Diseases and Insects of Shrubs
and Small Trees
Second in a series of articles...
Barberry, Japanese - Berberis thunbergi
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 that 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.
There are no major insect pests on barberry in Illinois
Boxwood - Buxus spp.
Dieback/canker - 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. Winter injury
makes boxwood more prone to this canker fungus. This disease also
makes the plant more prone to winter injury. 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.
Verticillium wilt – see Barberry
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. Leaves are usually the first
to die, followed by buds and then the smaller twigs. 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.
Yellownecked caterpillar, Datana ministra, eats a wide
range of trees and shrubs. Found mostly in the southern two thirds
of Illinois, they occasionally show up in the northern third of
Illinois. They travel and eat in mass (usually thirty to a hundred)
for protection. They also raise the front and rear portions of their
body in mass when a possible predator (birds & other insects)
appears. These insects over winter as pupae and the adult moths
emerge in early summer. Eggs are laid on the lower side of leaves.
Six to eight weeks later newly emerged caterpillars eat holes in
the leaves leaving the veins alone. As the caterpillars enlarge,
they eat the entire leaf leaving the just the petiole. There is
a yellow to orange band around the body just behind the head. This
is how they got their name. The stripes along the side of their
bodies also vary some. They may be yellow and black to orangish
and brown striped. The head is black.
Boxwood leafminer, Monarthropalpus flavus, over winters
as a partially grown larva in the boxwood leaves. Warm weather in
the spring helps the larva finish growing and become a pupa. A few
days before the adult emerges, the pupa wiggles out of the mine
to the surface of the leaf. A fly emerges from the pupa case. The
flies are about the size of a gnat. After mating, females lay slightly
more than two-dozen eggs inside the upper tissue of new leaves.
Several weeks later, the larvae emerge and begin feeding. It is
not uncommon to find multiple larvae in the same mine. Larvae grow
slowly through the summer. Damage starts to appear about midsummer
as yellow spots. Leaves may fall off early. In addition, heavy infestation
can cause plants to look thinned out – even killing branches.
Heavy infestation make the plants more prone to winter kill and
diseases. There are few natural predators but there are boxwoods
known to be resistant to this insect.
Oystershell scale, Lepidosaphes ulmi, is an armored scale
(its shell is a hardened waxy material). It attacks a wide range
of plants. The scale got its name because it resembles an oyster
shell. It is found in every state but it is found more often and
in heavier numbers in the northern states. The scale often blends
in with the color of the plant’s bark. The scale is often
“discovered” because branches are dying back from the
feeding injury. There are two commonly found races of oystershell
scale in Illinois. They are brown oystershell scale and gray ostershell
scale. The scale overwinters as eggs under the mother’s shell.
Crawlers of the brown race occur on dogwood when Vanhoutte spirea
is in full bloom (about mid June in Northern Illinois). The crawlers
of the gray race, which feeds on other hosts, emerge later.
October/November 2004: Diseases and Insects
of Shrubs and Small Trees | Does Your Ash Tree
Have the Emerald Ash Borer? | Harvesting
and Storing Pumpkins, Winter Squash, and Gourds