cob and tassel smut onmaize
C
ob and tassel smut, also known as
head smut is caused by the fungus
Sphacelothecareiliana
(syn.
Ustilago
reiliana
) and occurs in South Africa,
Australia, Mexico, New Zealand, Asia and
southeasternEurope.
Cob and tassel smut was a major threat to
maize production in South Africa during
the late 70’s and early 80’s reducing yields
due to serious epidemics. Resistant maize
hybrids were introduced to control these
early epidemics.
In the last five to six seasons, coband tassel
smut epidemics have occurred sporadically
with significant seasonal variations. Epide-
micshavebeenprominent incertainareas in
Mpumalangawhere thediseasehas resulted
in major yield losses in outbreak seasons.
The primary reason for these outbreaks
is the introduction of susceptible hybrids
and weather conditions that have been
favourable for infestation of the seedling
and systemic growth of the organism in the
maizeplant.
Symptoms
Disease symptoms develop on maize ears
and tassels. Powderysporemassesdevelop
on infected ears, which appear round and
lack silks (
Photo 1
). As soon as the infected
ear ruptures, a black powdery spore mass
can be seen from opened ear husks. Tassel
infections can vary from a few individual
spikelets (
Photo 2
) on the tassel (florets)
to the entire tassel, which may form leafy
structures. Plants with tassel symptoms
will almost alwayshave infectedears. Other
symptoms include aborted ears, stunt-
ing (often more than 50%) and excessive
tillering.
Economic importance
Infection inadevelopingmaizeear destroys
the grain entirely. A 10% infection results
in a 10% yield loss. Harvested grain is
often so contaminated with spores that
it is downgraded when delivered to the
silo, resulting in further losses. Continued
planting of susceptible maize hybrids
and favourable conditions for disease
development will result in higher initial
inoculum levels which may put maize
production in infested areas at risk.
Epidemiology
S. reiliana
issoil borneandsporesarewind-
dispersed. Spores hibernate in soil andmay
remain viable in the soil for many years.
Soil temperature is critical for infection
(21°C - 28°C) of maize seedlings. Moderate
to low soil moisture levels (below 50% field
capacity) and acid soil conditions (pH 5,5)
favours infection.
The fungus’ mycelia grow systemically
behind thegrowthpointofmaizeplantsuntil
sexual maturity of the crop, after which the
fungus erupts in the ear and tassel tissues.
These infected maize tissues develop galls
and produce spores that are again released
into the air to repeat the cycle.
Control measures
Resistant hybrids
Public and private breeding programmes
identified useful sources of resistance and
were responsible for reducing the impact of
the epidemics observed during the 1970’s
and the 1980’s. The disease was brought
under control until recent outbreaks were
observed. Selection of resistant hybrids is
themost sustainable control option for this
disease. Cultivar reactions for the short and
long season hybrids have been screened
over a number of seasons and localities.
Results indicate adequate variation in
susceptibility to cob and tassel smut.
Soil fertility
Soil nitrogendeficiencies result in increased
incidence of cob and tassel smut. Correct
fertilisation and nitrogen applications will
limit infections. Liming to increase soil
pH will help to reduce infections by this
pathogen. Adequate plant nutrition reduces
stress levels which increase the incidence
of cob and tassel smut due to slowgrowth.
Sanitation systems
Rotation with any other crop will reduce
inoculum levels in soil. Rotations must
at least be away frommaize for a number
of seasons to be effective as the spores
survive for up to sevenyears in soil. Burning
all infected plants will reduce inoculum
build-up in soil.
Plantingdate
Later planting,when soil temperatureshave
increased and rainfall is more frequent,
will reduce levels of cob and tassel smut
infection. This is due to plants being less
stressed and vigorous seedling growth
reducing cob and tassel smut infections.
Fungicides
In South Africa, fungicide seed treatments
have been registered for cob and tassel
smut control. The efficacy of such
treatments is also dependant on prevailing
soil environmental factors. Seed companies
are continuously evaluating new seed
treatment fungicides for efficacy to cob and
tassel smut.
Quarantine
In various countries strict quarantine
measures have been used
with great success to reduce
inoculumbuild-up anddisea-
se spread.
BRADLEY FLETT,
ARC-GrainCrops Institute
1: Sori of cob and tassel smut develop on
infected ears, which appear round and lack
silks.
Photo: Pannar Seed
2: Tassel infections of cob and tassel smut in-
fected spikelets.
Photo: Pannar Seed
April 2014
66
ON FARM LEVEL
Cob smut / Tassel smut /Maize / Fungus
Pest control
