A new leaf disease resembling early blight has been observed in South African potato fields, with researchers urging growers to remain alert. The pathogen, Colletotrichum coccodes, was previously considered mainly a post-harvest concern but is now appearing on potato foliage in the field.
Dr Elsie Cruywagen, specialist researcher in crop protection at the Agricultural Research Council (ARC), said at the Potatoes SA Innovation Symposium 2025 in Pretoria: "We are increasingly seeing symptoms on green stems and leaves that resemble early blight, but with no Alternaria in the samples – only Colletotrichum coccodes." She noted that testing and 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." Producers are encouraged to submit samples showing such symptoms.
Dr Cruywagen also addressed potato late blight, caused by Phytophthora infestans. While under control in South Africa, the disease can become epidemic under favourable conditions. She explained that only the A1 mating type occurs locally, preventing sexual reproduction and slowing pathogen evolution. "If you see light-green spots with sporulation on the underside of the leaf, you need to act quickly," she said.
Early blight (Alternaria solani) and brown spot (Alternaria alternata, also called malroes or "crazy blight") remain pressing issues. Fungicide testing by the ARC and Potatoes SA across 13 of the country's 16 potato-growing regions has revealed declining sensitivity to several active ingredients. "We are already seeing a loss of sensitivity to several active ingredients," Dr Cruywagen said. She added that some fungicides are incompatible, limiting management strategies.
No resistance has been observed to contact fungicides or multi-spectrum products. "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," she advised.
Nutrient deficiencies, including a lack of manganese or magnesium, can mimic early blight or brown spot symptoms. Dr Cruywagen urged growers to confirm diagnoses with laboratory testing where needed.
With shifting pathogen populations and climate pressures, disease management requires adaptation. "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 observing unfamiliar symptoms on potato plants or tubers are encouraged to send samples to the ARC's Plant Health and Protection Research Institute in Roodeplaat, Pretoria.
Source: AfricanFarming