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April 2015
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Ear rots of maize:
A continuous threat to food safety and security
D
iplodia ear rot caused by
Stenocarpella maydis
, Fusarium
ear rot caused by
Fusarium verticillioides
and Gibber-
ella ear rot caused by the
Fusarium graminearum
species
complex are the major ear rot diseases occurring in
South Africa.
These diseases have been identified as recurring problems through-
out maize producing areas. The fungi causing these diseases also
cause maize stalk rots which result in plant lodging. Maize ear rots
result in yield losses and grain quality reduction through discoloured
kernels, observed during the grain grading process and production
of mycotoxins.
Stenocarpella maydis
,
F. verticillioides
and the
Fusarium gramine-
arum
species complex produce mycotoxins that are toxic to hu-
mans and livestock. It is important to remember that environmental
conditions favourable for ear rots do not correspond to conditions
causing stalk rots, even though the same fungi are involved.
Each ear rot disease must be seen as an individual disease as cli-
matic and/or environmental conditions for the development of each
disease varies. These diseases will be discussed separately.
Diplodia ear rot
Symptoms in maize
Diplodia ear rot symptoms (
Photo 1
and
Photo 2
) associated with
infections during early ear development are the yellowing and dry-
ing of husk leaves while stalks and leaves remain green. Infection
generally begins at the ear base and ramifies upwards (
Photo 3
). The
entire ear becomes overgrown with a white mycelial growth.
A cross section of an infected ear shows black spore-producing bod-
ies at the kernel bases. Late season infections may occur when ker-
nel moisture is low and symptoms are less obvious. Embryos only
become infected and slightly discoloured, but no ramification of
the rest of the kernel occurs. Such symptomless infections are lo-
cally referred to as “skelm Diplodia”.
Life cycle and epidemiology
Stenocarpella maydis
spores are transmitted by air, seed and soil.
Airborne spores result in heavy infections of up to 10 m from the
inoculum source and the number of successful infections is re-
duced with distance. Single spores travelling long distances may
lead to trace infections 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
S. maydis
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 and germinated spores
may enter tissue and lie dormant until conditions are favourable for
fungal growth through the tissues.
Mycelial colonisation
is accompanied by cell wall degradation ahead
of the growing pathogen. This is due to enzymes that are secreted.
Fungal ramification of maize ears 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 subsequent season.
Economic importance
Yield losses caused by diplodia ear rot have not yet been quanti-
fied because the harvest method, the make of the harvester, speed,
and settings among other factors, all affect the percentage of
rotten kernels that remain in the grain bin or trailer. Rotten kernels
that are light are blown out during the harvesting process, the per-
centage 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, serious grain quality re-
ductions occur. Secondly, where a high percentage of rotten kernels
are discarded in the harvest process, it will improve grain quality,
but manifest itself as yield loss.
A number of mycotoxins have been isolated and identified from
S. maydis
infected grain. Diplodiatoxin, dipmatol and three chae-
toglobosins have been found in
S. maydis
infected material. Re-
cently diplonine, a neurotoxin produced by
Stenocarpella maydis
,
was isolated and identified by researchers at the ARC-Grain Crops
Institute (ARC-GCI) in collaboration with toxicologists at the ARC-
Onderstepoort Veterinary Institute.
Planned research includes the development of techniques to identify
and quantify these toxins in maize samples to determine the levels
of each mycotoxin in various tissues produced by different isolates.
Not all
S. maydis
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%. Techniques enabling researchers to quantify
the presence of these toxins will assist in determining which grain/
isolates are toxic and which are not.
Animals, particularly cattle, being fed Diplodia-infected rations
must be monitored. As soon as symptoms of reluctance to move,
standing with wide-based stance, poor coordination, walking stiff-
legged with a high stepping gait, falling, paralysis, constipation,
salivation and tremors are observed, animals must be removed im-
mediately and fed on healthy rations. Recovery rates are high, but
animals will die if kept on the infected ration.
Diplodia ear rot / Fusarium ear rot / Gibberella ear rot
ON FARM LEVEL
Integrated pest control
BRADLEY FLETT
and
EDSON NCUBE,
ARC-Grain Crops Institute