Septoria leaf spot on tomatoes

Septoria leaf spot and early blight are the two most common causes of tomato defoliation in the eastern US, and they frequently appear on the same plant at the same time. Many gardeners report "early blight" when the actual culprit is Septoria, early blight, or both. Getting the identification.

Septoria leaf spot and early blight are the two most common causes of tomato defoliation in the eastern US, and they frequently appear on the same plant at the same time. Many gardeners report "early blight" when the actual culprit is Septoria, early blight, or both. Getting the identification right doesn't change the immediate management much — the cultural and fungicide approaches are nearly identical — but it helps you understand the disease cycle and why some seasons are worse than others.

I don't grow tomatoes at my Long Island property, so this guide draws on University Extension research.

The pathogen

Septoria leaf spot is caused by Septoria lycopersici. Per Penn State Extension, the fungus overwinters in infected plant debris in soil and on volunteer tomatoes or nightshade weeds. Pycnidia (spore-producing bodies) in infected debris produce conidia that splash onto lower leaves at the start of each season.

Unlike early blight, which affects many hosts in the Alternaria genus, Septoria lycopersici is relatively host-specific to tomatoes and a few closely related solanaceous plants.