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February 2016
Control measures
Stubble reduction
Control measures include reduction of infested surface stubble by
means of grazing, burning, baling or ploughing in of surface maize
stubble. As the fungus (
Stenocarpella maydis
) survives on maize
stubble (
Photo 4
) and survives poorly in soil, any management prac-
tice that reduces levels of infected surface stubble will reduce inocu-
lum concentrations 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 spe-
cific season. Where stubble is present the following season, the risk
of Diplodia ear rot may increase to its original level, should weather
conditions be favourable.
Crop rotation
Crop rotations reduce Diplodia ear rots by reducing inoculum levels
in two ways. Firstly, a non-host for the fungus will not allow the fun-
gus to persist for the season where maize is not grown. Secondly,
a greater period (a season or two) between maize crops allows for
natural breakdown of maize stubble, which again reduces survival of
the fungus. Leguminous crops such as soybeans, dry beans, ground-
nuts and cowpeas are very good rotation crops.
Other rotation crops that reduce diplodia ear rots are wheat and oats.
Sunflowers do not significantly reduce diplodia ear rots under ex-
perimental conditions, but the reason has not yet been determined.
Early harvesting
Early harvesting will reduce diplodia ear rots as it reduces time avail-
able for the fungus to grow on the ear. The fungus (
Stenocarpella
maydis
) can grow on maize ears in the field until 11% moisture. Late
or winter rains keep ears wet and increase the chance for fungal
growth. In certain cases it would pay to harvest early at higher mois-
ture levels and artificially dry grain. This is one reason why Diplodia
ear rot may not be considered the major ear rot problem in the USA
where maize is harvested early and dried artificially.
Hybrid resistance
Selection of cultivars is very important in control of Diplodia ear rots.
However, it appears that there is widespread confusion regarding
resistance and use of resistance. There are no hybrids on the market
that do not get Diplodia ear rot at all; however, some get more than
others under specific climatic conditions. This reaction is affected
by different climatic conditions which is important to consider when
selecting resistant hybrids.
Fusarium ear rots
Symptoms
Fusarium ear or kernel rot, is caused primarily by the fungus
Fusarium verticillioides
as well as
F. proliferatum
and
F. subgluti-
nans. F. verticillioides
also causes stalk and root rot, as well as seed-
ling 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 5a
and
Photo
5b
).
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
(
Photo 6
).
F. 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%.
Economic importance
Fusarium ear rot can result in yield and grain quality reductions. In-
fections associated with ear damage are often localised on ears and
infected grain is harvested during the harvesting process.
1a and 1b: Initial Diplodia ear rot develop-
ment symptoms are yellowing and drying
of husk leaves while stalks and leaves
remain green.
2: The maize ear eventually becomes over-
grown with a white mycelial growth.
3: A cross section of a Diplodia infected
maize ear shows black spore-producing
bodies at the kernel bases.
4: Pycnidia of
Stenocarpella maydis
on
maize stubble.
1a
3
4
1b
2