A new leaf disease resembling early blight has appeared in South African potato fields, prompting calls for farmers to be vigilant. The pathogen, Colletotrichum coccodes, was previously considered primarily a post-harvest issue, but it is now increasingly affecting potato foliage in the field.
By Jasper Raats, senior journalist at African Farming and Landbouweekblad
“We are increasingly seeing symptoms on green stems and leaves that resemble early blight, but with no Alternaria in the samples – only Colletotrichum coccodes.” This is according to Dr Elsie Cruywagen, a specialist researcher in crop protection at the Agricultural Research Council (ARC), who spoke at the Potatoes SA Innovation Symposium 2025 in Pretoria.
The ARC is urging producers to submit samples showing such symptoms, as testing for pathogens and further research are ongoing. “We don’t have all the information yet, but the fact that the pathogen is now present on living plants is an important development for us,” she said.
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Potato Late Blight
Dr Cruywagen also addressed common leaf diseases, including potato late blight, caused by the pathogen Phytophthora infestans, infamous for its role in Ireland’s Great Famine in the mid-1840s. While the disease remains under control in South Africa, it has the potential to reach epidemic proportions under favourable conditions.
The good news, she said, was that only the A1 mating type occurs in South Africa, which means the pathogen cannot reproduce sexually. This slows its evolution and allows control with currently available fungicides.
Symptoms include light-green spots with sporulation on the underside of the leaf. “If you see this, you need to act quickly,” she warned.
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Fungal Pressure Is Increasing, and So Is Resistance
Diseases of more immediate concern are early blight (Alternaria solani) and brown spot (Alternaria alternata, also known as malroes in Afrikaans, or “crazy blight”).
The ARC, in collaboration with Potatoes SA, tested fungicides on samples from 13 of the 16 potato-growing regions in South Africa. “We are already seeing a loss of sensitivity to several active ingredients,” Dr Cruywagen said. In addition, some fungicides are incompatible, which further limits farmers’ options.
Yet there is hope. No resistance to contact fungicides or multi-spectrum products has been observed so far. “The key is integrated disease management. Rotate your fungicides, avoid repeated use of the same group, and don’t base your entire spray programme on a single formulation.”
Nutrient deficiencies such as a lack of manganese or magnesium can cause symptoms that resemble early blight or brown spot. “Make sure of your diagnosis; use laboratory testing if necessary,” Dr Cruywagen advised.
With climate change and shifting pathogen populations, disease management in potatoes is entering a new era. “The evolution of blight diseases is forcing us to adapt,” Dr Cruywagen said. “We must be innovative, use data and work together to keep production sustainable.”
Farmers who notice unfamiliar symptoms on their potato plants or tubers are encouraged to send samples to the ARC’s Plant Health and Protection Research Institute in Roodeplaat, Pretoria.
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