A vast number of fungi can cause spots on the leaves of plants. Most of them are of little consequence. A typical spot 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.
Blackspot is a common disease on roses. The spots appear on the leaves and are up to 1/2 inch across with yellow margins. Severe cases cause leaves to drop. To control blackspot, plant resistant cultivars, and destroy all dropped leaves and prunings. Mulch to prevent dirt and spores from being splashed up onto plants. Bicarbonate sprays can be very helpful in preventing leaf spot diseases.