Diseases
Inputs/Production
Februarie 2014
44
FOLIAR DISEASES
OF SUNFLOWER:
MOSES RAMUSI AND BRADLEY FLETT, ARC-GRAIN CROPS INSTITUTE,
POTCHEFSTROOM
Foliar diseases of sunflower are caused by various pathogens which
include
Alternaria helianthi
,
Puccinia helianthi
and
Septoria helianthi
.
These diseases cause various symptoms on foliage which may decrease
sunflower yields in seasons conducive to the development of these
diseases.
Environmental conditions for the development of epidemics of these
leaf diseases vary and it is therefore important that correct disease
identification is critical in developing appropriate disease management
strategies for each.
Alternaria leaf blight
Although a number of Alternaria species have been reported to cause
diseases on sunflower, only
A. helianthi
, which is the most common
Alternaria
spp. of sunflower, may be responsible for significant yield
losses.
Alternaria occurs in all sunflower producing areas and is currently
a potential disease threat to sunflower production in South Africa.
Alternaria survives between sunflower crops in and on infested crop
debris, on weed hosts and on seed. The disease starts when spores land
on leaves or stems, germinate in the presence of moisture, and directly
penetrate and infect the leaves.
Symptoms
Symptoms of
A. helianthi
appear as circular, dark brown to black lesions
with concentric rings ranging from 0,2 mm to 0,5 mm in diameter
(
Photo 1
). Lesions will eventually enlarge in size and coalesce causing
blighting of leaves. Some lesions can be identified by distinct yellow
halos, particularly on young plants. If the disease is severe, plants may
be defoliated prematurely and die or lodge.
Brown rust
Rust is caused by the fungus
P. helianthi
and was recently detected in
some parts of the Free State. This rust does not require an alternate
host; it overwinters only on sunflower plant debris as teliospores which
germinate in spring to produce basidiospores. Spores are disseminated
through rain or irrigation water and wind. Disease development occurs
between temperatures ranging from 25°C to 27°C, accompanied by at
least 12 hours of wet foliage.
Symptoms
Rust is characterised by cinnamon-coloured spots or uredial pustules,
which primarily occur on the leaves, but also on the stems, petioles,
bracts and the back of the head under severe infections (
Photo 2
).
Photo 1: Typical leaf symptoms of Alternaria leaf blight on sunflower caused by
A. helianthi
.
Photo: Charlie Block
Photo 2: Typical leaf symptoms of rust on sunflower caused by
P. helianthi
.
Photo 3: Typical leaf symptoms of Septoria leaf spot on sunflower caused by
S. helianthi
.
Photo: Charlie Block
1
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