Hardly any disease causes gardeners as much grief as late blight. Overnight it seems to destroy a healthy tomato plant, and the potato crop rots in the ground. It is the reason the great famine in 19th-century Ireland arose at all, and to this day it is the most feared disease in the potato and tomato bed.
But once you know its one weak point, you can effectively get ahead of it. Because behind both names sits the same pathogen, and it needs one thing without fail: moisture on the leaf. That is exactly where protection starts. Grasp this connection and you keep your tomatoes and potatoes healthy while next door the haulm is already turning black.
One pathogen, two hosts
Late blight is caused by Phytophthora infestans, a fungus-like water mould. Botanically it is not a true fungus but an oomycete; in the garden, though, you treat it like a fungal disease. Both hosts, potato and tomato, belong to the nightshade family, and the pathogen uses both.
It usually breaks out first in the potato patch, and the spores blow from there onto the tomatoes. That explains the most important rule: do not plant potato and tomato next to each other. The dead, infected potato plant is the most common source of infection for your tomatoes. Whoever grows both keeps a deliberate distance.
Why the disease has two names
The double name tells its story. On the potato it is called haulm blight, because the above-ground haulm is attacked first and dies off brown. On the tomato it is called fruit blight, because there it mainly makes the typical brown, hard patches on the fruit. The pathogen is the same in both cases.
Its cycle begins in the soil. Phytophthora infestans overwinters on infected potato tubers that are left in the ground or rot in store. From such tubers, and from so-called volunteers, that is potatoes sprouting again in spring, the first infection starts. From there the pathogen spreads by spores that wind and rain splashes carry over long distances. A single warm, wet spell in high summer can catch a whole stand this way.
How to prevent the blight
Keep the foliage dry
Always water from below, straight at the root, never over the leaves. Best in the morning, so whatever does get wet dries off. The pathogen cannot colonise dry foliage.
Roof over your tomatoes
A simple rain roof or a tomato house keeps the rain off and is the single most effective protection outdoors. Under the roof the leaves stay dry even in damp weather.
Plant airy and pinch out
Give the plants plenty of space and pinch out the tomato consistently. An airy, open plant dries off quickly again after dew and rain.
Remove the lowest leaves
Take off the lowest leaves up to about 30 cm once the plant stands strong. Then no spores reach the foliage through splashing soil, and the air moves through better.
Keep your distance from the potatoes
Never set tomatoes near the potato bed. And at the first brown spots, cut off the potato haulm; that protects the tubers and takes the base away from the pathogen.
Remove anything affected at once
Cut out diseased leaves and shoots straight away and put them in the residual waste, not on the compost. Every affected leaf left hanging scatters new spores.
Recognise it on leaf, fruit and tuber
The disease shows differently depending on the plant part. On the leaf, greyish-green, watery-looking patches appear first, quickly turning brown-black, often starting at the leaf margin. In damp weather a delicate, whitish fungal down sits on the leaf underside, the surest sign. On the stems, dark, elongated streaks form.
On the tomato fruit, hard, brown, leathery patches form, often starting from the shoulder of the fruit. On the potato, the drama plays out below ground: infected tubers show a brown, rusty rot that pulls from the skin inward.
The right variety takes off the pressure
The pathogen cannot be fully prevented, but you can put up a lot against it with robust varieties. In recent years tomatoes have been bred that are noticeably less susceptible.
How to lower the risk for good
- Choose tolerant tomato varieties
Varieties like Tomato 'Phantasia', Tomato 'Philovita' or Tomato 'Primabella' shrug off the attack for a long time. Out in the open, that is half the battle.
- Mulch against splash water
A layer of mulch prevents spores splashing from the soil onto the lower foliage when you water and when it rains. That cuts off an important route of infection.
- Keep the rotation
Do not grow tomato and potato in the same spot year after year. A break of several years denies the pathogen its base in the soil.
- Start early and healthy
Strong, well-fed plants defend themselves better. Aim for balanced feeding without a nitrogen surplus, because soft foliage falls ill more easily.
Tidy up at the end of the season
The best prevention for next year happens in autumn. Because the pathogen overwinters on tubers in the soil, a clean autumn decides the infection pressure of the coming season.
Get every potato out
Dig the bed over thoroughly and pick up even the small tubers. Every forgotten tuber can sprout again as a volunteer in spring and become the first source of infection.
Do not compost affected haulm
Diseased potato and tomato haulm belongs in the residual or organic waste bin, not on the open garden compost. There the pathogen would survive.
Store no diseased tubers
Sort out infected tubers strictly. A single rotting tuber quickly infects the whole box in store.
The blight needs wet foliage. Keep the leaves dry and roofed, and you have already half beaten it.
The core rule against late blight
Frequently asked questions
Can I still eat affected tomatoes or potatoes?
Fruit and tubers with brown rot belong in the bin; they taste rotten and spoil fast. Still healthy, firm tomatoes on an affected plant you can pick and use quickly. Healthy tubers from an affected stand keep worse, so eat them soon.
Does the disease really jump from potato to tomato?
Yes. Both are nightshades, and the same pathogen attacks both. It usually starts in the potato patch, and from there wind and rain carry the spores onto the tomatoes. That is why you should not plant the two next to each other.
Does removing affected leaves still help?
Only at the very start and only as a support. If you remove the first spots at once, you delay the spread. Once the plant is heavily affected, the blight can no longer be stopped, and then only prevention next year helps.
Why does it often break out after a thunderstorm?
Because thunderstorms deliver exactly what the pathogen needs: wet foliage in mild warmth. If it stays muggy afterwards, the spores germinate on the damp leaves. A few warm, wet days in high summer are enough for an outbreak.
How do I tell late blight from early blight?
Late blight makes watery, brown-black spots, spreads very fast and carries a whitish fungal down in the damp. Early blight shows dry, round spots with concentric rings, usually first low on the plant, and is far less dangerous.
Are some tomato varieties really resistant?
Yes, there are clearly more tolerant varieties such as Tomato 'Phantasia', Tomato 'Philovita' or Tomato 'Primabella'. They are not spared entirely, but they fall ill later and more mildly, a big advantage out in the open.
Can I take seed or tubers from an affected stand?
From potatoes, better not, because the blight can survive in seemingly healthy tubers. Use certified seed potatoes. Tomato seed from fully ripe, healthy fruit is fine, on the other hand; the pathogen practically does not transmit through the seed.
