Dark brown spots develop on the leaves starting at the tip or the leaf margins. In humid climates, these spots become water-soaked lesions. A white fungal covering can be seen on the underside of the leaves. As the disease progresses, entire leaves become necrotic, turn brown and die off. Similar lesions develop on stems and petioles. The potato tubers have greyish-blue spots on their skin and their flesh also turns brown, making them inedible. The rotting of the infested fields emits a distinctive odor.
This fungus is an obligate parasite. This means that it must overwinter in plant debris and tubers as well as on alternative hosts to survive. It enters the plant via wounds and rips in the skin. Fungal spores germinate at higher temperatures during the spring and spread through wind or water. The disease is more severe in periods of cool nights (below 18°C), warm days (18-22°C), and extended wet conditions such as rain and fog (90% relative humidity). In these conditions, a late blight epidemic can occur.
Apply copper-based fungicides before dry weather. Foliar sprays of organic coating agents can also prevent the infection.
Always consider an integrated approach with preventive measures together with biological treatments if available. Fungicide applications are important to control late blight, particularly in humid areas. Contact fungicides that coat leaves are effective before infections and do not trigger resistance in the fungi. Fungicides containing mandipropamid, chlorothalonil, fluazinam, or mancozeb can also be used as a preventive treatment. Seed treatment before sowing with fungicides like mancozeb also work.