Diseases and Insects of Shrubs
and Small Trees
First in a series of articles
Azalea/Rhododendron - Rhododendron spp.
Botrytis/gray mold –Botrytis
cinerea is a fungus that attacks a wide range of woody and herbaceous
plants. The fungus can attack
leaf buds, flower buds, leaves, flowers, stems and fruit. On most
woody plants, Botrytis is most often noticed in the open flowers.
It also causes buds to die (bud blast). Plants growing in stressful
conditions such as damp moist areas are most prone to infection.
The disease can over winter in dead as well as live plant material.
On Rhododendrons/azaleas, the disease can cause flower, twig, and
seedling blight. Brown lesions/spots in open flowers are a good
indication that Botrytis has infected the flower. The more petals
a flower has, the more likely it will get Botrytis following rainfall
or overhead watering. The fungus can infect and grow between 32°F
and 80°F. Good air circulation and other good cultural
practices can help reduce Botrytis infection.
rhododendron - Phytophthora spp. & Botryosphaeria
dothidea attack both leaves and woody stems. Spores of the
Phytophthora pathogen in the soil are splashed by rain onto the
wet weather helps infection to occur in a few days. The pathogen
survives in the soil on infected dead leaves. Phythophthora is
a water mold and needs plenty of moisture to cause infection and
to kill. The infected plant tissue is often invaded by Botryosphaeria
dothidea – a canker causing fungus. Botryosphaeria dothidea
is usually a very opportunistic canker organism. Phytophthora can
also cause collar rot and root rot of rhododendrons & azaleas.
To reduce the chance of these plants getting dieback, plant in
soils with good drainage, let soils dry between watering, use an
organic mulch so soil can not be splashed onto the plant, and remove
all fallen leaves.
Black vine weevil - Otiorhynchus sulcatus (sometimes
called the taxus weevil) larvae feed on feeder roots of plants.
Large numbers can
eat enough roots to cause the plants to decline and even die. However,
in most cases the plant is able to grow new feeder roots faster
than the black vine weevils eat them. The adults, which are all
females (there are no males needed for reproduction), emerge in
Northern Illinois during mid to late June. They emerge earlier
in the rest of the state. The wings are fused so they cannot fly.
They are often moved from one location to another on infested plant
material or in the soil around the plant’s root system. The
adults feed on plant foliage into the fall. The feeding damage
appears as notches along the edge of the leaves. (On yews, the
black vine weevil adults not only notch the needles, but sometimes
eat the tip end of the needle off.) Adults feeding on plants close
to a building often migrate into buildings as temperatures drop.
Inside the home, the insects are just a nuisance. Black vine weevils
feed on a wide range of plants including herbaceous plants such
as strawberries, hostas and lily-of-the-valley. In addition, the
strawberry weevil may feed on azaleas/rhododendrons causing similar
damage. The black vine weevil feeds at night so insecticides should
be applied just before dusk. The insect hides during the day – usually
in plant litter or mulch around the base of the plant. This adult
weevil is black with “gold” flecks scattered on its
fused wings. The black vine weevils begin to lay eggs about two
weeks after emerging from the soil as adults. Insecticidal control
relies on eliminating the adults before they lay eggs. They can
lay eggs throughout the summer months. After the eggs hatch the
larvae start feeding on feeder roots. The weevil often over-winters
as partially grown larva. Some adults may over-winter, if they
are in a heated building.
Two-spotted spider mite - Tetranychus urticae is
green to greenish yellow with two dark spots on its back. The females
in leaf litter or under the bark of several different trees and
shrubs. In addition to attacking azaleas and rhododendrons, this
mite attacks many other woody plants as well as numerous herbaceous
plants. Reproduction can occur throughout the growing season as
long as the weather is warm enough. Beneficial mites may be present
at the same time as the two-spotted spider mite. Plant feeding
mites such as the two-spotted spider mite move slowly and if crushed – leave
a greenish stain. Beneficial mites move fairly rapid and when crushed
leave a reddish stain. Check for mites by vigorously shaking/tapping
a branch onto a piece of white paper. Watch closely. The mites
will move but the debris will not. Do not treat if beneficial
mites are present since they are natural control for plant mites.
Treat if only the plant mites are present.
August/September 2004: Pet
Waste and Water Quality | Diseases and
Insects of Shrubs and Small Trees
| Fall Garden Wrap-Up | Managing
Thatch in Home Lawns