In this article, we look at diseases that can affect sunflower crops in South Africa. While some are more common than others, all those listed here can cause severe yield and economic losses.
Foliar diseases of sunflower are caused by various pathogens, including Alternaria helianthi, Puccinia helianthi and Septoria helianthi. They cause various symptoms on foliage, which may result in lower sunflower yields during seasons that are favourable to their development.
The ideal environmental conditions for the development of these diseases vary, and so correct disease identification is crucial for developing appropriate disease-management strategies for each.
ALTERNARIA LEAF SPOT
Alternaria leaf spot is a major defoliating fungus that occurs in warm, humid climates all around the world, according to the Agricultural Research Council (ARC). There are many species that cause leaf spot, but A. helianthi is the most common. A. helianthi overwinters on infected plant residue, the ARC explains, but wild sunflowers can also be reservoirs. A. helianthi seeds are also wind-borne.
The optimal temperature for the germination of the fungus is above 26°C, and at least four hours of wetness are required. Seedlings are the most prone to infection.
What to look for
Alternaria causes linear spots on the stem of the plant, as well as sunken, water-soaked lesions on the back of the head, says the ARC. However, the council warns that all Alternaria species cause similar symptoms, making it difficult to identify the particular species in-field.
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