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April 2015
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ON FARM LEVEL
Integrated pest control
Fusarium ear rots
Symptoms
Fusarium ear rot (
Photo 4
and
Photo 5
), is caused primarily by the
fungus
Fusarium verticillioides
, formerly known as
F. moniliforme.
F. verticillioides
also causes stalk and root rot, as well as seedling
blight of maize.
Two major symptom types of this ear rot can be noted in the field.
The first are symptoms observed in association with maize stalk
borer feeding channels (
Photo 6
and
Photo 7
).
F. verticillioides
, in particular, is generally associated with insect
or bird damage on maize ears. The fungus appears as pink/white
mycelial growth on damaged kernels. The second symptom type
is evident as pink or streaked kernel discolouration not related to
kernel damage.
Fusarium verticillioides
may infect kernels without showing any
visible symptoms. It has been known for clean (first grade) grain
samples to have symptomless infections of up to 90%.
Disease cycle
Fusarium verticillioides
overwinters in maize debris and survives
in maize stalks as thickened hyphae in moist soils that have poor
aeration and little or no competition with other fungi and bacteria.
The soil-borne hyphae germinate and infect the germinating seed
and roots and move up the plant through systemic growth. The fun-
gus also produces airborne spores from sporulation on the previous
crop residue.
The mode of kernel infection by
F. verticillioides
is both through sys-
temic infections from contaminated seed and through the silk chan-
nel by airborne spores. Silk colonisation by
F. verticillioides
starts
from the tip of the ear downward.
Infection is enhanced by late-season rainfall and the physiological
state of the silks after pollination. Direct invasion of kernels can also
occur through weak points such as stress cracks in the pericarp and
through the pedicel.
Insect transmission is primarily due to the stalk borers,
Chilo partel-
lus
and
Busseola fusca
. Stalk borers feed on infected tissue, move
to new plants or plant parts and continue feeding, while leaving the
fungal spores in their frass. The fungus is released back to the soil
through infected stalks or infected seed.
Fusarium verticillioides
is
favoured by dry, hot climatic conditions such as those prevailing
primarily in the north-western parts of the South African maize pro-
ducing area.
Economic importance
Fusarium ear rot can result in yield and grade reductions. Infections
associated with ear damage are often localised on cobs. The symp-
tomless nature of certain infections by these fungi results in infected
grain passing unnoticed. A major economic implication of Fusarium
ear rot is the ability of these fungi to form mycotoxins in infected
maize. The most important being fumonisins which are toxic to
chickens, pigs and horses.
Horses are extremely sensitive to fumonisins and a level above
five parts per million (ppm) in their feed will result in a fatal disease
called
leucoencephalomalacia
or hole in the head syndrome. Guid-
ance levels for fumonisins in pig and chicken feeds are set at maxi-
mum allowable (safe) levels of 10 ppm and 50 ppm, respectively.
Research has also implicated this mycotoxin as one of the causes of
human oesophageal cancer, which is common in certain regions of
Africa, Europe, China and the USA.
Control measures
Control
Maize hybrids differ in their susceptibility to Fusarium ear rot. For ex-
ample, studies currently underway at the ARC-GCI show that geneti-
cally modified maize hybrids that contain the insecticidal proteins
for the control of maize stalk borer (Bt maize) have significantly less
Fusarium ear rot symptoms compared to their non-Bt isohybrids.
Agricultural practices such as planting hybrids that are adapted to
local climatic conditions, use of hybrids with tight husks, control of
ear feeding insects, avoiding excessive plant populations, maintain-
ing adequate levels of nitrogen and other essential growth nutrients,
crop rotation and sub-soiling in compacted soils to minimise plant
stress, are some of the possible means that can be followed to re-
duce Fusarium ear rot.
Standard grain storage procedures that prevent the development of
fumonisin mycotoxins in stored grain, such as drying maize kernels
to moisture levels below 16% after harvest, may be recommended
where high infection levels are expected. Stored grain should be
aerated regularly to lower moisture content and temperature to de-
sired levels.
Adjusting the combine harvester to avoid kernel damage during
harvesting reduces mycotoxin contamination. However, due to the
common occurrence of these fungi in nature, the use of sanitation
practices have not been very successful in disease reduction.
Graminearum ear rot
Symptoms
Graminearum ear rot (
Photo 8
), also known as Gibberella or red ear
rot, is caused by the fungus
Fusarium graminearum
and other fungi
belonging to the
Fusarium graminearum
species complex which
also causes root rot, crown rot, stalk rot and seedling blight of maize.
Recent research has shown that three of the recently identified
16 species within the
Fusarium graminearum
species complex
occur on maize roots and include
F. boothii
,
F. meridionale
and
F. graminearum s.s.
and only
F. boothii
occurs on maize ears in
South Africa.
Disease symptoms are dark red discolouration of the whole or part
of the maize ear. Early infections result in complete ear rotting, with
husks adhering tightly to the ear. Graminearum ear rot usually pro-
gresses from the tip of the ear downward.
Survival
This fungus survives primarily on the surface of maize stubble
throughout winter. Survival on other organic matter that may be in
or on the soil has recently been shown to result in alternate sources
for the survival of this pathogen. This includes crops such as lucerne
and grass cover crops. This explains the presence of Gibberella ear
and stalk rot on lands that have been cropped to “non-hosts” for a
number of years.
Survival structures may develop and mature on organic material
and/or maize stalk tissue under warm, wet conditions. Ascospores
are exuded from the perithecia and are taken up into air currents,
from where these spores can then be deposited on and infect other
maize plants.
Ear rots of maize:
A continuous threat to food safety and security