Insect and Disease Fact Sheet Compliments of
New Century 
1-877-79TREES
Maintaining Healthy
Rhododendrons & Azaleas in the Landscape
Rhododendrons and azaleas, which are closely
related, are among the most popular flowering shrubs in Ohio. In many urban
landscapes, these shrubs thrive and have relatively few serious health problems
once they are established. However, they
do have some “special requirements” that must be met to, insure good health.
These special conditions match those in areas where rhododendrons and azaleas
are native. Rhododendrons and azaleas grow on forest floors in many parts of
the world, in shaded habitats, with acidic soils rich in organic matter. Soils
are often covered with a surface layer of decaying leaf litter.
Matching these conditions where native
rhododendrons and azaleas thrive is the key to their good health in the
landscape. Plant in
areas with good soil drainage, acid pH and partial shade that is sheltered from
direct afternoon sun and winter winds.
Some of the more common problems that can occur on rhododendrons and
azaleas in Ohio include iron deficiency, winter injury (burn), black vine
weevil, and Phytophthora root rot.
Iron Deficiency
Yellowing of rhododendron leaves is typical of
iron deficiency. The yellowing is
between the veins and more severe on younger leaves. This problem generally results from plants
growing in soils of improper pH. Rhododendrons must be grown in acidic soil
that is high in organic matter. If the
pH is above 6.0, soil amendments such as sulfur, iron sulfate or ammonium
sulfate must be incorporated into the root area to lower pH. It will be difficult to overcome the
deficiency problem in soils high in lime or calcium, even with soil
amendments. In such cases, mulch the
plant heavily with a good grade of sphagnum peat. Bark mulch mixed with the peat provides mulch
with good aeration and drainage. It also
suppresses root rotting organisms. If
kept moist, plants will root into this mulch.
Use a complete acid fertilizer that contains iron.
Winter Burn
Leaf-drying and browning can occur on
rhododendron leaves as a result of winter exposure. The leaves, even though they may be rolled-up
at times, are subject to drying out in dry winter air. The solution is to protect the plant from the
drying wind. Plant rhododendrons behind
buildings or other plants that can serve as wind shields. Put wind shields in place around the plants
during the winter months. Mulching as
described above is critical to preventing winter injury.
Black Vine Weevil
The adult weevils feed on rhododendron leaves
producing a C-shaped notching in the leaf margin (figure 3). These insects can be quite damaging. Most of the damage comes from weevil larvae
feeding on the roots. Affected plants
lose vigor, and may die eventually. The
insecticide Orthene is registered for use on
rhododendrons, and it will control black wine weevil adults. The key to
control, of course, is to kill the adults early in the season before they lay
too many eggs.
Figure 1. C-shaped
notching in leaf margin caused by Black Vine Weevil.
Phytophthora
Root Rot (Rhododendron Wilt)
This disease is caused by a soil-borne fungus (Phytophthora spp.). Generally, it is a problem where wet
(saturated) soil conditions occur frequently.
Early symptoms of the disease consist of retarded growth, drooping of
foliage (perhaps on one or two branches only) and yellowing of leaves (figure
4). Infected roots appear dark and
“mushy”. As the disease progresses, a
browning discoloration of the wood may extend upward from the base on affected
branches (figure 5). Plants in poorly
drained soils are very subject to waterlogging which
makes them highly susceptible to this disease.
If this occurs, plants may die quickly.
Figure
2. This plant is wilting because it is
infected with fungal root and crown rot.
Whereas infected plants cannot be cured, root
rot may be tolerated by the plant if improvements in soil drainage and aeration
are made as soon as possible. Young
plants can be lifted and replanted.
Before replanting, improve the drainage and aeration of the soil. Use drainage or add porous materials in a
layer beneath the root zone. Plant in a
raised bed and do not mound the soil up around the crown. Mulching with tree bark provides biological
control. The mulch must be applied to a
depth of two inches and reapplied as it decomposes.
Figure 3. Brown
discoloration at the crown in rhododendron caused by fungal crown rot.
If plants cannot be lifted and replanted, try to
improve drainage and lessen the occurrence of over watering by redirecting rain
runoff, placement of drain tiles, and changes in irrigation programs. If the plants die from root rot, it would be
unwise to replant another rhododendron in the site without considerable
improvement in the soil conditions.
Caroline and English Roseum are rhododendron
cultivars with some resistance to this disease
Information obtained
through the Ohio State Extension Factsheet
HYG-3043-96
Insect and Disease Fact Sheet Compliments of
New Century
1-877-79TREES