Diseases and Insects of Shrubs
and Small Trees
Third in a series of articles
Bridal Wreath & Vanhoutte Spirea - Spiraea
prunifolia & Spiraea x vanhouttei Dieback/cankers
- Nectria cinnabarina is a saprophytic fungus that will
invade and kill stressed plant tissue. Under the right conditions,
the canker will kill the entire plant. In dead areas on the woody
tissue during the growing season, spore structures develop. The
spore structures vary in color from coral pink to pinkish orange
to purplish red. As they age, the color changes to tan, brown and
almost to a black like appearance. During summer months and into
fall, additional spore structures that are round and orange red
in color develop among the other spore structures. These can persist
into the winter. Wet weather helps disperse the spores in these
structures. The spores infect dead buds and other winter injured
plant parts. The spores also infect through pruning wounds. Keep
plants healthy by growing the plants in the right location and use
good cultural practices in maintaining them.
Powdery mildew - Podosphaera clandestine is a fungus
that has subgroups that are host specific. Powdery mildew fungi
are white and look like powdered sugar. Most of the fungus grow
on the leaf surface, but some of it may penetrate the inner cells
of leaves and buds and even small twigs. In Illinois, the weather
conditions are the same for almost all the powdery mildew fungi.
The fungi need three consecutive days and nights that are warm &
dry during the day and cool and humid at night. Freestanding water
on the leaves actually inhibits powdery mildews from growing. However,
freestanding water encourages many other foliar diseases to grow.
If infection starts in the spring or early summer, consider treatment
with a fungicide to prevent further infection and severe symptoms.
If infection starts in late summer or fall, do not treat. Some powdery
mildews can overwinter on dead leaves as well as live tissue. The
disease is air borne.
Verticillium wilt - Verticillium spp. are fungi that live
in the soil and attack roots of many woody and herbaceous plants.
The fungus invades the root and may travel up the xylem or may release
spores which move upwards more quickly. In either case, this infection
results in the development of brown streaks in the sapwood. In some
plants, the color will vary from brown to green to yellow to purple
and various combinations of all these colors. All gymnosperms (e.g.,
pine, spruce, juniper) and monocots (grasses) are immune to the
two Verticillium species found in North America. Although not immune,
there are several dozen types of woody dicots that are considered
resistant to infection. Verticillium wilt can cause a rapid death
in plants or a slow death. When the plants die quickly, removal
and destruction of the plants is the best recourse. For plants dying
slowly, prune out the dead and dying, fertilize correctly, and water
properly. Make sure plants have adequate drainage. When putting
new plants in, make sure they are the right plant for the growing
site and plant and maintain using optimum cultural practices.
Spirea aphid - Aphis citricola is found in every state
on spireas. The aphid occurs in large numbers and its feeding often
causes the leaves to distort, curl and pucker. The aphid during
the warm weather gives birth to live young that are all females.
The new aphids can soon produce their own live young. In northern
states, as cooling weather and shortening days occur, the aphids
are born in a sexual state and lay eggs that over winter on any
spirea that is available. Over crowding causes the aphid to give
birth to winged aphids so that they can fly to other branches and
spireas. Heavy rains can wash the aphids off the plants resulting
in their deaths. Occasionally in Illinois (usually during hot muggy
weather) a fungus will attack and kill the aphids. In addition,
many predators feed on aphids. The aphid has a dark head and thorax
while the abdomen is pale green.
Burning bush (also called Winged Euonymus) -Euonymus
Cold injury – Winter injury may be caused by very low temperatures
as well as drought stress. With excessively low temperatures, the
moisture in the cells freezes (due to chemical compounds in plants,
moisture freezes at various degrees below freezing). Drought stress
already has resulted in limited moisture in the plant cells. Dry,
freezing winds during the winter reduces the moisture level even
farther, often resulting in dead plant tissue. Diseases can help
magnify or increase susceptibility to winter kill. Nectria canker
kills the sapwood tissue thus reducing or even cutting off moisture
to tissue further out on the plant. Winterkill also makes plants
more prone to infectious diseases and insect problems.
Dieback/canker – See bridal wreath spirea. In addition Botryosphaeria
dothidea will infect and kill for similar reasons.
Winged euonymus scale - Lepidosaphes yanagicola occasionally
occurs in the southern half of Illinois on burning bush. It is an
armored scale. And will attack several trees as well. This scale
can cause premature leaf drop, branch die back and cause the plant
to become more prone to winter injury. It is found between the “wings”
– the bark ridges. It does not move to the plant’s leaves.
The scale over winters as an adult and lays its eggs in June. Eggs
may be laid for up to a month. Mating occurs before frost.
Euonymus scale - Unaspis euonymi – females are black
and males are white. The scale causes the foliage to develop yellowish
green spots. Heavy infestation results in early foliage drop and
often stems are killed. Eggs survive by over wintering in the female
body. The eggs hatch about early June in Northern Illinois. Crawlers
emerge and move onto new growth or can be blown by wind to other
December 2004 - January 2005: Choosing
a Christmas Tree Variety | Diseases and Insects of Shrubs and
Small Trees | Catalogs are Arriving, Plan Your
Spring Garden Now | Keeping Holiday Plants
| Prevent Ice and Snow Damage to Trees and