Blight

Phytophthora infestans

also called Late blight, Potato blight, Tomato blight

Dark spreading lesions on leaves and stems of tomatoes and potatoes that collapse the plant in warm wet spells. Grow resistant varieties, space well, and remove affected plants at once.

Water mold No cure Airborne sporesRain splashSpread in water Spreads to neighbors

What you'll see

Dark brown-black blotches with a faint yellow-green rim, usually starting at leaf tips and margins.
In humid mornings a fine white mold rings the underside of each lesion.
Brown-black lesions run up the stems and leaf stalks, which is when collapse becomes certain.
The whole haulm goes from spotted to slumped and foul-smelling in three or four days.
Fruit and tubers develop firm, sunken, red-brown marbling under the skin, unlike the concentric target rings of early blight.

Why it happens

A water mold, not a true fungus, that survives in living tissue only — in infected tubers left in the ground or in a store, in a cull heap, or on volunteer potatoes. Spores are carried on the wind for miles, then need a wet leaf to germinate. Once inside, it kills tissue outright rather than merely spotting it.

Warm, humid spells — two consecutive days of at least 10°C with humidity above 90 per cent for six hours, the classic Smith period. Outbreaks run from midsummer onwards in a wet season, and travel fast through crowded, damp foliage.

Treating it

There is no cure for an infected plant; act to save the crop, not the leaves.
On potatoes, cut the haulm to the ground at once and bin or burn it, then leave the tubers in the ground for two to three weeks before lifting so spores on the soil surface have died.
On tomatoes, pull the plant entirely — outdoor tomatoes rarely recover — and pick any sound green fruit to ripen indoors.
Never compost blighted material or leave tubers in a cull heap; both start next year’s outbreak.
Disinfect canes, ties and greenhouse surfaces before the next season.

Keeping it away

Grow blight-resistant varieties such as the Sárpo potatoes and modern blight-resistant tomatoes.
Plant early potatoes and lift them before the main blight season arrives.
Grow tomatoes under cover, keep the leaves dry and water only the soil.
Space and stake generously, and remove lower leaves so air moves through the row.
Rogue out volunteer potatoes each spring and never plant into ground with tubers left in it.

Who gets it

Potatoes and tomatoes above all, with aubergines, tomatillos and a few related weeds occasionally taken.