Rhododendrons encounter diseases, fungi, and climatic issues that can result in brown spots, or even entirely brown leaves. Discovering the cause will make treatment options much more effective, and employing basic preventive practices will enable you to avoid the problems in the future. Drought could have been a major factor in your Rhododendrons brown leaves. During Summer, it is recommended that you check plants under eaves where rainfall may not penetrate even on the wettest days. Analyze summer watering systems to be sure plants are getting well soaked down into the roots. A deep watering twice a month in summer will protect rhododendrons better than frequent light sprinkles grazing the top of the roots. Rhododendrons planted in too much sun can show drought symptoms even if they are watered. The general rule is that they need protection from direct hot sun. Late afternoon sun is the most damaging. Filtered light is best, or morning light with afternoon shade. If too much fertilizer has been applied, and has not washed down into soil, you'll see accumulated salts (a whitish crust) from fertilizer. This too can result in brown leaf edges. Water deeply to reduce salts and avoid over application of fertilizer. Rhododendrons grow well if fertilized before they bloom, and immediately after they bloom. Other causes: There are some fungi that periodically affect Rhododendrons. One common occurrence is referred to as, "Phytophthora Dieback.” There are several different species of phytophthora, each with slightly different signs and symptoms. In general, though, this fungus affects newer foliage with brown spots at first, followed by more spots on older foliage and stems. Eventually, leaves will dry out and fall off. The warmer the weather, the faster the spread. The best treatment is to prune away all infected parts of the plant . If a more significant infection is underway, purchase a fungicide safe for rhododendrons at a local gardening supply store and cover the plant thoroughly. Preventative Practices Disease prevention practices will prevent the spread of phytophthora fungi. When watering, avoid splashing or sprinkling; instead, position the hose at the roots. Most diseases and fungi spread via moisture, so avoid any activities that spread moisture from plant to plant or leaf to leaf. Additionally, over-fertilization often results in an increased likelihood of infection. As with fertilizing any plant, applications of fertilizer should occur after soil tests confirm a definite need.  For more information see

http://www.extension.umn.edu/garden/yard-garden/flowers/azaleas-rhododendrons-for-minnesota/