Mountain laurel leaves turning brown are a sign of infection by cercospora leaf spot, a disease that affects nearly all mountain laurels. It causes circular spots between 2 and 4 inches wide that initially show as medium or dark brown spots on both sides of the leaves. The lesion on the upper surface then transforms to more of a grayish-brown color, while the margin remains dark brown or purple. Fruiting bodies of the fungus will be scattered across the brown spots.
Environmental conditions are crucial in the healthy growth of the mountain laurel. The plant has a reputation for being difficult to establish in a garden setting and, should the soil, irrigation or sunshine not be ideal, the leaves of the plant could suffer, turning brown. It grows best in partial shade and although it is somewhat drought-resistant, the mountain laurel demands moist, well-drained soil. The plant thrives in U.S. Department of Agriculture hardiness zones 4 through 9.
An evergreen shrub that grows to between 12 and 15 feet with a width of 12 feet, the mountain laurel is often infected by borers, scale insects and lacebugs that feed on the branches and foliage of the plant, weakening the shrub and turning the leaves yellow and brown and sometimes defoliating the entire laurel. The lacebugs (Stephanitis pyrioides) deposit waste material on the surface of the leaves that appear as tiny, brown spots.
Leaf Scorch and Sun Scald
Leaf scorch turns the tips of the mountain laurel brown and may also result in twig dieback. Damage to the roots caused by borers or too much fertilizer is a typical cause of leaf scorch, as the roots are unable to supply proper nutrition and water to the leaves. Sun scald during the winter, caused by water evaporating from the leaves on warm, sunny, winter days, is also a likely culprit when the leaves of a mountain laurel turn brown.