More Haloes on Milkweed Leaves
The small black dots in the lower part of the leaf may look like ozone injury, but look closely. They are surrounded by discolored zones or haloes. The haloes in this picture are yellow, not purple like in the previous image. Yellowing indicates a loss of chlorophyll and the cells adjacent to the lesion are becoming "sick" or infected.
There are many leaf diseases. Fungi, bacteria, and viruses can all cause problems for plants. Because of this, knowing the exact cause of a leaf spot disease is often difficult.