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Februarie 2016
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ear rots in maize production
T
he major maize ear rot diseases occurring in South Africa
are Diplodia, Fusarium and Gibberella. The diseases have
been identified as recurring problems throughout maize pro-
ducing areas. Maize ear rots result in grain quality reduction,
yield losses, livestock and potential human toxicity problems.
The fungi causing these maize ear rots can also cause maize stalk
rots which result in lodging of plants. It is important to remember
that environmental conditions favourable for ear rots do not corre-
spond to conditions causing stalk rots, even though the same fungi
are involved.
Diplodia, Fusarium and Gibberella must be seen as individual dis-
eases as climatic and/or environmental conditions for the develop-
ment of each of these diseases varies. Therefore, these diseases will
be discussed separately.
Diplodia ear rot
Symptoms
Diplodia ear rot symptoms associated with infections during early
ear development are yellowing and drying of husk leaves while
stalks and leaves remain green (
Photo 1a
and
Photo 1b
). Infection
generally begins at the ear base and ramifies upwards. The entire ear
becomes overgrown with a white mycelial growth (
Photo 2
).
A cross section of an infected ear shows black spore-producing
bodies at the kernel bases (
Photo 3
). Late season infections may oc-
cur when kernel moisture is low and symptoms are less obvious.
Embryos become infected and slightly discoloured but no ramifica-
tion of the rest of the ear occurs. Such symptomless infections are
locally referred to as ‘skelm Diplodia’.
Economic importance
Yield losses caused by Diplodia ear rot have not yet been quantified
because harvest method, make of harvester, harvest speed and har-
vester settings all affect the percentage rotten kernels that remain in
the grain bin or trailer.
Rotten kernels that are light are blown out during the harvesting pro-
cess, the percentage depending on abovementioned factors. This
implies that where Diplodia ear rot infections are serious, damage
is twofold.
Firstly, if a low percentage of rotten kernels are discarded during
the harvesting process, grain quality reductions are noted. Second-
ly, where a high percentage of rotten kernels are discarded in the
harvest process this will improve grain quality, but manifest itself
as yield loss.
Diplodiosis, a nervous disorder of cattle and sheep, results from the
ingestion of ears infected by
S. maydis
(Diplodia ear rot). Cases of
diplodiosis occur from six days to two weeks after the animals are
placed on fields with infected maize ears. The disease is character-
ised by reluctance of the animals to move, a wide-based stance, in-
coordination, tremors, paralysis and death.
Myelin degeneration
(
status spongiosis
) is the major histopathologi-
cal change observed in affected animals. The disease also causes
abnormal foetal development and foetal death. Field outbreaks of
diplodiosis in southern Africa are favoured by late heavy rains and
occur during the late winter months (July to September).
The practice of using harvested maize fields for winter grazing is a
major contributing factor to outbreaks of this mycotoxicosis. Myco-
toxins are secondary metabolites produced by certain fungi and are
toxigenic to animals or humans. In addition to diplodiatoxin, new
metabolites, namely dipmatol, diplonine and chaetoglobosins K and
L, have been isolated recently from
S. maydis
infected crops.
To date, none of the pure metabolites has been administered to
ruminants in order to reproduce the disease. Until such time that
the toxic metabolite(s) responsible for this disease are fully under-
stood, diplodiosis will continue to be reproduced experimentally
only by feeding naturally infected maize or pure cultures of the fun-
gus or their extracts to ruminants.
Not all fungus isolates are toxic. For example, two isolates from
the same field may be fed to ducklings with one being toxic and the
other not. This makes decisions difficult on whether or not to use
Diplodia-infected grain for feed. Milling infected grain is thought
to reduce the heat-sensitive toxin and ensure that total Diplodia-
infected grain content of feed rations is lower than 10%.
Animals, particularly cattle, being fed Diplodia-infected rations must
be monitored. As soon as symptoms of reluctance to move, stand-
ing with wide-based stance, poor co-ordination, walking stiff legged
with a high stepping gait, falling, paralysis, constipation, salivation
and tremors are observed, animals must be removed immediately
and fed on healthy rations. Recovery rates are high, but animals will
die if kept on the infected ration.
Life cycle and epidemiology
S. maydis
spores are transmitted by air, seed and soil. Airborne
spores result in heavy infections up to 10 m from the inoculum
source and the number of successful infections is reduced with dis-
tance from the source.
Single spores travelling long distances may lead to trace infec-
tions which may then develop into an epidemic focal point. Infected
maize seed is an important inoculum source which may result in
seedling and crown rot diseases. However, the majority of Diplodia
infected kernels do not germinate.
Spores land behind leaf or ear sheaths where they germinate and
infect stalks or ears. Spore germination is inhibited by exposure to
sunlight and desiccation. Free water is necessary for germination.
Germination may take up to seven days after spore germination.
Germinated spores may enter tissue and lie dormant until conditions
are favourable for fungal growth through the tissues.
Mycelial colonization is accompanied by cell wall degradation
ahead of the growing pathogen due to enzymes that are secreted by
the fungus. Fungal ramification of maize ears generally begins at
the shank. Cob tissue colonisation begins at the attachment with
embryonic tissues and proceeds into the endosperm. Similarly
stalk ramification may also occur. These infected tissues develop
fruiting bodies (pycnidia), which produce spores during the subse-
quent season.
ON FARM LEVEL
Diplodia / Fusarium / Graminearum
Integrated pest control
BRADLEY FLETT,
ARC-Grain Crops Institute, Potchefstroom