Septoria leaf spot

What you see

Small round spots, 2-4 mm, with a pale grey centre and a dark brown margin, peppered across the lowest leaves first. Look closely at a spot centre and you will see minute black dots - the fruiting bodies. Leaves yellow, then brown, then hang dead on the stem while the plant above keeps growing.

Conditions

Warm and wet, 20-25C with long leaf wetness. Splashing rain or overhead watering moves it up the plant one tier at a time. Overcrowded, unstaked plants that never dry out are the classic victims.

Treatment

Strip and bin every spotted leaf the moment you see it - do not compost. Copper or a Bacillus subtilis biofungicide will protect the clean leaves above but cannot cure a spotted one. Water the soil, never the foliage, and only in the morning.

Prevention

Three-year rotation away from tomatoes. Mulch heavily so rain cannot splash soil onto leaves. Stake and prune for airflow, and clear all tomato debris in autumn - the fungus overwinters on it.

Months: May, June, July, August, September, October

Plants

Tomato, Aubergine, Potato

Remedies

Bordeaux mixture (copper), Bacillus subtilis biofungicide, Mulch

The thing people get wrong

This is the disease people mistake for early blight and then treat wrongly. Septoria spots are small, numerous and uniform; early blight makes larger lesions with concentric rings like a target. Septoria almost never touches the fruit - so if your plant is losing leaves from the bottom but the tomatoes are perfect, it is this, and the answer is airflow and mulch rather than panic.