
large
number
of
spores
expected
to
be
released
from
the
nearly
infinite
number
of
points
at
the
origin
of
the
rust,
Central
America
and
the
West
Indies,
would
have
no
absolute
theoretical
limit
of
dispersal.
The
chances
are
that
spores
often
complete
the
3500
miles
journey,
but
are
almost
certainly
non
-viable
on
arrival.
Experiments
carried
out
at
Ibadan
have
shown
that
the
uredo-
spores
of
Puccinia
polysora
failed
to
germinate
after
exposure
to
C.
for
longer
than
12
hr.
At
some
stage
of
the
journey
over
mid
-Atlantic
the
spores
would
without
doubt
be
subjected
to
similar,
and
even
lower,
temperatures
for
long
periods.
Present
opinion
tends
to
discount
wind
as
a
medium
of
dispersal
of
fungus
spores
over
large
areas
of
ocean
and
this
is
supported
by
present-
day
species
isolation
throughout
the
world.
Presumably
P.
polysora
was
in
America
for
many
years
prior
to
its
discovery
in
1897
and
in
all
the
years
that
maize
has
been
present
in
the
African
continent
the
rust
has
failed
to
establish
itself
there
by
normal
processes.
Experiments
on
the
inoculation
of
grain
all
gave
negative
results.
These
experiments,
however,
were
not
sufficiently
comprehensive
and,
owing
to
the
difficulty
of
simulating
natural
conditions
of
seed
contamination
experimentally,
are
far
from
being
conclusive.
The
possibility
of
intro-
duction
by
spores
adhering
to
grain
imported
by
a
sea
-route
would
be
Puccinia
polysora.
II.
R.
H.
Cammack
31
greater
in
rusts
in
which
the
viability
of
the
uredospore
was
longer.
P.
polysora,
in
laboratory
tests
at
Ibadan,
showed
a
reduction
of
viability
of
5o
%
after
27
days
and
total
death
after
5o
days.
The
chances
of
survival
during
a
long
sea
voyage
are
small.
The
observations
of
Meijers
(1938)
and
Humphrey
&
Cromwell
(193o)
indicate
that
the
possibility
of
introduction
by
this
method
cannot
be
discounted
completely.
It
is
unlikely
that
the
rust
was
introduced
on
infected
Tripsacum
laxum
in
1945.
The
remaining
original
material
which
was
not
destroyed
was
multiplied
and
distributed
throughout
Nigeria
and,
in
the
years
between
its
introduction
and
the
appearance
of
P.
polysora
on
Zea
mays
in
Nigeria,
no
rust
was
observed
on
the
T.
laxum,
nor
on
Z.
mays.
The
imported
T.
laxum
was
clonal
material
from
Trinidad,
in
which
area
it
is
commonly
infected
with
a
rust
currently
classified
as
P.
polysora.
Cross
-inoculation
studies
at
Ibadan
have
repeatedly
failed
to
produce
infection
on
T.
laxum
with
inoculum
obtained
from
infected
Z.
mays,
and
it
is
possible
that
the
respective
strains
are
incompatible
on
the
two
hosts.
No
similar
cross
-
inoculations
have
been
done
in
Trinidad
to
support
this
theory.
On
the
basis
of
existing
evidence,
it
is
most
likely
that
the
rust
was
introduced
by
air
-transport.
Nevertheless,
this
must
remain
a
supposition,
because
the
possible
methods
of
introduction
cannot
be
reproduced
experi-
mentally.
The
problem
is,
however,
one
of
great
interest
with
respect
to
plant
quarantine,
since
not
only
is
the
rust
now
present
throughout
all
tropical
maize
-growing
areas,
but
disease
saturation
has
occurred.
REFERENCES
BISBY,
G.
R.
(1935).
Are
living
spores
to
be
found
over
the
(Atlantic)
Ocean?
Mycologia,
27,
84-85.
CAMMACK,
R.
H.
(1955).
Observations
on
Puccinia
polysora
Underw.
in
West
Africa.
First
Ann.
Rep.
W.
African
Maize
Rust
Research
Unit,
1953,
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