SA Graan Julie 2016

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Brown rust of sunflower:
A potentially devastating disease
Brown rust is caused by the fungus
Puccinia helianthi Schw., which
is capable of causing yield losses
in specific sunflower production
areas of South Africa where the disease
may reach epidemic proportions.
Severe infection of sunflower rust causes a
decrease in seed size, head size, oil content
and yield. Rust can occur anytime during
the growing season as long as the environ-
mental conditions are favourable.
Annual sunflower disease surveys con-
ducted by the ARC-Grain Crops Institute
(ARC-GCI) during the 2012/2013, 2013/2014,
2014/2015 and 2015/2016 seasons reported
brown rust to occur in all major sunflower
producing areas, particularly in the North
West Province and Free State.
Severe symptoms of brown rust which can
cause an estimated yield loss of between
10% and 40% in each field were observed
in and around Bothaville, Ottosdal, Vrede-
fort, Kroonstad, Rustenburg, Senekal and
Lichtenburg during the 2015/2016 survey.
Epidemiology of the
disease
Rust overwinters as teliospores on plant
debris and do not require an alternate host
to complete its life cycle. These teliospores
germinate in spring to produce basidio-
spores which infect volunteer sunflower
seedlings.
This initial infection results in the formation
of pycnia which in turn leads to the develop-
ment of aecial pustules. The aecia produce
aeciospores which spread by wind to other
sunflower plants and will eventually pro-
duce uredinia.
Uredinia will then produce urediniospores
which are disseminated by wind and can
infect stems, bracts, petioles and leaves of
sunflower plants. Conditions favourable for
rust are warm temperatures (12°C - 29°C)
and either rain/irrigation or dew.
In dry years, if night temperatures are
low enough to promote dew formation on
leaves, infection may occur. High nitrogen
fertilisation and seeding rates results in
excessive foliage, which increases humid-
ity within the canopy and this favours rust
development.
Symptoms of the disease
Rust is characterised by cinnamon-coloured
spots or uredial pustules, which primarily
occur on the leaves but also on the stems,
petioles, bracts and back of the sunflower
head.
The uredial pustules on the underside of
the leaf turn black with the advent of cool
temperatures as the brown urediospores
are replaced by black overwintering teli-
ospores (Photo 1). The upper side of the leaf
show aecial pustules alone or in clusters
(Photo 2).
Disease control
Planting rust-resistant hybrids reduces dis-
ease development, therefore producers
experiencing rust problems should look out
for rust resistant hybrids available in the
market. Crop rotation is important because
it prevents disease build-up by interfering
with the life cycle of the pathogen.
Reduced irrigation and nitrogen fertiliser
application also minimise disease infec-
tion as these conditions favour disease de-
velopment.
Rust overwinters on volunteer sunflower
plants, therefore destruction of volunteer
plants occurring close to commercial fields
early in spring reduces sources of primary
inoculum.
Timely and proper application of registered
fungicides (Triazole and Strobilurin) will
limit new infections and prevent epidemics.
Producers are always advised to seek ex-
pert advice before applying fungicides and
they should keep to the recommended label
rate and time of application.
Producers that observe brown rust symp-
toms in their sunflower fields are requested
to contact the authors at 018 2996100 as we
are always looking out for isolates to collect.
For more information on this
topic, contact Moses Ramusi
at .
77
July 2016
ON FARM LEVEL Integrated pest control
Yield losses / Brown rust / Sunflower
MOSES RAMUSI and BRADLEY FLETT, ARC-Grain Crops Institute, Potchefstroom
1: Infected sunflower leaf showing brown
urediospores and black teliospores on the
underside of the leaf.
2: Infected sunflower leaf showing aecial
pustules on the upper side of the leaf.
1 2