Rhododendron leaves will turn brown for many reasons. If the tips are brown as well as the edges it could be fertilizer burn. These shrubs are not heavy feeders and have shallow roots. Fast release granular plant foods can burn them. They also droop and drop leaves that turn brown if they are planted too deep or buried in mulch. Rhododendrons, azaleas and camellias need to be just below the surface of the soil with less than one inch of mulch on top of the roots. Finally, too much cold wind or hot sun will damage the foliage with brown markings. Usually this shows up as patches of brown of the leaves and often one side of the plant is more damaged than the other with the inner more protected leaves less brown. Hope this helps and remember the most cold and wind resistant rhododendron is the PJM rhodie.