Cercospora species and Colletotrichum gloeosporioides as well as are a couple of fungi that cause leaf spots on rhododendron and azalea (Rhododendron). In addition, Pestalotiopsis species often invade leaves damaged by winter desiccation and exacerbate marginal spot/blotch symptoms.
Azalea cultivars of Rhododendron species are most susceptible to Cercospora and Colletotrichum anthracnose leaf spot. Pestalotiopsis gray blight invades wounded foliage on all Rhododendron species.
Small, lop-sided to round tan-brown spots with yellow margins appear throughout the year on leaves infected with Cercospora species. Heavily infected leaves drop prematurely. The lesions caused by Colletotrichum gloeosporioides appear on azalea leaves. They are round brown areas visible on both the upper and lower surfaces of infected leaves.
Leaf blotch and marginal browning symptoms caused by winter desiccation begin to appear in late winter, although the full extent of the damage is often not apparent until early April. Pestalotiopsis gray blight worsens browning of lesions, which often coalesce causing large portions of the leaf to have a gray cast. Defoliation often follows extensive leaf discoloration.
Knowledge of the life cycles of leaf spot fungi that commonly infect rhododendron and azalea foliage is incomplete. Infections by Cercospora species and Colletotrichum gloeosporioides usually begin on immature azalea foliage in the spring even though symptoms may not appear until the winter and spring on one-year-old leaves. During wet springs fruiting structures of Cercospora leaf spot form in the leaf spots on attached one-year old leaves, while the fruiting structures of azalea anthracnose form only on leaves that have fallen during the spring. Spores blow and splash onto nearby foliage. If the leaf is wet for several hours, the spores germinate and penetrate immature leaves. At least two months passes between infection and the appearance of leaf spot symptoms. On the other hand, Pestalotiopsis species causes gray blight, which infects the injured leaves and worsens damage beyond what would normally happen by these factors alone. Fruiting structures of the gray blight fungus develop under the surface of dead portions of infected leaves and release spores during cool, wet periods all through the growing season. Wind and rain spread spores to nearby foliage. Pestalotiopsis spores invade wet leaves damaged by winter sunscald, dehydration damage, and physical wounds.
Rhododendron leaf spots seldom cause significant damage to the health of infected plants unless they are young or weakened by other harmful agents. Maintain plant vitality with proper fertilization, irrigation during dry periods, mulching, and attention to soil pH levels is the best way to minimize these diseases. Prune plants to promote, sunlight penetration, air circulation and rapid drying of foliage. Also, minimize leaf wetness by irrigating before midday so the leaves dry rapidly in the afternoon. Removal of infected fallen leaves reduces the amount of the inoculum present for new infections. Rhododendron leaf spot diseases are usually more severe after wet springs, but they rarely warrant fungicide controls. Fungicide sprays protect the new green shoots and leaves. Begin sprays as the buds swell and reapply 2-3 more times at label intervals to maintain protection during vulnerable periods.
Written by: Dan Gillman
Photos: Gary Moorman, Penn State University and R. K. Jones, Diseases of Woody Ornamentals and Trees. APS Press.