The UConn Plant Diagnostic Lab (https://plant.lab.uconn.edu/) receives many samples of rhododendrons as well as emails from clients wondering why their plants have dead stems and dieback. Rhododendrons are very common landscape plants loved by many homeowners, but they can have a few disease issues. One common disease that we see frequently is Botryosphaeria dieback.

Botryosphaeria dieback is caused by the fungal pathogen, Botryosphaeria dothidea. Symptoms include wilting leaves that roll inwards along the midvein with dead leaves remaining attached to stems. Leaves begin to turn dark green to brown and eventually die because stem cankers block and girdle the stem. Reddish-brown and sunken cankers are found on stems and branch tips. Stem discoloration can extend to the wood and pith, and a cross section of the stem often shows a darkened wedge pointed towards the center of the stem. The infection can extend down branches into the main stem, killing the plant. On older plants, cankers and dead wood can develop black pycnidia, which are fruiting structures of the fungus.

Botrysphaeria dieback on rhododendron. Photo by Elizabeth Bush, Virginia Polytechnic Institute & State University, Bugwood.org

Infections can occur through natural wounds, pruning wounds, leaf scars, or dead branches. Exposure to conditions such as heat or drought stress can make plants more susceptible to infection. Plants exposed to full sun are frequently infected.

Botryosphaeria can also be confused with symptoms of phytophthora infection. Botryosphaeria cankers usually develop more slowly than phytophthora cankers. Phytophthora cankers can extend to the leaves and form brown wedges of discoloration at the bases of leaves and down the midvein. Winter injury symptoms can also look like Botryosphaeria, including tip dieback and leaves rolled along the midvein. Young transplants that have not been acclimated to winter conditions or entered dormancy are often affected by winter injury. Winter injury can be avoided by planting cold tolerant cultivars and following good cultural practices.

Note the reddish color of stem tips and leaves along the midvein. Photo by David L. Clement, University of Maryland, Bugwood.org

Planting site characteristics such as light exposure and soil drainage are important things that diagnosticians consider when a plant is showing symptoms of disease. It is important to know how often your rhododendron plants are getting watered and if the soil has good or poor drainage. Proper irrigation and fertilization can help prevent plants from becoming susceptible to infection. Rhododendrons should be planted in partial sun and watered during periods of drought. Fertilizer applications in late summer or early fall can cause plants to push out new growth that does not harden off properly for winter conditions. These stems will likely suffer from cold injury and become entry sites for infection in the spring.

Managing Botryosphaeria dieback includes the preventative controls listed above. Additionally, all diseased or dead tissue should be pruned away during dry weather. Make pruning cuts at least 6 inches below where symptomatic tissue ends and sanitize tools between cuts with 10% bleach solution or 70% alcohol. Remove all diseased tissue to prevent the disease from overwintering or spreading to healthy plants. Either place in trash or burn it. Following good cultural practices is always recommended when managing disease. Avoid overhead or sprinkler irrigation, as this causes prolonged leaf wetness and aids in disease spread. Avoid wounding plants and encourage good air circulation by clearing the area around plants of any debris or weeds. Sanitation and cleanliness of the planting site is important and often overlooked. Remove any leaves or branches that have fallen to the ground throughout the growing season and do one last sweep in the fall. Fungicide applications are not effective or recommended for use against this disease, so preventative and cultural controls are most important for management.

Lillian Borbas

Rhododendron Problems

Rhododendron damage by. Carol D. Quish, UConn HGEC

Rhododendron damage by. Carol D. Quish, UConn HGEC

The rhododendron population in Connecticut is not looking very good this spring. Lots of dead branches and entire bushes have succumbed to a cold winter, desiccation, disease and insect damage. Cultural problems may also cause their decline.

What I am seeing in the UConn Home and Garden Education Center are examples of branches killed by the Rhododendron Borer (Synanthedon rhododendri).  The branches look drought stressed and foliage turns from green to yellow to drying and brown. There might be an entry hole spilling out sawdust and frass (insect excrement) from the larva chewing inside the tunnel within the branch. Look for the material on the ground below where the insect pushed it out of the tunnel.

