Late blight in tomatoes and potatoes
Late blight is the most feared disease of tomatoes – and it can destroy an entire harvest within a few days. It is caused by the fungus Phytophthora infestans, which appears explosively in warm, humid weather. Tomatoes and potatoes are affected alike; often the pathogen jumps from infected potatoes onto the tomatoes.
Once it has broken out, the disease is almost impossible to stop. That is why prevention is the decisive lever here – above all, keeping the leaves dry.
In short
- Brown spots on leaves and stems, a white fungal layer on the leaf underside.
- Hard, brown patches on the fruits, which then rot.
- Spreads extremely fast in wet, warm weather.
- Prevent: rain cover, water at the base, distance from potatoes, airy position.
How to spot late blight
First, grey-green to brown spots appear on the leaves and quickly enlarge. In damp weather a whitish fungal layer forms on the leaf underside. Dark brown, sunken patches develop on the stems. The fruits get hard, brown, bumpy patches and eventually rot – even green tomatoes are affected.
Why it occurs
The fungus loves moisture and warmth. Long leaf wetness from rain, dew or overhead watering, high humidity and temperatures around 15–25 °C are ideal conditions. The spores are spread by wind and water splashes and survive on plant debris as well as on potatoes in the soil.
Preventing late blight
- Rain protection: a tomato roof or a covered spot keeps the leaves dry – the single most effective measure.
- Water at the base: never over the leaves, in the morning rather than the evening.
- Airy position: generous spacing, remove the lower leaves up to 30–40 cm, pinch out side shoots.
- Distance from potatoes: never plant tomatoes next to potatoes.
- Mulch: prevents spores from splashing up from the soil.
- Robust varieties & crop rotation: choose resistant varieties, change the spot every year.
What to do in case of infestation
- Remove immediately: cut off affected leaves and fruits generously and dispose of them in the household waste – not on the compost.
- Harvest healthy fruits: pick still-green, healthy tomatoes and let them ripen indoors in a dry place.
- Keep dry: change watering, thin out the stand.
- Copper products only preventively and as a last resort – there is no cure.
Tip
A simple roof over the tomatoes often works wonders: if the leaves stay dry, the fungus has little chance – even in rainy summers.
Caution
Never compost affected material in your own garden and never plant tomatoes near potatoes – the pathogen migrates in especially easily from there.
Frequently asked questions
Can I still eat affected tomatoes?
Do not use fruits with brown rotten patches. You can harvest healthy, firm fruits from affected plants and let them ripen.
Does the disease come back every year?
The spores survive in the soil and on plant debris. Consistent hygiene, crop rotation and a rain roof reduce the risk considerably.
Healthy, strong young plants with good roots start the season more resistant. We raise our tomato seedlings to be robust.