Septoria leaf spot
A fungal disease that covers lower leaves in small round spots with dark edges and pale centres, then strips plants of foliage from the bottom up.
Also known as: Septoria blight, Leaf blight
Symptoms and identification
Many small, circular spots, 3 to 6 mm across, appear on lower leaves first. They have grey or tan centres, dark brown margins and tiny black specks in the middle. Heavily spotted leaves turn yellow, wither and drop, leaving stems bare and fruit exposed to sunburn.
Causes
The fungus survives on plant debris and in the soil over winter. Spores splash up onto lower leaves in rain or overhead watering, and the disease spreads fastest in warm, wet or humid weather.
Organic treatment
Pick off the lowest spotted leaves as soon as they appear and bin them, not compost them. Spray a copper-based or sulphur-based fungicide approved for organic use every 7 to 10 days during wet spells.
Prevention
Water at the base of the plant and mulch to stop soil splashing up. Stake or trellis plants for airflow, remove fallen leaves at the end of the season and rotate susceptible crops so they do not return to the same bed for three years.
Chemical treatment (use with care)
Use a fungicide labelled for Septoria leaf spot on that crop, starting at the first spots and following the label for interval and harvest wait.