How to identify the major leaf-spot diseases

Leaf spots -- discrete, localized discolored areas on leaf tissue -- are caused by dozens of different fungal and bacterial pathogens. They vary in color, shape, size, and distribution, and the differences between them carry management implications. Not all leaf spots require treatment. Many are.

Leaf spots — discrete, localized discolored areas on leaf tissue — are caused by dozens of different fungal and bacterial pathogens. They vary in color, shape, size, and distribution, and the differences between them carry management implications. Not all leaf spots require treatment. Many are cosmetic and do not threaten plant health. Others — black spot on rose, Entomosporium on photinia — can defoliate plants repeatedly and cause significant decline.

Septoria leaf spot

Hosts: Tomato, hydrangea, many vegetables and ornamentals.

Appearance: Per Penn State Extension, Septoria on tomato produces small (0.1–0.25 inch), round spots with a dark brown to black border and a lighter tan-gray center. Tiny black specks (pycnidia — spore-producing structures) are visible in the center of spots under a hand lens. Spots coalesce and leaves yellow and drop.

On hydrangea: Per Penn State Extension, Septoria hydrangeae produces similar spots — circular, with a brown border and lighter center, often with a yellow halo.

Spread: Water splash from soil or infected lower leaves spreads conidia (spores). Per Penn State Extension, infection starts on the lower leaves first, progressing upward.

Management: Remove and dispose of infected leaves. Per Penn State Extension, fungicides containing chlorothalonil or copper are preventive — they prevent new infections but do not cure existing spots. Apply before symptoms appear in problem years.