The larva overwinters in the branch, pupating in the spring close to the exit hole. Once changed into the adult phase, it is a clear winged moth. The adult moths begin to emerge in late May staying active through June. Adults do not feed, only mate, then the female lays eggs on rhododendron near wounds, bark crevices, branch crotches and pruning sites. Once eggs hatch into larvae, they chew into the branch. One generation occurs per year. Prune off the damaged branch below the tunnel and discard in the trash. An insecticide containing permethrin sprayed on the bark of larger branches and trunk of the shrub during the end of May through early June will target the adults and prevent egg laying.

Winter injury is another leading cause of dieback. Here in CT, we had a couple of mild temperature years. This past year was colder than normal with some rapid temperature swings.  A lot of wind was also present drying the evergreen broad leaves resulting in desiccation. Prevention for next year includes wrapping plants or erecting wind breaks. Also water in the fall to have plants fully hydrated before freezing weather sets in. Use an anti-desiccant in fall to protect leaves from losing too much moisture through the winter. Remove damaged limbs now to help plants heal. No need for the shrub to support dead branches.

Fungus attack is another possible reason for dead branches. Botryosphaeria and Phytophthora are two fungi that affect the water uptake of the plant. Fungicides are not very effective against either disease. Prevention is the best defense. Do not let soil become waterlogged. Phytophthora loves wet soil. Rhododendron wants well drained soil. Botryosphaeria prefers rhododendrons stressed by drought. Leaves are affected near the tips or on the margins with spots forming on the leaf. Leaf stem, twigs and then branches are then infected. Pruning infected parts is the recommended control measure.

The best defense against all diseases and insects is a healthy plant and good cultural conditions. Provide what the plant likes and problems will be less likely. Plant rhododendrons in light shade as an understory plant. Do not plant them too deeply, keep the basal flare at soil surface level.  Soil should be well drained, acidic and with a high organic matter level. Keep roots cool with a one inch layer of mulch but never let mulch touch the bark at the base of the shrub. Avoid harsh wind-swept sites to avoid desiccation.

-Carol

rhododendron by Carol Quish, UConn HGEC

rhododendron by Carol Quish, UConn HGEC

Rhododendron Leaf Curl

Checking the winter landscape recently, I noticed the rhododendrons looking most distressful. Their leaves were curled under into green bean looking tubes droopily hanging from the branches. It was during the very cold weather experienced here in Connecticut last week. I have seen this other years in the same plant so I know they will recover. The evergreen leaves will uncurl and perk up once the temperatures rise to about 35 degrees F.

I did a little research to find rhodies are ‘thermotropic’; sensitive to temperature changes and respond with leaf movement. Charles Darwin wrote a book in 1880 titled ‘The Power of Movement in Plants’ in which temperatures causing movement is covered. As air temperatures drop below 35 degrees, the curling and drooping begins. The lower the temps drop, the tighter the curl and more vertical the hanging leaf. Some people have come to recognize the actual temperature by how far their rhododendron leaves have curled and drooped. Different species of rhodies curl at varying temperatures, so you will have to watch your particular plant and the thermometer to develop this talent!

Rhody leaf curl is widely thought to be a protective measure taken by the plant to ward off the drying winter air and winds causing moisture loss. The rhododendron’s leaves have tiny valve openings on the undersides called stomata. This is where the plant releases moisture. When the leaf curls, the stomata are concealed. The vertical drooping catches less wind than a horizontal leaf, resulting in less drying.

Another theory proposed goes into more detail saying the curling is to protect the leaf from the sun. Rhododendrons naturally grow in part shade but in winter the deciduous trees are lacking leaves exposing the evergreen rhodies to more light than in summer. This causes the leaf temperature rise, thaw out and make food in the leaves. When night comes, the temperature drops, freezing the leaves and water in the leaf. Water expands as it freezes, forming ice crystals in the leaf cells, cutting the cell walls. Leaf curling reduces the amount of leaf tissue exposed to the sun therefore reducing the amount of photosynthesis taking place. It is the daily thawing and freezing causing the damage.

Time (and research) will tell which theory proves correct. Or, it may end up being a combination of the two. Either way, they both say that Rhododendron’s leaves curl in below freezing weather to protect the leaves from being damaged. I will watch them for a little sign of spring as I wait for them to uncurl and stay there!
-Carol

Rhododendron leaf curl - Carol Quish

Rhododendron leaf curl - Carol Quish