A rhododendron infested with lace bugs (Stephanitus rhododendri) will exhibit black discolouration on the underside of its leaves. The top portion of the leaf will appear mottled with spots of grey and white. Phytophthora spp. and Botryosphaeria dothidea fungus cause the foliage of the rhododendron to blacken and die.
The black speckling on the underside of the rhododendron leaf is caused from the excrement of the insect. The black nymph lace bug also congregates on the leaf's underside, which adds to the black discolouration of the foliage. The fungal infections Phytophthora spp. and Botryosphaeria dothidea cause the stems and foliage to die.
Applying horticultural oils to the top and underside of the rhododendron foliage helps control the lace bug. Systemic insecticides containing acephate also work well, according to the University of Connecticut's website. Prune the dead stems of rhododendrons infected with Phytophthora and Botryosphaeria dothidea. Apply fungicides that contain chlorothalonil, copper hydroxide or Bordeaux 4-4-100, according to the Clemson University Extension.
Planting the shrub in a shady location helps prevent infestations. Washing away the young nymphs and the black excrement in the spring helps prevent damage. To prevent the rhododendron from contracting Phytophthora spp. and Botryosphaeria dothidea, plant the shrub well-draining soil. Avoid excessive watering.