Disease (fungus, oomycete, bacterium, virus)

Potato late blight

Phytophthora infestans

A potato leaf with late blight: a large oily brown patch edged with white-grey down. — Late blight (Phytophthora infestans): an oily brown patch ringed with a fine white-grey down (sporulation), here on a potato leaf, in warm humid weather.
Photo: Björn Andersson (SLU) · CC BY-SA 4.0 · source

Role in the garden

A fungal disease, devastating in a wet summer, that destroys foliage and then tubers in a few days.

Practical guide

Where to find them

Potato and tomato.

Prevention

Choose the most resistant varieties: “Eden”, “Carolus”, “Sarpo Mira” for the potato. Space the plants for air; mulch to avoid splashing; water at the base, never on the foliage.

Early signs

In summer, after warm, humid weather, brown spots appear on the leaves. The plants collapse and eventually die; a white-grey down may appear on the underside of the leaves.

Action / control

You can only hope to limit the damage. Cut the potato stems as soon as it appears to save the tubers. Cut the attacked tomato leaves to slow the progression. Preventively: sprays of Bordeaux mixture (at a moderate dose) or horsetail decoction.

Sources

INRAE, Ephytia (protection des cultures)
Jardiner Autrement (SNHF, plan Écophyto)

Favourite plants / resources

Potato Tomato

Related plant entries

Tomato

Central plant of the tomato guild. The neighbouring basil is reputed to hinder aphids through its volatile compounds.

Potato

A nightshade with traditional planting (April) or grown under mulch (Charles Dowding) requiring zero earthing-up. Demanding in manure, it readily heads the rotation on an enriched bed (GAB/FRAB Bretagne). Not to follow or precede a tomato on the same bed (shared blight, Phytophthora infestans).