Septoria leaf spot is a fungal disease caused by Septoria lycopersici. It usually begins on the lower leaves, where small circular spots develop with dark margins and pale grey or tan centres. Tiny black fruiting bodies may appear in the centres of older spots.Spores spread mainly by splashing water from infected plant debris or leaves. Warm, wet or humid conditions favour infection, and repeated infections can work their way rapidly up the plant. Severe disease can defoliate tomatoes, exposing fruit to sunscald and reducing plant vigour and yield. The fruit itself is rarely infected.

How to Fix It
Remove infected lower leaves as soon as you spot them and dispose of them rather than composting them. At the end of the season, remove infected tomato debris from the garden. Rotate tomatoes and other susceptible solanaceous crops away from the affected area where practical.Keep foliage as dry as possible. Water at soil level, mulch beneath plants to reduce soil splash, provide good spacing and airflow, and stake or cage plants to keep foliage off the ground. Avoid working among plants while the leaves are wet.Biological fungicides containing Bacillus species or copper-based products may help protect healthy foliage, but they work preventively rather than curing already infected leaves. For a home garden, sanitation and keeping foliage dry are the most important controls.