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Brown rust of sunflower:
A scarce, but potentially damaging disease
B
rown 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 with 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 environmental conditions are
favourable for it. Annual sunflower disease surveys conducted by
the ARC-Grain Crops Institute (ARC-GCI) during the 2012/2013 and
2013/2014 seasons, reported brown rust to occur in some parts
of the Free State (Arlington and Bloemfontein) and North West
Province (Welgelegen) in isolated areas.
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 basidiospores which infect volun-
teer sunflower seedlings.
This initial infection results in the formation of pycnia which in turn
leads to the development of aecial pustules. The aecia produce
aeciospores which spread by wind to other sunflower plants
and will eventually produce 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 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 result in excessive
foliage, which increases humidity within the canopy and this
favours rust development.
Symptoms of the disease
Adequate rainfall or heavy irrigation associated with warm tempe-
ratures promotes rust development. Rust is characterised by cinna-
mon-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 teliospores (
Photo 1
). The upper side of the
leaf show aecial pustules alone or in clusters (
Photo 2
).
Disease control
Planting rust-resistant hybrids reduces disease development;
therefore producers experiencing rust problems should look out
for rust resistant hybrids available on 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 infection as these con-
ditions favour disease development. Rust overwinters on volun-
teer 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 expert advice before applying
fungicides and they should keep to the recommended label rate
and time of application.
Producers that observe brown rust symptoms in their
sunflower fields are requested to contact the authors
at 018 299 6100 as we are always on the lookout for
isolates to collect.
55
May 2015
ON FARM LEVEL
Integrated pest control
Brown rust / Sunflower / Yield losses
MOSES RAMUSI
and
BRADLEY FLETT,
ARC-Grain Crops Institute
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