Quick answer: Tomato blight causes brown, rotting, shrivelled leaves and fruit decay, usually appearing from early summer onwards in warm, wet weather1. Remove and destroy affected foliage fast, improve airflow, keep leaves dry, and choose blight-resistant cultivars where possible to protect your harvest1.
What tomato blight actually is
Tomato blight, also called late blight, is caused by a fungus-like organism named Phytophthora infestans14. It is one of the most widespread and damaging diseases affecting tomato crops4.
This same organism also attacks potatoes, so blight can spread between the two crops if you grow them near each other1. The disease is not caused by a true fungus but by an Oomycete, a fungus-like organism with its own distinct biology1.
When blight strikes
Blight is most common in warm, wet weather, and typically appears from early summer onwards1. In some years it can cause almost total yield loss, especially in susceptible cultivars grown outdoors1.
Tomatoes grown under cover are less likely to be affected, though they can still suffer badly at times1. If your growing season brings a run of humid, rainy days, treat that as your cue to check plants closely1.
Recognising the symptoms
The clearest signs are brown, rotting, shrivelled leaves, along with decay of the fruit itself1. Watch both foliage and fruit together, since blight rarely stays confined to just one part of the plant1.
Blossom end rot causes tomatoes to turn black and rot at the bottom end, opposite the stem, and it has a completely different cause3.
Blossom end rot is a physiological disorder linked to a calcium deficiency in the developing fruit, often triggered by low soil calcium or by soil moisture that swings between too dry and too wet3. Telling the two problems apart matters, because the fixes are not the same3.
Why wet, uneven conditions make blight worse
Blight thrives specifically in warm, wet weather, which is why prolonged damp spells are the riskiest stretch of the growing season1. Splashing water on foliage, and leaves that stay damp for long periods, give the disease exactly the conditions it needs1.
Steady moisture from a sub-irrigation reservoir, the design GardenStack planters use, keeps water at the roots rather than on the foliage, which helps avoid the wet-leaf conditions blight favours.
Fluctuating soil moisture is also named as a trigger for blossom end rot, a separate but related lesson: consistency at the root zone matters for more than one tomato problem3.
Control and prevention steps
Since outdoor tomatoes are most exposed to the warm, wet conditions blight needs, growing under cover reduces risk, though it does not eliminate it entirely1.
Choosing cultivars bred with greater resistance is worth considering if blight has been a repeat problem in your garden, since the RHS notes some cultivars are more susceptible than others1.
Good general tomato care also matters here: the same wet-or-dry soil swings tied to blossom end rot point to consistent watering as a habit worth building regardless of which problem you are guarding against3.
Common mistakes to avoid
- Confusing blossom end rot (fruit blackening opposite the stem, from calcium and moisture issues) with blight (leaf and fruit rot from Phytophthora infestans)13.
- Growing tomatoes and potatoes close together without accounting for the fact that the same pathogen attacks both1.
- Assuming greenhouse tomatoes are immune. They are less likely to be affected but can still suffer badly1.
- Ignoring symptoms that appear early in the season. Blight is noted as an early-summer-onwards problem, not something that only shows up late1.
A safety note
Frequently asked questions
Is tomato blight the same as blossom end rot?
No. Blight is a disease caused by a fungus-like organism that rots leaves and fruit, while blossom end rot is a calcium-related disorder that blackens the bottom of the fruit. They look different and need different responses.
Can tomato blight spread to other plants?
The organism behind tomato blight also affects potatoes, so keeping these crops apart is a sensible precaution if you grow both in the same garden.
Does growing tomatoes in a greenhouse stop blight?
It reduces the risk since blight favours warm, wet outdoor conditions, but greenhouse tomatoes can still be badly affected, so regular checks are still worthwhile.