Late blight

The causal agent of late blight is the oomycete Phytophthora infestans. Classified within the kingdom Chromista and the class Oomycetes, it is not a true fungus but a filamentous protist. It remains one of the most destructive plant pathogens for solanaceous crops worldwide, particularly potato and tomato.

Symptoms appear as water-soaked lesions on leaves that rapidly turn dark brown or black, often accompanied by a white mildew-like growth on the undersides of the leaves under humid conditions. The pathogen also infects tubers, leading to reddish-brown dry rot and secondary bacterial decay.

The life cycle involves the production of sporangia, which can germinate directly or produce motile zoospores. These spores are dispersed by wind and splashing water, allowing for rapid movement between plants. The pathogen survives in infected tubers in storage or cull piles, which serve as the primary inoculum for the next season.

Epidemics are driven by cool, moist conditions with temperatures between 15°C and 20°C and high relative humidity. Under these conditions, the pathogen reproduces exponentially, potentially destroying a potato field in less than two weeks if environmental conditions remain favorable for sporangial production.

Control strategies require an integrated approach to manage the disease effectively. This includes:

• Application of preventive and systemic fungicides.
• Planting certified, disease-free seed tubers.
• Early harvesting and destruction of infected haulms.
• Monitoring weather patterns to predict infection risks.