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February 2016
Graminearum ear rot
Symptoms
Graminearum ear rot, also known as Gibberella or red ear rot, is
caused by the
Fusarium graminearum
species complex which also
causes stalk rot, root rot and seedling blight of maize. In South Africa
the primary pathogen causing ear rot in the
Fusarium graminearum
species complex is
F. boothii
.
Disease symptoms are dark red discolouration of the whole or part
of the maize ear (
Photo 7
and
Photo 8
). Early infections result in com-
plete ear rotting, with husks adhering tightly to the ear. Graminearum
ear rot usually progresses from the tip of the ear downward.
Economic importance
Graminearum ear rot is increasing in economic importance in the
South African maize production areas and may become a major
threat to the maize industry. Previously reports from certain areas
where cool, wet, late season conditions were experienced, imply
sporadic and localised outbreaks of this ear rot disease.
Recent studies have indicated an increased spread and severity of
this disease as well as the stalk rots caused by the
Fusarium gramine-
arum
species complex. In certain cases severe yield and quality re-
ductions were observed. The major concern is toxicity, associated
with this disease.
F. graminearum
is known to produce a number of
important mycotoxins, which cause major problems, especially for
pig producers.
Recently, high levels of these mycotoxins in dog food have resulted
in feed refusal and mycotoxicoses in dogs. These mycotoxins are
zearalenone (ZEA), deoxynivalenol (DON) and nivalenol (NIV). Toxi-
genic symptoms in pigs, range from hormone induced syndrome
caused by ZEA, which reduces the reproductive performance of the
animals, to feed refusal due to high levels of DON and/or NIV.
Cattle appear to be much more resistant than pigs to the hormonal
effects of ZEA, whereas chickens do not seem to be affected. The
most conspicuous changes in pigs due to ZEA are enlargement of
the uterus and mammary glands and atrophy of the ovaries.
Feed refusal is a result of the unpalatability of the feed and may be
reflected in decreased weight gains and slower growth rates. Vomit-
ing may occur in animals that consume small quantities of infected
grain. Maize containing more than 5% infected kernels should not be
included in rations for pigs, although it may be diluted with sufficient
quantities of first grade maize.
Life cycle and epidemiology
The species within the
Fusarium graminearum
species complex
survive primarily on the surface of maize stubble throughout win-
ter. Species within the
Fusarium graminearum
species complex
have also been reported to survive on stubble or organic matter of
other crops previously planted as saprophytes. Survival structures
(perithecia) may develop and mature on maize stalks under warm
wet conditions. Ascospores are exuded from the perithecia and are
taken up into air currents where they can be transported long dis-
tances, from where these spores are then deposited on, and infect
other maize plants or other susceptible crops.
The species within the
Fusarium graminearum
species complex also
infects various other tissues and cereals such as wheat and barley,
which may help the pathogen to overwinter, causing even larger dis-
ease outbreaks the following season.
F. boothii
infects maize seed
and infection levels of up to 66% have been reported.
Seed to seed transmission, however, has not been clearly shown.
These spores infect the maize silks and grow down into the point of
the ear. The pathogen has also been reported to be transmitted by
birds and insects.
Graminearum ear rot severity is favoured by cool, wet weather with-
in three weeks of silking. This disease is common under irrigation
conditions in South Africa. Regions affected by sporadic outbreaks
of this disease are generally KwaZulu-Natal and Mpumalanga and
irrigation fields in the Limpopo and North West Province.
Control measures
Crop rotation
Rotation of maize with non-graminaceous crops decreases the inci-
dence of Graminearum ear rot. It is however important to note that
certain species within the
Fusarium graminearum
species complex
are able to utilise organic matter and other crop residues to overwin-
ter as saprophytes. More research on the efficacy and ability of these
species to survive on alternate sources of organic matter need to be
researched intensively to determine the potential of crop rotation on
reducing inoculum levels.
Stubble removal
As the fungus overwinters on maize stubble retained on the soil sur-
face, the removal of maize residues will reduce disease incidence the
following crop season. The ability for species within the
Fusarium
graminearum
species complex to survive on other sources of or-
ganic matter and stubble of other crop species will also influence
the role that removal of maize stubble will play in reducing inoculum
sources of this disease.
Cultivar selection
Reports in the literature indicate that hybrids vary in susceptibil-
ity to the disease. Although local hybrids are presently in the pro-
cess of being screened for resistance to this disease
no results are yet available. Field observations have
resulted in the identification of highly susceptible
hybrids.
7: Graminearum ear rot symptoms are typical dark red discolouration of the whole or part of the maize ear.
8: Kernels showing typical Graminearum ear rot symptoms.
7
8
ON FARM LEVEL
Integrated pest control
Ear rots in maize production