Pests, Insects and Disease
from: Lindgren, Dale and Wilde, Ellen. Growing Penstemons: Species,
Cultivars and Hybrids. 2003. Haverford, PA. Infinity Publishing. pp. 110113.
Penstemons are generally less susceptible to pests than most plants if given appropriate
growing conditions. Older foliage and stems are bitter and rabbits, deer, quail, slugs, snails etc.
generally do not like them if anything else is available. They may nip off stems and leave them, but
do not eat the basal leaves unless conditions are extremely dry. The following lists of insects
and diseases are seldom seen but have been reported on penstemons.
Although usually not severe, they can be quite damaging and unsightly in some situations.
The best defenses against insects and diseases are well-grown, healthy plants that are not under stress
and are in sites where air moves freely, the soil drains well, debris is regularly removed and
watering, if necessary, is done when the leaves will dry quickly. Overly fertilized plants with
lush growth are more subject to attack than those with normal growth. Integrated Pest
Management strategies are recommended as a first choice if you do have problems. You can read
about them in Organic Gardening books and on the Internet. Check with local plant specialists for
recommendations if chemical controls are needed. Follow all label directions carefully when using
chemicals to control pests on penstemons.
Insects
Aphids are small, less than 1/16 inch long, soft-bodied insects that suck juices from
penstemon foliage. There are many species and they come in various colors and multiply rapidly.
Symptoms of aphid feeding include curling leaves and leaf discoloration. They excrete a sticky
residue which may attract ants, flies and other insects. They are more apt to occur in crowded
borders and where air cannot move about the plants. They can spread diseases as well as
distorting leaves, so should be attended to as soon as noticed. A strong spray with 2 tablespoons
of liquid soap/gallon once or twice should get rid of them.
Borers are larvae that tunnel into flower stalks. Their presence is indicated by a hole with
sap or a mealy material near it. Cut off the affected stem and crush it.
Cutworms can destroy plants by cutting them off near ground level. Young plants are
most susceptible. They are usually not a problem except in gardens that are watered frequently.
There are several granular products that can be sprinkled on the soil if they do become a
problem.
False Plant Bugs are about 1/8 inch long and gray. They feed on young new shoots and
can also destroy buds and blooms. Use insecticidal soap spray.
Flea Beetles of several species are insects up to 1/8 inch long that can be seen jumping
off foliage when the plants are disturbed. They may chew holes in the leaves and even devour
whole leaves and plants. They usually do not bother healthy plants but may be found when plants
are stressed. Many times, just shaking the plants gently will dislodge the beetles and they can be
swept up or stomped. If they have laid eggs and the larvae are in the ground you will have to
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look for and go after them several times until no more appear. If you choose to use insecticides,
apply at the first sign of them. Several applications will probably be needed. The plants should
then be well-watered to alleviate stress.
Grasshoppers are plant feeders that may be a problem to all plants in dry years in the
mid-west and west. They usually need to be attacked on a large scale when they first appear as
they are very mobile. Most insect sprays do not affect mature grasshoppers.
Lacebugs are small insects, 1/8 inch long, that have clear wings with many veins as
adults while their immature form is dark, spiny and wingless. They suck sap from plant parts and
excrete a sticky fluid. Use insecticidal soap for control.
Leafhoppers are small, fast-moving, wedge-shaped insects that also suck sap from plants
and are usually found on the underside of the leaves. They can spread some virus infections and
if found should be treated immediately with an insecticide.
Lygus Bugs are small and brown, about 3/16 inch long, and damage plants by sucking
out sap. They kill young growing tips and deform flowers. Use insecticidal soap for control.
Pittosporum Pit Scale is probably the most serious problem that can occur on
penstemons because the insect is highly mobile, particularly among the Dasanthera and
Caespitosi sections of penstemons and many other genera. The scale is flat with a tan to whitecolored hard shell, approximately ¼ inch long. The toxin injected into the plant causes a 'pitforming' response. The stems become swollen, distorted, stunted and discolored. If you see it
when it is just getting started, immediately cut out any parts infected and get rid of them. Look
for the active insect stage in late winter and early spring and spray all plants in the area with a
refined horticultural oil in two applications a week to ten days apart. Systemic insecticide and
insecticidal soap can also be used.
Spider Mites are minute members of Arachnida, not a true insect. They suck sap from
leaves, leaving a whitish stippling on them that can lead to drying and death. They may also form
a dense webbing. Often a strong spray of water, being sure to reach the underside of the leaves,
will remove them. The plant should be watered well to restore vigor since healthy plants are less
likely to be attacked.
Thrips are small 1/16 inch long slender insects that scurry around on plant parts. They
like to hide in tight areas of the plants. They feed by rasping plant material and sucking the
released sap. Nymphs are creamy white but adults come in various colors. All above ground
parts of the plants are susceptible, but flowers and buds are particularly attacked. Infested buds,
flowers and other parts become stippled and distorted and can have a sand-papered appearance,
with small green fecal spots. Gently shake the plant to dislodge them and stomp or sweep up. If
this fails to control them, use insecticidal soap spray for control.
Verbena Bud Moth larvae have a habit of tying leaves and buds together with a silky
web. They may also move into seed capsules and feed on the developing seed. Remove as soon
as spotted.
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Diseases
Leaf Spot attacks stems and leaves. Fungi are the cause and dark spots develop from
which the middle often drops out, leaving a hole. Removal of affected leaves and stems,
cleaning up in the fall and, only if it becomes a serious problem, application of an appropriate
chemical control, are recommended. Many new varieties of cultivars and hybrids have been
selected for disease resistance.
Among Dasanthera Black Spot is likely to occur in the Northwest if there is a warm and
wet spring.
Powdery Mildew appears as a whitish, almost velvety growth on the surface of leaves. It
can cause leaves to become distorted, discolored and die. Such leaves should be promptly removed.
It is more prevalent in damp climates and as summer temperatures cool. Mid-western species are the
most susceptible. Keep foliage dry as much as possible. Use a fungicide only for severe cases.
Stem and Crown Rots can be caused by a single disease organism or by a complex of
diseases. Plants wilt and die quickly. Remove plants and do not replace with anything related. The
best protection against this too frequent problem is to be sure soil is not over-watered and drains
well.
Damping Off is primarily a problem with germinating seeds. Clean, sterile rooting
mixtures and containers, good air movement, bright light and temperatures below 60 degrees F. will
help avoid the problem.
Viruses have occasionally been reported to infect penstemons. Symptoms include
stunting and leaf distortion. They are spread by aphids and leafhoppers. Remove and dispose of
plants properly as soon as discovered. Never propagate infected plants. No cures have been
found.
Nematodes can infect penstemon roots in rare cases. Irregular swellings are found on the
roots of the plant when it is dug up after it has turned yellow or wilted and died. Plant should be
removed and disposed of properly. Replace with other plants that are not susceptible.
Rust appears as spots filled with orange to reddish dust-like material which is actually a
mass of microscopic spores. Most gardeners tolerate it and only try to control it with a fungicide in
severe infections.