patches of disease or other damage on plant foliage.

Merriam-Webster Online Dictionary
leaf spot (noun)
any of various plant diseases characterized by discolored often circular spots on the leaves
leaf spot (Wikipedia)
Leaf spot
'Cercospora capsici.jpg
Leaf spot caused by Cercospora capsici
Causal agentsfungi or bacteria

Leaf spots are round blemishes found on the leaves of many species of plants, mostly caused by parasitic fungi or bacteria.

A typical spot is "zonal", meaning it has a definite edge and often has a darker border. When lots of spots are present, they can grow together and become a blight or a blotch. Fungal spots are usually round or free-form in shape.

In most cases, leaf spots are considered to be only a cosmetic problem, but they can have economic effects on nurseries or in floriculture.

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