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Pests, Insects and Disease
from: Lindgren, Dale and Wilde, Ellen. Growing Penstemons: Species, Cultivars and Hybrids. 2003. Haverford, PA. Infinity Publishing. pp. 110-113.
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
Aphidsare 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.
Borersare 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.
Cutwormscan 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 Bugsare 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 Beetlesof 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 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.
Grasshoppersare 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.
Lacebugsare 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.
Leafhoppersare 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 Bugsare 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 Scaleis 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 white-colored hard shell, approximately ¼inch long. The toxin injected into the plant causes a 'pit-forming' 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 Mitesare 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.
Thripsare 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 Mothlarvae 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.
Diseases
Leaf Spotattacks 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 Offis 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.
Viruseshave 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.
Nematodescan 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.
Rustappears 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.