Late Blight

Problem type: Disease

Name of problem: Late blight

Plant name(s): Potato, tomato

Symptoms / Characteristics:
In potatoes, symptomatic leaves exhibit irregular, grey- green, water-soaked lesions that are often surrounded by yellowish halos. These lesions often appear on lower leaves first, but may appear anywhere on the plant where conditions are conducive to its growth. Under moist or high humidity conditions, a white mildew-like growth may border the lesions of infected leaves. This symptom is typically used as a diagnostic symptom of late blight. Lesions may also spread throughout the plant, ultimately resulting in defoliation and plant death. Reddish, depressed lesions appear on the surface of infected tubers, often near the eyes. Beneath the skin, a brownish, cork-like rot may penetrate approximately 2 cm into the infected tuber. A distinctive odor surrounding heavily infected plants is attributed to the rapid breakdown of plant tissue. This cannot be used as a diagnostic feature, as a similar odor may also occur after severe frosts and chemical vine killing.

In tomatoes, identical leaf lesions may be observed as well as the characteristic white, fungal-like growth on the leaf undersides. These water-soaked lesions also appear on infected fruit, beginning at the green-fruit stage. Fruit lesions eventually darken and become wrinkled with very distinct edges.

Late blight of potatoes and tomatoes is a fungal-like disease caused by Phytophthora infestans. Prolonged periods of cool temperatures and high moisture facilitate infection and subsequent disease spread. The late blight organism overwinters on potato tubers (cull piles and potato seed). Special spores (oospores) resulting from sexual reproduction can overwinter in the soil. Tubers typically become infected by water-runoff from infected leaves or by contact with infected leaves during harvest. Spores on infected tubers and seedlings are spread by wind to neighbouring potato and tomato fields. A similar Phytophthora species is known to attack peppers.

Control / Preventions:
Destroy all possible sources for the late blight fungus, including cull piles and volunteer plants, and plant only disease free seed. Ensure adequate spacing between plants to reduce canopy density and promote air circulation. Prolonged leaf wetness is a contributing factor towards the development of the disease. Avoid excessive irrigation and apply water only during the day in order to ensure adequate drying of the leaves. Adequate hilling provides extra protection for the tubers and reduces the potential for infection. Kill the potato vines approximately two weeks before harvest. This will reduce tuber infection by promoting the complete drying of the leaves and subsequent death of spores prior to harvest. Place only dry tubers in storage and maintain proper storage conditions, including cool temperatures and good air circulation. A protectant fungicide may be applied if optimal late blight conditions are forecast to persist. Subsequent applications may be necessary throughout the growing season. Thorough coverage is crucial, especially later in the season when a thickened canopy creates a high-humidity microclimate that is ideal for disease development. Protection during this stage of growth is particularly important. Some potato varieties have more resistance than others, but there is no variety with complete immunity. Once infection has occurred, daily removal of symptomatic plant parts may reduce further disease spread, especially during periods of dry weather.