An understanding of the level of tolerance of specific plants to geographically local insect pests and diseases is critical in designing a landscape that minimizes the use of pesticides. Selecting plants that are not susceptible to pest problems decreases the potential need for pest control treatments while choosing pest prone plants increases the potential need for pest control practices and the likelihood of regular applications of pesticides.
For further information, see the documents Pest Tolerance of Ornamental Plants, Introduction to IPM for "Urban" Landscapes, Landscape Design for IPM and Low Maintenance, and the IPM prescriptions for some common insect pests and diseases. Please note that the information provided below is only intended as an example to illustrate the importance of plant selection. Most of the examples apply in particular to the Pacific Northwest, although some (such as rose diseases) are common problems anywhere the species is grown. It is important to understand the specific pest and disease problems for each biogeographic region and their level of occurrence for plants grown in that climatic area.
Most of the pest-related images below have
been taken from Landscape Plant Problems: A Pictorial Diagnostic
Manual, a 1996 publication produced by Washington State University
Cooperative Extension and the Washington Department of Ecology. To
order and for more online and printed WSU Cooperative Extension materials,
visit their website at http://caheinfo.wsu.edu.
Aggressive Ornamental Plants
There are a number of common ornamental plants
that spread so aggressively as to warrant noting and avoid their use in
all but the most prudent situations. At best, some species will outcompete
other desirable plantings in a localized setting, and at worse, some very
widely used species have become major problems by escaping into natural
areas where they can completely exclude the native plant communities in
significantly large areas. For example, some species that have been
used as ornamentals and are now problematic in the Pacific Northwest include
English ivy, purple loosestrife, English hawthorn, yellow flag iris, English
laurel, creeping buttercup, black locust, and vinca.
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| English ivy is a very common but high maintenance ground cover. It's climbing habit and fast growth rate require regular pruning to keep it from smothering all other plants in the area. Especially in the maritime Pacific Northwest climate, English ivy spreads aggressively and invades woodlands, displacing native plants and killing trees. | ||
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Purple loosestrife (Lythrum salicaria) is a prime example of an introduced ornamental plant becoming a noxious weed. It is attractive in the ornamental garden but spreads into wetlands where it can rapidly take over extensive areas. Please see Fish and Wildlife Service's National Wildlife Refuge website and the Manitoba Purple Loosestrife Project's Purple Loosestrife InfoCentre for more information and further links. |
Common Insect Pest and Disease Problems
Below are a few examples
of commonly used plants that have notable pest or disease problems.
In some cases, these problems occur anywhere throughout the country (e.g.,
rose diseases), whereas in other cases they are more regional (e.g., root
weevils and lace bugs on rhododendrons in the Pacific Northwest).
Please note that links to prescriptions have been provided for further
information if they are currently available online at IPM Access.
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| Roses are host
to numerous diseases and insect pests wherever they are grown. Shown
on the top row of photos is black spot,
which can infect both leaves and stems. Below left is powdery
mildew. Below right is rust.
Common insect pests not shown are aphids
and Japanese beetles. Roses are also a favorite food of deer.
Hybrid tea roses are most prone to disease, while many species roses and heirloom varieties exist that are highly disease resistant. |
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Verticillium wilt is becoming increasingly common, especially on Japanese maples and other Acer spp. Dieback of individual branches is a characteristic symptom. Verticillium is a water borne disease that persists in the soil and after an area has become infected maples and other susceptible species (e.g., redbud, smoke tree) should not be used. |
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| Shown above is brown rot on flowering cherry. Ornamental plums and all other Prunus spp. are also susceptible. Top photo shows diseased blossoms, lower left shows infected fruit, lower right shows characteristic "flagging" of leaves from infected twigs that have died back. Cankers on bark of trunk and limbs is also common, and they sometimes exude a resinous substance. Prunus spp. are also prone to numerous other diseases and insect pests and their use in the landscape should be limited in favor of other less disease prone small flowering trees (e.g., Japanese snowbell, yellowwood, redbud, goldenraintree, Japanese pagodatree). | |
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| Anthracnose has become increasingly prevalent on flowering, Pacific, and other Cornus spp. Characteristic symptoms include small, brown, elongated spots along the veins, margins and eventually entire leaf; premature leaf drop; infected leaves often persist into winter. Cornus kousa is a good alternative to flowering dogwood and corneliancherry (Cornus mas) is also resistant. | ||
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| Root weevils can be a significant problem on rhododendrons and also evergreen viburnams. The adults feed on the leaf margins at night, causing the notching as seen above, but the larvae (below right) are what cause the damage by feeding on the roots. Predatory nematodes have become a widely used and viable alternative to conventional insecticides. | |||
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| Lace bug is another common pest of rhododendron. They cause damage to the leaves by sucking the plant juices from the undersides. A light stippling on the top sides of the leaves is caused when populations are at damaging levels. | |||
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| Powdery mildew is exceedingly common on some varieties of deciduous azaleas, especially when growing in the shade and where air circulation is poor. A number of deciduous azalea varieties have either little or no problem with powdery mildew and these should be selected over the susceptible varieties. | Scale is a common insect pest of deciduous azalea. |
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Spider mites can be a very serious problem on a number of broadleaf evergreen species, especially in dry, dusty conditions. Damaging population levels cause stippling and a dramatic bleached appearance on the leaves. |
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| Sequoia pitch moth is a common pest of pine and other conifers in the West (although it has never been observed on Sequoia). The larvae burrow into the sapwood of the lower trunk and limbs and exude large pitch masses from the wounds. Their presence is a sign of stress from either too much or not enough moisture. | European pine shoot moth mine the buds and candles of pines and also cause large pitch masses. Left photo shows larval damage, adult moth at right. | |
Native plant species often make excellent ecological and aesthetic contributions to ornamental and other types of landscapes, but they can also have diseases and insect pests! Below are some examples of useful natives in the Pacific Nortwest, even though diseases can sometimes be a problem.
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Leaf spot on salal (Gaultheria shallon). |
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