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Oktober 2016
70
sunϐlower disease surveys
T
he sunflower (
Helianthus annuus
L.) is the most important
cultivated oilseed crop in South Africa. Annual production
ranges from 500 000 tons to 700 000 tons. Sunflowers are
hosts to many pathogens, which cause various diseases
such as Sclerotinia head rot, Alternaria leaf spot, bacterial head rot
and brown rust (
Photo 1
,
Photo 2
,
Photo 3
and
Photo 4
).
These diseases possess potential risks to production and may re-
duce yields, depending on seasonal conditions and geographic lo-
cality. Increased enquiries from producers prompted the necessity
for seasonal surveys to determine which diseases predominate in a
particular area and on which cultivars.
Materials and methods
73 sunflower production sites including the ARC-Grain Crops In-
stitute (ARC-GCI) national cultivar trials and producers’ fields were
screened and diseases quantified over a period of three seasons,
2012/2013, 2013/2014 and 2014/2015 respectively.
The ARC-GCI replicated national cultivar trials, cultivars SYN 4045,
PHB 65A25, SYN 4200 and PAN 7049 were surveyed, as well as
producers’ fields. 30 plants were randomly selected at three points
within the field and surveyed for disease incidence and severity. A
questionnaire regarding sunflower farming practices was completed
with producers during the surveys.
Results and discussions
The 2012/2013 disease survey results of the cultivar trials indicated
that percentage average disease caused by Alternaria on all four cul-
tivars was 65%, whereas in all producers’ fields surveyed it was 60%.
Other prominent diseases in producers’ fields such as brown rust
(84%) in Arlington, Sclerotinia head rot (31%) in Delmas and Phoma
stalk rot (96%) in Naboomspruit were also observed.
In the 2013/2014 season the average severity of Alternaria leaf
spot was 26,44% and Sclerotinia head rot was 16,8%. In produc-
ers’ fields average disease severity caused by bacterial head rot in
Dwaalboom was 32,22% and in Arlington 75% caused by brown
rust, in all areas surveyed it was 45,67% caused by Alternaria leaf
spot and 38,90%by Sclerotinia head rot.
During the 2014/2015 season the average Alternaria leaf spot
ranged between 42% in producers’ fields and 47% in cultivar trials.
Although powdery mildew was detected in several fields during the
2014/2015 survey, severity was low.
The questionnaire results showed that diseases, poor emergence
and drought were the major sunflower production constraints
(
Graph 1
). The severity of diseases caused by Alternaria obtained
from both cultivar trials and producers’ fields was higher in the
2012/2013 and 2014/2015 seasons when compared to the 2013/2014
season.
However, in contrast, the severity of Sclerotinia head rot was
lower in 2012/2013 and 2014/2015 seasons compared to the
2013/2014 season.
Overall results showed a shift in disease distribution, incidence
and severities on sunflower crops when comparing the 2012/2013,
2013/2014 and 2014/2015 seasons’ surveys. In the 2012/2013
and 2014/2015 seasons there were high disease incidences and
severities of Alternaria leaf spot, primarily in the North West Prov-
ince (
Table 1
,
Table 2
and
Table 3
), whereas, in the 2013/2014 sea-
son the severity of Alternaria leaf spot decreased considerably while
the distribution increased to all major sunflower production areas
(Table 2).
Although no significant difference between infected cultivars was
recorded during the 2012/2013 season, disease severity based on
locality differed significantly (Table 1).
ON FARM LEVEL
Head rot / Leaf blight / Rust / Sunflower
Integrated pest control
MOSES RAMUSI
and
BRADLEY FLETT,
ARC Grain Crops Institute, Potchefstroom
1: Typical symptoms of Sclerotinia head rot on sunflower caused by
Sclerotinia sclerotiorum
.
2: Typical symptoms of Alternaria leaf blight on sunflower caused by
A. helianthi
.
Photo: Charlie Block
3: Typical symptoms of bacterial head rot on sunflower caused by
P. atrosepticum
.
4: Typical symptoms of brown rust on sunflower caused by
P. helianthi
.
1
2
3
4