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)

A leaf spot is a limited, discoloured, diseased area of a leaf that is caused by fungal, bacterial or viral plant diseases, or by injuries from nematodes, insects, environmental factors, toxicity or herbicides. These discoloured spots or lesions often have a centre of necrosis (cell death). Symptoms can overlap across causal agents, however differing signs and symptoms of certain pathogens can lead to the diagnosis of the type of leaf spot disease. Prolonged wet and humid conditions promote leaf spot disease and most pathogens are spread by wind, splashing rain or irrigation that carry the disease to other leaves.

Leaf spot
'Cercospora capsici.jpg
Leaf spot caused by Cercospora capsici
Causal agentsFungi or bacteria
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