Mint is a fast growing herb that grows best in full sun to partial shade. Although it likes moist soil, overwatering leads to disease. You should give it 1 or 2 inches of water every week, depending on the temperature and soil. Proper sunlight and watering will prevent or minimize most diseases, but there are several reasons why your plants might develop yellow leaves.
If the leaves curl, pucker or turn yellow, examine them for aphids that might be eating the leaves. Aphids are small green or black insects with soft bodies. You can get rid of aphids by spraying the mint with a jet of water that washes the insects away, or by using insecticidal soaps.
Verticillium wilt is caused by a fungus in the soil. Symptoms of this diseases include curling and twisting of the upper leaves and stunted growth. Infected leaves turn yellow and bronze. Once the fungus gets into the soil, it can stay there for years. Native spearmint is fairly resistant to this wilt, but Scotch spearmint is not.
Powdery mildew causes a gray, powdery fungus to grow on the leaves. Leaves then turn yellow and fall off. Several chemicals can be used to control this disease, but many of them can't be used within 30 days of harvest.
Rust is another fungus that attacks spearmint and peppermint plants. Symptoms include rusty-colored "blisters" on the bottoms of the leaves. Leaves turn yellow and fall off. Mint can be flame weeded with propane to control rust, or chemicals can be used.