Nothing is wrong with this leaf, no disease. This is the under-surface of a fern leaf. Inside these spots, called “sporangia” (singular, “sporangium”), the fern produces spores. Each spore is a single cell, surrounded by a coat made by a partner cell. When released to the wind, a spore can travel great distances from the parent fern. If it lands on moist and otherwise suitable soil, it can grow into a new fern plant. You can see several examples of sporangia, of different shapes, colors, and spatial arrangement on fern leaves of different species, in NPG's Plant Evolution Garden on this campus.