Brown spots are signs of bacterial infection from the overgrowth of microbia that naturally exist in the soil. Overhead watering, excessive moisture from rain and dew, and poor air circulation around plants can contribute to the problem. A healthy plant like mint can usually survive, unlike sweet basil. It is important to avoid the splashing of wet soil onto the plant by watering gently around the stem and drip line and water early in the day. Make sure the plant gets 6 hrs. of good sun each day. Remove any leaf litter or yellow leaves, and disinfect pruning tools with a 1:10 solution of bleach to water. There is a broad spectrum organic fungicide on the market called "Serenade." Its ingredient, bacillus subtilis, is an effective control of leaf spot. Red veins on peppermint are normal. Crinkly leafed spearmint has green veins. Hope this helps!