18 / 44
Desember 2017
16
Research on and control
of Diplodia in maize
D
iplodia stalk and ear rot is caus-
ed by the fungus S
tenocarpella
maydis
and can be encountered
throughout the world. Diplodia ear
rot is usually noted in seasons with early
drought followed by excessive and extend-
ed rainfall during the maturation stage of
the maize plant. Diplodia stalk rots become
common in seasons where early season
rainfall is followed by a midseason drought
period during grain fill.
Diplodia ear and stalk rot disease produc-
es specific mycotoxins which have been
shown to affect animals differently in vari-
ous studies, many of which were originally
determined in South Africa where field
symptoms of diplodiosis were initially re-
ported.
Diplodiosis is defined as a nervous disor-
der of cattle and sheep resulting from the
ingestion of mouldy cobs infected by
S. maydis
.
The past
The first record of diplodiosis in South
Africa is a report by Van der Bijl in 1914 in
the region of the Mooi River in the KwaZulu-
Natal province. He reported an outbreak of
‘sickness’ in cattle which was characterised
by paralysis following grazing on harvested
maize fields.
Over the past three decades in South Africa,
various reports of diplodiosis in livestock
were received from veterinarians and pro-
ducers. However these outbreaks have not
been confirmed to be caused by Diplodia
contaminated residues.
The present
Stenocarpella maydis
(
Diplodia
)
ear rot
Drought during the early season, followed
by rain during the late season, can lead to
Diplodia ear rot epidemics, especially where
high inoculum sources are present on stub-
ble covering soil.
This fungus has the ability to produce spore
producing structures that can survive on
maize stubble through the winter while pro-
ducing spores during spring.
These spores then infect plants through-
out the growing season. After rain or
during high humidity, these structures re-
lease spores in the air, which land on maize
plants and infect the base of the ear/leaf
junction and ramifies upwards into the ear.
The entire ear becomes overgrown with a
white mycelial growth (
Photo 1
).
If a cross section is made of an infected ear,
black spore-producing bodies at the kernel
bases can be seen (
Photo 2
). Late season in-
fections may occur when kernel moisture is
low, but these symptoms are less obvious.
Infections that show little or no symptoms
are locally referred to as ‘
skelm
Diplodia’.
Diplodia ear rot can re-occur (epidemic) in
certain areas and infected grain is then har-
vested with the healthy grain, thereby re-
ducing grain quality. Reduced grain quality
will have negative financial implications as
this reduces the price the producer receives
for his grain. During such an epidemic when
early infections are present, yield losses can
be of great economic importance.
Stenocarpella maydis
(
Diplodia
)
stalk rot
This fungus is common in all maize produc-
ing areas and in seasons with early rains
and persisting late season droughts, this
disease becomes very damaging, resulting
in lodging and poor grain fill.
Diplodia stalk rot reduces yield by reducing
nutrient and moisture uptake to ears during
grain fill. This sink (the ear) extracts sugars
from the stalk which further predisposes the
stalk to fungal growth and further reduces
nutrient uptake.
This continual sink-source cycle reduces
yield. The onset of windy conditions whilst
plants are drying results in lodging (
Photo 3
)
and further economic losses as ears have
to be picked by hand. Estimated annual
yield losses of 5% to 20% may occur due to
Diplodia stalk rot and lodging.
The fungus overwinters in a mycelial form in
maize stubble (buried or on the soil surface)
throughout the winter. Under warm, moist
conditions, pycnidia develop which release
spores which are spread by wind and rain.
Infections of plants occur mainly through
the crown and roots and occasionally at the
nodes between the crown and ear. Infection
usually takes place two to three weeks af-
ter silking under favourable conditions. Dry
early season conditions followed by rain
during silk formation favour Diplodia ear
rot, whereas a wet early season followed by
drier conditions or heat stress is likely to re-
sult in more severe Diplodia stem rot.
Stalk rot symptoms appear several weeks
after silking. Leaves of infected plants wilt,
become dry and appear greyish-green.
Lower internodes become brown and
spongy. Small, black fruiting bodies (pyc-
nidia) cluster near the nodes of the rind.
The rind may also be covered by a white
mycelial growth. The stalk pith discolours
and disintegrates with vascular bundles re-
maining intact. This weakening of the stalk
predisposes plants to lodging during strong
winds and rain prior to harvest.
Control measures for
Diplodia ear and stalk rots
It is critical that Diplodia ear and stalk rot
control is seen holistically and that other
control measures are included in an inte-
grated control programme to manage both
Diplodia ear and stalk rots.
Stubble reduction/retention
Control of Diplodia ear rot includes surface
stubble reduction by means of grazing,
burning, baling or ploughing in of surface
maize stubble. As the fungus (
S. maydis
)
survives on maize stubble and survives
poorly in soil, any management practice
that reduces levels of infected surface
stubble will reduce inoculum concentra-
tions in the field.
The removal of stubble for a single season
and then resorting back to stubble retention
practices, only reduces Diplodia ear rot for
that specific season. Where stubble is pre-
sent the following season, the risk of Diplo-
dia ear rot will increase to its original level,
should weather conditions be favourable.
Stress reduction
Avoid planting unrealistically high plant
populations on marginal soils and in areas
where there is a high probability of drought,
leaf or alternate stalk rot disease conditions.
Ensure plant nutrition is adequate and bal-
anced relative to the yield potential of the
land or area to be planted.
ON FARM LEVEL
Diplodia stalk and ear rot
Integrated pest control
DR BELINDA JANSE VAN RENSBURG,
ARC-Grain Crops, Potchefstroom and
PROF VINESH MAHARAJ,
Department of Chemistry, University of Pretoria