Cercospora leaf spot on Rhododendron. In addition there are many different fungi that have been found less frequently in association with leaf spots on rhododendron in the PNW. Brown spot often occurs in rainy season and humid weather. This looks like the typical damage done by one of the fungal leaf diseases of Rhododendron. There are several types of fungus that can cause this. Ovulinia petal blight Ovulinia azaleae Ovulitis azaleae [anamorph] Leaf Spots Various Fungal Species. Rhododendrons can be affected by a condition called "physiological leaf spot." Cercospora handelii When the fungus kills an area of the leaf the dead tissue drops out, making it look like something is eating the leaf. Fungal Disease Powdery Mildew . Even pests like aphids, bark beetles, spider mites, scales, weevils, and caterpillars can attack this plant. The spot's upper surface may feel bumpy due to the fungal fruiting bodies (pycnidia). Unfortunately, the lovely plant is prone to disease and pests. Exobasidium leaf spot Exobasidium burtii. Several fungi cause leaf spots on rhododendron. Cases of severe infection may result in early leaf drop, reducing the general health of the plant. Infected leaves display dark brown spots which may cover up to half of the leaf surface. Rhododendron Leaf Spot. This condition is caused by poor root health, improper irrigation as in over- or under-watering, drought and extreme heat exposure. But when you can see brown spots where the leaf tissue hasn't fallen away yet, you know that it's a fungal problem not an insect problem. They often infect through wounds. When Rhododendron occurs, there will be small brown spots on the leaf surface, which will gradually form irregular large spots, and then the leaves will fall off, affecting the flowering of the year and the development of flower buds in the next year. In general, most fungal leaf spots are not threatening to the health of the plant, although under severe conditions some defoliation can occur. There are various other rhododendron diseases, like leaf spots, and root and crown rot. Cercospora leaf spot) Circular to irregular brown spots up to 1/2 inch in diameter form on leaves, especially lower leaves. Any rose fungicide will do: myclobutanil (systhane), fungus fighter, roseclear etc. While leaf spots can be controlled with a fungicide, root and crown rot can be prevented to some extent by planting rhododendrons in well drained soil. Symptoms Spots are dark brown, zonate, and frequently cover half the leaf area. Rhododendrons are lovely plants with enchanting blooms that range from dark red to lavender. There are a number of different Rhododendron diseases, with the most serious being root rot and dieback of the branches. Lophodermium leaf spot Lophodermium schweinitzii. With the humid summers here in SC, I always seem to have at least one Rhododendron with this problem. The plants are able to survive and bloom for many years. Rhododendron : Leaf spot (revision date: 4/11/2018) Use Integrated Pest Management (IPM) for successful plant problem management. Brown spots on rhododendron leaves may not be a sign of disease. Dark-brown, pimple-like fungal fruiting structures form within the spots. Brown Spots On The Leaves. Spots on rhododendron leaves. Glomerella canker and dieback Glomerella cingulata Colletotrichum gloeosporioides [anamorph] Gray blight Pestalotia sydowiana. Spots may become tan in the center and may have a yellow halo. Leaf and flower gall Exobasidium vaccinii = Exobasidium azaleae. Some of the most common dangers to your rhododendron include aphids, root rot and Japanese beetles. symptoms are pale spots on upper leaf surface, with corresponding brown/grey patches underneath: Control with fungicide from late May onwards. A second type of Rhododendron issue is brown spots on the leaves. Other diseases of Rhododendron include petal blight, leaf gall, and leaf spots.Growing these plants under ideal cultural practices can help prevent most Rhododendron … Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.or. Biology. What causes it? Throughout the year, fungal spots (Cercospora species, Septoria species, Phyllosticta species and Colletotrichum species) of various colors appear on azalea and rhododendron leaves.The diseases caused are usually minor, only affecting the aesthetic value of the plant. Infected leaves may fall.