Brown spots on tomato leaves are almost always caused by a fungal disease, usually blight or septoria leaf spot. Remove the affected leaves completely and straight away, water only at the base rather than over the foliage and, if the attack is severe, treat with a fungicide approved for tomatoes.
How do I tell blight from other spots?
Blight first shows as dark, water-soaked patches at the leaf edges; they spread quickly and often have a paler halo. Septoria spots are smaller and round, often with a dark edge and a paler centre. If in doubt, compare several affected leaves: a single spot rarely tells you enough.
What conditions encourage the disease?
Leaves that stay wet for a long time are the main cause. Rain, watering from above in the evening or plants set too close together without air circulation create exactly the damp microclimate in which the spores multiply. Temperatures between 18 and 25 degrees speed up the spread further, which is why the attack becomes visible very quickly in warm, humid summer weeks.
How do I treat affected plants properly?
- Remove affected leaves and shoots completely and put them in household waste, not on the compost, so the spores do not spread.
- Water only at the base, never over the leaves, ideally early in the morning.
- Leave enough space between plants so the leaves dry quickly after rain.
- If the attack is severe, use an approved fungicide in a targeted way and according to the instructions, rather than spraying as a precaution.
Can brown spots be prevented?
Prevention is more reliable than cure. An airy position, regularly removing side shoots for better air flow and a mulch that stops soil splashing up all clearly reduce the risk. The page Maluna features shows how Maluna keeps track of this kind of care for your own tomato.