Bleach can be helpful in disinfecting pruning tools.
There are several different rhododendron diseases, the most serious of which are root rot and branch dieback. Other rhododendron diseases include petal blight, leaf bile, and leaf spot. Growing these plants under optimal cultural practices can help prevent most rhododendron diseases and pests.
Root and crown rot is the most serious disease of rhododendron in many parts of the country. It is caused by various species of Phytophthora Oomycetes, also known as water molds. Fungicides do not cure this disease, so it is important to prevent it with proper planting. This disease is a problem when the soil has excessive amounts of water.
Rhododendrons should be planted in soil with lots of organic matter and should not be planted too deep so that their canopy is not covered. It is also important that the soil is well drained. The first sign of this disease is usually wilting, with blackened or waterlogged roots. It may be possible to save the plant by repotting it with improved drainage and organic mulch.
If the plant dies, it is not a good idea to immediately plant another in its place, as the fungus normally remains in the soil. Even resistant varieties will succumb in these conditions. It may be possible to continue growing rhododendrons at this location if the soil is improved.
Another serious rhododendron disease is branch dieback caused by a fungus called Botryosphaeria dothidea. This disease is first visible on apparently healthy plants when dying branches appear. If the bark is scraped with a knife, there will be a reddish-brown discoloration on the dying branches.
Dieback is a disease of rhododendron that is difficult to control. Infected branches should be pruned well below the discolored wood and then discarded. Sanitize pruning tools between cuts with a 10% bleach solution or 70% isopropyl alcohol and apply a copper-containing fungicide.
Leaf gall is a common spring disease of rhododendrons, but it is not particularly serious. affected plants have leaves, shoots, or stems that develop swollen, distorted growths in April or May. Infected leaves are sometimes covered with white spots. This is caused by various species of the Exobasidium fungus. Control consists mainly of removing the galls and eliminating them.
Leaf spot is caused by a group of rhododendron diseases consisting of various fungi. Septoria, Colletotrichum, Phyllosticta, and Cercospora species are responsible for this primarily cosmetic disease. Remove fallen leaves and be careful when watering to keep them dry. Severe cases can cause premature leaf drop and can be treated with fungicides.
Another common rhododendron disease is petal rust, caused by the fungus Ovulinia azaleae. This manifests as spots on the flowers that cause them to rot. It can be easily spread from one flower to another by rain, wind, and insects.
This fungus lives in the soil, so replace the ground cover with a mulch that has not been contaminated with petal rust. An important additional treatment and control measure is to destroy all flowers showing signs of infection. Also, avoid watering from above and only water plants from below.
