Yield losses can be severe when conditions are favourable and infections
occur at an early plant growth stage. Rust does not only reduce crop
yield, but is also capable of reducing oil, seed size, test weight and kernel-
to-hull ratios.
Septoria leaf spot
Septoria leaf spot is caused by
Septoria helianthi
which can be seed-
borne and also survive on infected sunflower crop residue. This disease
can appear anytime during the growing season and is favoured by
moderately high temperatures and abundant rainfall. Septoria survives
on seed and can also survive on sunflower debris.
Symptoms
Septoria leaf spot develops initially on the lower leaves and spreads
to the upper leaves. The spots begin as water-soaked areas, which are
angular with tan centres and brown margins. A narrow yellow halo often
surrounds young spots. However, mature leaf spots may contain tiny
black specks, the fungal fruiting bodies (pycnidia) (
Photo 3
).
The presence of pycnidia is the best means of distinguishing leaf spots
caused by Septoria from those caused by Alternaria. Severe infections
can cause defoliation, but if this affects only the lower leaves on mature
plants, the yield impact may be minimal.
Management
Similar management practices can be applied to control all three foliar
diseases (Alternaria leaf blight, Septoria leaf spot and rust). Cultural
management practices, such as crop rotation, destruction of plant
debris and tillage operations that bury and rapidly promote residue
decomposition, can be used to minimise disease infection.
Crop rotation helps reduce populations of many important sunflower
pathogens in the soil. The diseases can also be managed by using
resistant or tolerant varieties and the correct plant spacing.
Chemical seed treatment with fungicides significantly reduces seed-borne
leaf diseases. In South Africa there are no registered foliar fungicides on
sunflowers that can be used legally to control these diseases.
For more information on sunflower Alternaria leaf blight, brown rust
and Septoria leaf spot, contact the authors at 018 299 6100 or
and
.
Alternaria leaf blight, brown rust
and Septoria leaf spot
3
45
February 2014
