In mid to late summer, browning lower leaves can be a sign of dehydration, so it is important to keep up watering through the entire growing season. When a plant does not need a leaf any more, the leaf does tend to be shed by the plant and the first part of that is a browning or yellowing off. Simply remove the discolored leaves with a pinch or a small pair of garden nippers.
Earlier in the season, yellowing or browning leaves that the plant is discarding can be from a variety of natural causes. These include temperature extremes that may have damaged the leaves in question, and water extremes such as heavy rainfall or periods of dryness.
Other leaf discolorations may indicate a problem with the plant’s uptake of nutrients from the soil, or very rarely , a bacterial or fungal condition.
Usually the plant works through minor issues naturally and all we can do as gardeners is make sure that water and nutrition are good, and remove unsightly leaves. In more serious issues, the entire stem or stems is affected. If the plant is growing well above the disfiguration, remove the leaves and watch it to monitor the over all health of the plant.