Late blight

Phytophthora infestans

Symptoms

Leaf

• Green-brown watery spots, not sharply defined
• Later the spots turn brown
• Lesions surrounded by white fungal mycelium on the underside of the leaf

Stem

• Long brown spots develop on the stem
• Sporulation on the stems is also possible
• Infected stems are more likely to break
• Fungus can remain in the stem for a long time
• Infection via leaf axil or infected seed tuber

Infection and spread

• Epidemic develops from dumps, groundkeepers and infected seed tubers
• Survives in infected tubers (mycelium) and zoospores in the soil
• Sporulation is dependent on relative humidity and temperature
• Infection possible at any time during the growing season

Possible confusion with

• Grey mould (Botrytis cinerea)
• Early blight (Alternaria solani)