Late blight

Phytophthora infestans

also called Potato blight, Tomato blight

The one that takes a tomato crop in a week: greasy gray-green blotches, white mold underneath, and a whole plant black within days. There is no cure — pull it and protect the rest.

Water mold No cure Airborne sporesRain splashSpread in waterSoil-borne Spreads to neighbors

What you'll see

Greasy, water-soaked gray-green blotches on the leaves, often starting at the tip or the margin, with no yellow halo around them.
A ring of fine white mold on the underside of the blotch in humid weather, exactly at the edge of the dead tissue.
Dark brown or black lesions running up the stems and leaf stalks, which is what kills the plant rather than the leaf damage.
Firm, greasy brown patches on green tomatoes that spread from the shoulder and turn the fruit inedible.
Potato tubers with a coppery-brown marbled rot under the skin, going soft and foul in store.

Why it happens

A water mold, not a true fungus, and the same organism that caused the Irish famine. It cannot survive a hard winter on its own; it comes back each year from infected potato tubers left in the ground or in a cull pile, and from spores blown in on weather systems from other gardens and fields. Once it lands on a wet leaf it is inside within two hours, and spore-producing within four days.

It needs cool nights and wet leaves — the classic trigger is two consecutive days with a minimum above 10 °C and humidity over 90 per cent for eleven hours. That means a damp, close spell in July and August, an overhead-watered bed, or fog and dew in a coastal summer.

Treating it

There is no cure for an infected plant. Pull it whole, bag it, and put it in the bin or burn it — do not compost it and do not leave it lying in the row.
On potatoes, cut the haulm off at the ground the moment blight reaches the stems and leave the tubers underground for at least two weeks so the spores on the surface die before you lift.
Pick any sound green tomatoes immediately and ripen them indoors; fruit already marked will rot however carefully you store it.
Stop overhead watering entirely, and do not work among wet plants — you will carry it down the row on your sleeves.
Copper fungicide only protects tissue it has already coated, so it is a shield for healthy neighbors, never a treatment for a plant that is showing symptoms.

Keeping it away

Grow resistant cultivars — the Mountain Magic and Defiant tomatoes and the Sarpo potatoes are genuinely resistant, not merely tolerant.
Lift every potato, including the small ones, and never leave a cull pile or a volunteer to overwinter the disease for you.
Space widely, stake and prune the lower leaves so the canopy dries by mid-morning.
Water at the base, early in the day, and mulch to stop soil splashing spores upward.
Watch a regional blight-warning service and get the copper on before the humid spell, not after it.

Who gets it

Tomatoes and potatoes above all; occasionally peppers, aubergines and petunias, which share the family.