FOLIAR DISEASES OF SUNFLOWER: Alternaria leaf blight, brown rust and Septoria leaf spot

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).

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 seedborne 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 RamusiM@arc.agric.za and FlettB@arc.agric.za.